What is a dictionary of units of measurement?

It is a reference that defines each unit rather than only converting it — what the unit is, where its value comes from, and how it relates to the SI base unit. This page covers 127 units across 8 categories, and every size shown is computed from the same factor the converters use.

Why do some converters disagree about the same unit?

Because some units genuinely have more than one value. The Indian bigha differs by state, the marla was rounded differently in Indian and Pakistani surveys, and the foot-pound names both a unit of energy and (as the pound-foot) a unit of torque. A converter that prints one number for these is hiding a choice it made. Where a unit is region-variable we publish the per-state basis instead of a national average.

Is a newton-metre the same as a joule?

As a unit of energy, yes — 1 N·m = 1 J exactly. As a unit of torque, no. Torque is a rotational quantity and does not convert into joules, kilowatt-hours or calories, even though the number and the symbol look identical. The same trap exists in US customary units between the foot-pound (energy) and the pound-foot (torque).

Which unit values are exact and which are approximate?

Values fixed by international agreement are exact: 1 inch = 25.4 mm, 1 foot = 0.3048 m and 1 pound-force = 4.4482216152605 N have been exact by definition since the 1959 international yard and pound agreement. Values derived from those are exact too. Traditional and regional units — bigha, biswa, katha, dhur — are not standardised and vary by jurisdiction.

A Dictionary of Units of Measurement

A reference to 127 units across 8 categories — area, length, volume, energy, time, speed, angle and data size. Each entry gives the unit's symbol, its size in the SI base unit, and the definition that value comes from. Every figure below is computed from the same factor the live converters use, so the reference and the tool can never drift apart. Last updated 2026-08-01.

Most conversion sites print a number and leave you to trust it. Where a unit genuinely has more than one value — and several below do — this page says so and gives the basis rather than an average.

Why a units reference needs a basis, not just a number

A conversion factor is a claim about the world, and for a surprising number of units that claim is contested. Three patterns cause almost all of the disagreement between converters:

The unit varies by place. The Indian bigha has no national value — it differs by state, and in parts of Uttar Pradesh by district. A converter that prints a single bigha figure has quietly picked one jurisdiction. We publish the per-state basis instead.
One name, two physical quantities. A newton-metre of energy is exactly one joule. A newton-metre of torque is a different quantity that does not convert into joules at all, even though the number is identical. The same is true of the foot-pound (energy) and the pound-foot (torque), which is why torque-wrench specifications and work calculations must not be mixed.
The unit was redefined. Values fixed by the 1959 international yard and pound agreement are exact by definition — 1 inch is 25.4 mm and 1 foot is 0.3048 m, not approximations. Older tables sometimes still carry pre-1959 survey values, which differ in the far decimals and matter over long distances.

The reference

Each table lists every unit in that category with its size in the category's base unit, written out in full rather than rounded. A value fixed by international agreement is shown to its last defined digit — an acre is exactly 4,046.8564224 m², not "about 4,046.86". Where a unit is defined as a fraction that never terminates, such as a degree in radians, the figure is marked ≈ because no decimal can state it exactly. Follow the category link under any table to convert between any pair of its units.

Area — 31 units, measured in square metres (m²)

Unit	Symbol	Size in square metres (m²)	Definition
Gaj	gaj	0.83612736	A gaj is a square yard (9 sq ft) — the everyday unit for plots and houses across North India.
Marla	marla	25.29285264	A marla is 272.25 sq ft (30.25 gaj) — exactly 1/160 acre, and the size used in official land records in Indian Punjab/Haryana and in Pakistan. Many Pakistani housing societies sell a smaller 225 sq ft marla, so check which one your papers use.
Kanal	kanal	505.8570528	A kanal equals 20 marla (5,445 sq ft). 8 kanal make 1 killa (acre). Common in Punjab, Haryana, J&K, HP and across Pakistan — though societies using the smaller 225 sq ft marla count a kanal as 4,500 sq ft.
Killa	killa	4,046.8564224	A killa is the local name for an acre (4,840 gaj) in Punjab and Haryana — 8 kanal or 160 marla.
Murabba	murabba	101,171.41056	A murabba ("square") is 25 killa (25 acres) — the large land block used in Punjab/Haryana revenue records.
Decimal	decimal	40.468564224	A decimal is 1/100 of an acre (48.4 gaj) — widely used in eastern India (Bihar, Bengal, Odisha, Jharkhand).
Cent	cent	40.468564224	A cent is 1/100 of an acre — identical to a decimal — and is the common small land unit in South India.
Guntha	guntha	101.17141056	A guntha is 1/40 of an acre (121 gaj) — the standard unit in Maharashtra, Karnataka and Andhra/Telangana.
Acre	ac	4,046.8564224	An acre is 4,840 square yards (43,560 sq ft) — the standard imperial land unit.
Rood	rood	1,011.7141056	A rood is a quarter of an acre — 1,210 square yards (10,890 sq ft), or 40 square perches. It is the middle figure in the old "acres, roods and perches" way of writing a land area, so 3 roods means three-quarters of an acre.
Square Perch	perch²	25.29285264	A square perch is 1/160 of an acre — 30.25 square yards (272.25 sq ft). Also called a square rod or square pole, it is the smallest of the three units in "acres, roods and perches", and 40 of them make one rood.
Hectare	ha	10,000	A hectare is 10,000 square metres (about 2.471 acres) — the metric land unit.
Square Foot	ft²	0.09290304	A square foot is the area of a 1 ft × 1 ft square — the common unit for built-up area.
Square Yard	yd²	0.83612736	A square yard (9 sq ft) is identical to a gaj — used for plots and carpet area.
Square Metre	m²	1	The square metre is the SI unit of area — a 1 m × 1 m square.
Square Centimetre	cm²	0.0001	A square centimetre is a 1 cm × 1 cm square — 1/10,000 of a square metre.
Square Millimetre	mm²	0.000001	A square millimetre is a 1 mm × 1 mm square — the smallest common metric area unit.
Square Inch	in²	0.00064516	A square inch is a 1 in × 1 in square — 6.4516 cm².
Square Kilometre	km²	1,000,000	A square kilometre is a 1 km × 1 km square — 100 hectares.
Are	are	100	An are is 100 square metres (a 10 m × 10 m square) — the metric unit the hectare (100 ares) is built from, and the historic basis of the "cent"/"decimal".
Decare	daa	1,000	A decare is 1,000 square metres (10 ares, one-tenth of a hectare) — a metric land unit used in Norway, Turkey and the Balkans, and close to the Middle-Eastern dunam.
Square Mile	mi²	2,589,988.110336	A square mile is a 1 mi × 1 mi square — 640 acres (2.589988 km²), the large imperial/US unit for districts, farms and map areas.
Acre	ac	4,046.8564224	An acre is 4,840 square yards (43,560 sq ft) — the standard imperial land unit.
Square Yard	yd²	0.83612736	A square yard (9 sq ft) is identical to a gaj — used for plots and carpet area.
Square Foot	ft²	0.09290304	A square foot is the area of a 1 ft × 1 ft square — the common unit for built-up area.
Hectare	ha	10,000	A hectare is 10,000 square metres (about 2.471 acres) — the metric land unit.
Square Karam	karam²	2.81031696	A square karam (sarsahi) is a Punjab/Haryana land unit — the square of a 5.5 ft karam, so exactly 30.25 sq ft (2.81 m²). 9 sarsahi = 1 marla.
Ground	ground	222.967296	A ground is a Tamil Nadu / South-India land unit equal to 2,400 sq ft (222.97 m²) — common in Chennai property listings.
Lessa	lessa	≈13.37804	A lessa (lecha) is an Assam land unit of 144 sq ft (12 ft × 12 ft, 13.38 m²): 20 lessa = 1 katha and 100 lessa = 1 bigha in Assam. Multi-source verified for Assam; the unit differs in other states.
Chatak	chatak	4.1806368	A chatak is a Bengal-region land unit (West Bengal, Bangladesh) equal to 45 sq ft (4.18 m²) — a fixed subdivision of the rural Bengal katha (1 katha = 16 chatak = 720 sq ft).
Pura	pura	≈5,351.215	A pura (পূৰা) is an Assam land unit equal to 4 Assam bigha = 57,600 sq ft (5,351.215 m²). Multi-source verified from Wikipedia, land-record references and real property listings for Assam.

Time — 8 units, measured in seconds (s)

Unit	Symbol	Size in seconds (s)	Definition
Nanosecond	ns	0.000000001	A nanosecond is one-billionth of a second (1 s = 1,000,000,000 ns) — the timescale of CPU clock cycles and memory access.
Microsecond	µs	0.000001	A microsecond is one-millionth of a second (1 s = 1,000,000 µs; 1 ms = 1,000 µs) — used for high-resolution timers and signal timing.
Millisecond	ms	0.001	A millisecond is one-thousandth of a second (1 s = 1,000 ms) — the unit for web-page load times, API timeouts, ping/latency and animation frames.
Second	s	1	The second is the SI base unit of time. Everyday durations, timeouts and stopwatch readings are usually measured in seconds.
Minute	min	60	A minute is 60 seconds — the common unit for short everyday durations.
Hour	hr	3,600	An hour is 60 minutes (3,600 seconds) — the standard unit for scheduling and work.
Day	day	86,400	A day is 24 hours (86,400 seconds) — one full calendar day.
Week	wk	604,800	A week is 7 days (604,800 seconds).

Length — 30 units, measured in metres (m)

Unit	Symbol	Size in metres (m)	Definition
Kilometre	km	1,000	A kilometre is 1,000 metres — the metric unit for road and map distances.
Metre	m	1	The metre is the SI base unit of length, from which every other unit here is defined.
Decimetre	dm	0.1	A decimetre is one-tenth of a metre (10 cm).
Centimetre	cm	0.01	A centimetre is one-hundredth of a metre — the everyday small-length metric unit.
Millimetre	mm	0.001	A millimetre is one-thousandth of a metre.
Micrometre	µm	0.000001	A micrometre (micron) is one-millionth of a metre — used for cells, wavelengths and machining tolerances.
Mile	mi	1,609.344	A mile is 5,280 feet (1,760 yards), exactly 1,609.344 m — the imperial/US road-distance unit.
Yard	yd	0.9144	A yard is 3 feet, defined as exactly 0.9144 m.
Foot	ft	0.3048	A foot is 12 inches, defined as exactly 0.3048 m.
Inch	in	0.0254	An inch is exactly 2.54 cm (1/12 of a foot).
Nautical Mile	nmi	1,852	A nautical mile is exactly 1,852 m — one minute of latitude, used in sea and air navigation.
Furlong	fur	201.168	A furlong is 660 feet (1/8 of a mile) — still used in horse racing.
Chain	ch	20.1168	A Gunter's chain is 66 feet (100 links, 1/10 furlong) — the basis of old land surveys.
Link	li	0.201168	A link is 1/100 of a surveyor's chain — 7.92 inches (0.66 ft).
Rod	rd	5.0292	A rod (also called a perch or pole) is 16.5 feet, exactly 5.0292 m; 4 rods make a chain.
Pole	pole	5.0292	A pole is the same length as a rod or perch — 16.5 feet (5.0292 m).
Fathom	ftm	1.8288	A fathom is 6 feet (1.8288 m) — traditionally used to measure water depth.
Hand	hand	0.1016	A hand is exactly 4 inches (0.1016 m) — the standard unit for the height of horses.
Span	span	0.2286	A span is 9 inches (0.2286 m) — the distance across a spread hand; 4 fingers = 1 span.
Finger	finger	0.1143	A finger (cloth measure) is 4.5 inches — 1/8 of a yard, 11.43 cm. It is NOT the same as a fingerbreadth/digit (about ¾ inch).
Cubit	cubit	0.4572	A cubit is about 18 inches (0.4572 m) — elbow-to-fingertip. Ancient values varied; 18 inches is the common English value used here.
Rope	rope	6.096	A rope is 20 feet (6.096 m) — a traditional English surveying length.
Ken	ken	≈1.818182	A ken is a Japanese unit of 6 shaku, standardised since 1891 as 60/33 m ≈ 1.818 m.
League	lea	4,828.032	A statute (land) league is 3 miles — 4,828.032 m.
Point	pt	≈0.0003527778	A DTP point is 1/72 of an inch (≈0.3528 mm) — the unit used for font sizes.
Astronomical Unit	AU	149,597,870,700	An astronomical unit is the average Earth–Sun distance, defined as exactly 149,597,870,700 m.
Light-year	ly	9,460,730,472,580,800	A light-year is the distance light travels in vacuum in one Julian year — exactly 9,460,730,472,580,800 m (≈9.46 × 10¹⁵ m).
Decametre	dam	10	A decametre is 10 metres — the metric step between the metre and the hectometre.
Hectometre	hm	100	A hectometre is 100 metres — the metric step between the decametre and the kilometre.
Nautical League	nl	5,556	A nautical league is three nautical miles — exactly 5,556 m — a traditional sea-distance unit.

Volume — 24 units, measured in litres (L)

Unit	Symbol	Size in litres (L)	Definition
Millilitre	mL	0.001	A millilitre is one-thousandth of a litre — equal to one cubic centimetre.
Centilitre	cL	0.01	A centilitre is one-hundredth of a litre (10 mL).
Litre	L	1	The litre is the common metric unit of volume — exactly 1,000 cubic centimetres.
Cubic Metre	m³	1,000	A cubic metre is a 1 m × 1 m × 1 m cube — 1,000 litres.
Cubic Centimetre	cm³	0.001	A cubic centimetre (cc) is a 1 cm cube — identical to a millilitre.
Cubic Inch	in³	0.016387064	A cubic inch is a 1 in cube — exactly 16.387064 mL.
Cubic Foot	ft³	28.316846592	A cubic foot is a 1 ft cube — exactly 28.316846592 litres (1,728 cubic inches).
US Gallon	US gal	3.785411784	A US (liquid) gallon is 231 cubic inches — exactly 3.785411784 litres; smaller than the imperial gallon.
US Gallon	gal	3.785411784	The US (liquid) gallon — 3.785411784 litres (231 cubic inches).
US Quart	US qt	0.946352946	A US liquid quart is 1/4 of a US gallon — 0.946352946 litres.
US Quart	qt	0.946352946	The US liquid quart — 1/4 US gallon, 0.946352946 litres.
US Pint	US pt	0.473176473	A US liquid pint is 1/8 of a US gallon (16 US fl oz) — 0.473176473 litres.
US Pint	pt	0.473176473	The US liquid pint — 16 US fluid ounces, 0.473176473 litres.
US Fluid Ounce	US fl oz	0.0295735295625	A US fluid ounce is 1/128 of a US gallon — 29.5735295625 mL; larger than the imperial fluid ounce.
US Fluid Ounce	fl oz	0.0295735295625	The US fluid ounce — 29.5735295625 mL (1/128 US gallon).
Imperial Fluid Ounce	imp fl oz	0.0284130625	An imperial (UK) fluid ounce is 28.4130625 mL — slightly smaller than the US fluid ounce.
US Tablespoon	tbsp	0.01478676478125	A US tablespoon is 1/2 US fluid ounce — 14.78676478125 mL (3 teaspoons).
US Teaspoon	tsp	0.00492892159375	A US teaspoon is 1/6 US fluid ounce — 4.92892159375 mL.
US Gill	gi	0.11829411825	A US gill is 4 US fluid ounces — 1/32 US gallon, 118.29411825 mL.
US Bushel	bu	35.23907016688	A US bushel is a dry-volume unit defined as exactly 2,150.42 cubic inches — about 35.239 litres.
Oil Barrel	bbl	158.987294928	The oil barrel is exactly 42 US gallons — 158.987294928 litres (the standard for crude oil).
Microlitre	µL	0.000001	A microlitre is one-millionth of a litre (one-thousandth of a millilitre).
US Cup	US cup	0.2365882365	A US customary cup is 8 US fluid ounces — 236.5882365 mL. (The metric cup of 250 mL and the legal US cup of 240 mL differ slightly.)
US Fluid Ounce	US fl oz	0.0295735295625	The US fluid ounce — 29.5735295625 mL (1/128 US gallon, 1/8 US cup).

Energy — 12 units, measured in joules (J)

Unit	Symbol	Size in joules (J)	Definition
Joule	J	1	The joule is the SI unit of energy — the work done by a force of one newton acting over one metre (1 J = 1 N·m), and also one watt-second.
Newton-metre	N·m	1	A newton-metre is one newton applied over one metre. As a unit of ENERGY it is identical to the joule (1 N·m = 1 J); as a unit of TORQUE it measures rotational effort — a different physical quantity that does not convert into joules, kWh or calories.
Kilojoule	kJ	1,000	A kilojoule is 1,000 joules — the energy unit on food labels (shown next to kilocalories) and for everyday amounts of energy.
Megajoule	MJ	1,000,000	A megajoule is 1,000,000 joules (1,000 kJ) — used for larger energy quantities such as the energy content of fuels.
Watt-hour	Wh	3,600	A watt-hour is the energy of one watt sustained for one hour — exactly 3,600 joules. Commonly used to rate battery capacity.
Kilowatt-hour	kWh	3,600,000	A kilowatt-hour is 1,000 watt-hours — exactly 3,600,000 joules (3.6 MJ). It is the "unit" your electricity meter bills.
Megawatt-hour	MWh	3,600,000,000	A megawatt-hour is one million watt-hours — 3.6 × 10⁹ joules (3,600 kWh). Used for power-station output and grid-scale energy.
Foot-pound	ft·lbf	≈1.355818	A foot-pound is the energy of one pound-force acting over one foot — about 1.3558 joules. The same number (as the pound-foot, lbf·ft) is also the common US unit of TORQUE.
Megaelectronvolt	MeV	0.0000000000001602176634	A megaelectronvolt is the energy an electron gains across a one-million-volt potential — 1 MeV = 1.602176634 × 10⁻¹³ J (the elementary charge is exact since the 2019 SI redefinition). Standard in nuclear and particle physics.
Calorie	cal	4.184	A (small/gram) calorie is roughly the heat needed to raise 1 g of water by 1 °C — the thermochemical calorie is exactly 4.184 J. The food "Calorie" is actually a kilocalorie.
Kilocalorie	kcal	4,184	A kilocalorie (the food "Calorie", Cal) is 1,000 small calories — exactly 4,184 J. It is the energy figure printed on nutrition labels.
British Thermal Unit	BTU	1,055.05585262	A British thermal unit is roughly the heat needed to raise one pound of water by 1 °F — about 1,055.06 J (the International Table BTU). Common in heating, cooling (AC tonnage) and US energy work.

Speed — 7 units, measured in metres per second (m/s)

Unit	Symbol	Size in metres per second (m/s)	Definition
Metre per second	m/s	1	The SI unit of speed — metres travelled per second. 1 m/s = 3.6 km/h.
Kilometre per hour	km/h	≈0.2777778	Kilometres per hour — the metric road-speed unit. 1 km/h = 1/3.6 ≈ 0.2778 m/s.
Mile per hour	mph	0.44704	Miles per hour — the US/UK road-speed unit. 1 mph = 1.609344 km/h = 0.44704 m/s.
Foot per second	ft/s	0.3048	Feet per second — 1 ft/s = 0.3048 m/s = 1.09728 km/h.
Centimetre per second	cm/s	0.01	Centimetres per second — 1 cm/s = 0.01 m/s = 0.036 km/h.
Millimetre per second	mm/s	0.001	Millimetres per second — 1 mm/s = 0.001 m/s; used for slow, precise motion (feed rates, fluid flow).
Knot	kn	≈0.5144444	A knot is one nautical mile per hour — 1 kn = 1.852 km/h = 0.514444 m/s; used in sea and air navigation.

Angle — 6 units, measured in radians (rad)

Unit	Symbol	Size in radians (rad)	Definition
Radian	rad	1	The radian is the SI unit of angle — the angle subtended at a circle’s centre by an arc equal to the radius. A full turn is 2π rad.
Degree	°	≈0.01745329	A degree is 1/360 of a full turn. 1 rad = 180/π ≈ 57.2958°.
Gradian	gon	≈0.01570796	A gradian (gon) is 1/400 of a full turn — a right angle is exactly 100 gon. Used in surveying.
Turn	turn	≈6.283185	A turn (revolution) is one full rotation — 360° = 2π rad = 400 gon.
Arcminute	′	≈0.0002908882	An arcminute is 1/60 of a degree (1/21,600 of a turn) — used in astronomy and optics.
Arcsecond	″	0.00000484813681109536	An arcsecond is 1/60 of an arcminute (1/3,600 of a degree) — the resolution scale of telescopes.

Data size — 9 units, measured in bytes

Unit	Symbol	Size in bytes	Definition
Bit	bit	0.125	A bit is a single binary digit (0 or 1). 8 bits = 1 byte.
Byte	B	1	A byte is 8 bits — the basic addressable unit of computer storage.
Kilobyte	KB	1,000	A kilobyte is 1,000 bytes in the SI/decimal convention used by storage makers and networks. The 1,024-byte unit is the kibibyte (KiB).
Megabyte	MB	1,000,000	A megabyte is 1,000,000 bytes (decimal). The binary 1,048,576-byte unit is the mebibyte (MiB).
Gigabyte	GB	1,000,000,000	A gigabyte is 1,000,000,000 bytes (decimal). The binary unit is the gibibyte (GiB) = 1,073,741,824 bytes.
Terabyte	TB	1,000,000,000,000	A terabyte is 10¹² bytes (decimal). The binary unit is the tebibyte (TiB).
Kibibyte	KiB	1,024	A kibibyte is exactly 1,024 bytes (2¹⁰) — the binary (IEC) unit operating systems often label "KB".
Mebibyte	MiB	1,048,576	A mebibyte is exactly 1,048,576 bytes (2²⁰) — the binary unit often shown as "MB" by operating systems.
Gibibyte	GiB	1,073,741,824	A gibibyte is exactly 1,073,741,824 bytes (2³⁰) — the binary unit often shown as "GB" by operating systems.

Using this reference

Every unit above has a live converter. The converters run entirely in your browser — nothing you type is uploaded — and they show the same factor listed here, so you can check any result by hand. For the region-variable Indian land units there is a dedicated state-by-state guide, and the Punjab and Haryana system (marla, kanal, killa) is documented separately because its ratios are fixed rather than regional.

Sources and standards

Every size in the tables above is computed from that unit's definition in terms of its category's base unit, then re-verified automatically on each build — nothing is typed in by hand. The references below are the authorities those definitions come from. Where a unit is traditional or region-variable, such as the bigha or the katha, no international standard defines it and the land-record authority is cited instead.

Department of Land Resources, Ministry of Rural Development (India) — the central department responsible for land records and the Digital India Land Records Modernisation Programme, under which the state portals are maintained.
Bihar Bhumi — Revenue & Land Reforms Department, Bihar — the official Bihar land-record portal; the authority for the katha, dhur and dhurki values used in Bihar revenue records.
UP Bhulekh — Uttar Pradesh land records — the official Uttar Pradesh record-of-rights portal, used to corroborate bigha and biswa values for UP.
BIPM — The International System of Units (SI Brochure) — the defining document for the SI, published by the International Bureau of Weights and Measures. It fixes the base unit that this conversion resolves to, which is why the factor here can be exact rather than approximate.
NIST — Definitions of the SI base units — the US National Institute of Standards and Technology reference for the SI base units, used here as an independent cross-check on the base-unit definition.
NIST — Prefixes for binary multiples — the reference that separates the decimal prefixes (kB = 1,000 bytes) from the IEC binary ones (KiB = 1,024 bytes). This page keeps the two apart rather than silently mixing them, which is the usual source of disagreement between file sizes.

Questions

What is a dictionary of units of measurement?: It is a reference that defines each unit rather than only converting it — what the unit is, where its value comes from, and how it relates to the SI base unit. This page covers 127 units across 8 categories, and every size shown is computed from the same factor the converters use.
Why do some converters disagree about the same unit?: Because some units genuinely have more than one value. The Indian bigha differs by state, the marla was rounded differently in Indian and Pakistani surveys, and the foot-pound names both a unit of energy and (as the pound-foot) a unit of torque. A converter that prints one number for these is hiding a choice it made. Where a unit is region-variable we publish the per-state basis instead of a national average.
Is a newton-metre the same as a joule?: As a unit of energy, yes — 1 N·m = 1 J exactly. As a unit of torque, no. Torque is a rotational quantity and does not convert into joules, kilowatt-hours or calories, even though the number and the symbol look identical. The same trap exists in US customary units between the foot-pound (energy) and the pound-foot (torque).
Which unit values are exact and which are approximate?: Values fixed by international agreement are exact: 1 inch = 25.4 mm, 1 foot = 0.3048 m and 1 pound-force = 4.4482216152605 N have been exact by definition since the 1959 international yard and pound agreement. Values derived from those are exact too. Traditional and regional units — bigha, biswa, katha, dhur — are not standardised and vary by jurisdiction.