Acres, Roods and Perches Converter

How to read acres, roods and perches

Old land records almost never give a single number. They give three, running from largest to smallest, and the whole system rests on two figures worth memorising:

4 roods = 1 acre  ·  40 perches = 1 rood  ·  so 160 perches = 1 acre

That is genuinely all of it. A rood is a quarter of an acre. A perch is 1/160 of an acre — you will also see it called a square rod or a square pole, and those are the same area under different names. Because the middle figure can only run 0 to 3 and the last only 0 to 39, a well-formed entry never shows 4 roods or 40 perches; if it does, something has been transcribed wrongly.

The shorthand varies by clerk. 15a 3r 35p, 15.3.35 and "15 acres, 3 roods and 35 perches" all mean the same area. The middle form catches people out: it looks like a decimal number and is not one. Reading 15.3.35 as "15.3 acres" gives 15.3 where the true figure is 15.96875 — small enough to look plausible, wrong enough to matter.

A worked example, from a real enclosure award

Enclosure awards are where this notation bites hardest, because they list every allotment separately and then total them. Take an award entry of 15a 3r 35p. Convert the parts to acres and add:

15 + (3 ÷ 4) + (35 ÷ 160) = 15 + 0.75 + 0.21875 = 15.96875 acres
15.96875 × 4,046.8564224 = 64,623.24 m² = 6.4623 hectares

Going the other way needs one extra step, and it is the step converters get wrong. Convert the whole area into perches first, round that, and only then split off acres and roods. Do it in the other order — split first, round last — and 0.9999999 of an acre prints as "3r 40p" instead of carrying up to a clean 1 acre. The converter above rounds first, so it always carries properly.

Quick reference: acres, roods and perches in modern units

Every figure above is computed from one exact starting value — the acre is 4,840 square yards, and the yard has been exactly 0.9144 metres since 1959 — so the square-foot column is exact rather than rounded. Notice the near-miss on the 2a 1r 35p row: it comes to 0.9991 hectares, which is why one hectare is 2a 1r 35.37p and not the tidy figure it almost looks like.

The customary acre — the trap in older documents

This is the one warning worth carrying into an archive. The converter above uses the statute acre of 4,840 square yards, which is the acre in essentially all legal documents and every modern record. But several regions kept their own larger customary acre in everyday use, sometimes into the nineteenth century:

The practical tell is arithmetic that does not close. If a document lists parcels that add up to one figure but states a different total, or if a field is implausibly large for its neighbours, a customary acre is a likely explanation — as is a surveyor who simply worked in the local measure while the clerk wrote "acres". Irish records before the Ordnance Survey are the most common case by far. Check which acre a document uses before trusting any conversion of it, including this one.

Where the numbers came from

Four and forty look arbitrary until you see the stick they came from. The rod — also called a perch or a pole — was a real surveying pole 16.5 feet long. A square perch is that pole squared, which is why it works out at 30.25 square yards rather than a round number.

The rood came from ploughing. A rood is one furlong long by one rod wide: a furlong was the distance a team of oxen was expected to pull a plough before resting, so a rood is the strip of ground that a day's work covered. Four of those strips side by side make an acre, which is one furlong by one chain. None of these units was designed as a fraction of anything — they were descriptions of work, and the neat 4 and 40 fell out afterwards.

That history is also why the notation survives in the record long after it left daily use. Tithe apportionments of the 1830s and 40s, enclosure awards from the 1700s and 1800s, glebe terriers, estate surveys and pre-metric Ordnance Survey area books are all written this way, and they remain the primary sources for who held which field. Anyone tracing a farm, a parish boundary or a family's land ends up reading acres, roods and perches whether they intended to or not.

Primary sources: The National Archives — tithe records and apportionments; The National Archives — enclosure awards; NIST Special Publication 811 — the yard defined as exactly 0.9144 m, which makes every figure on this page exact.

Frequently asked questions

What does "3 roods 20 perches" mean?

It is a land area written the old way, in three parts. A rood is a quarter of an acre and a perch is 1/160 of an acre, so 3 roods 20 perches is three-quarters of an acre plus twenty perches — 0.875 acres in total, or about 0.354 hectares. The three parts always run largest to smallest: acres, then roods, then perches.

How many perches are in a rood?

Forty. And four roods make one acre, so there are 160 perches in an acre. Those two numbers are the whole system: once you know 4 and 40, you can convert anything by hand. A perch is also called a square rod or a square pole, and all three names mean exactly the same area.

How big is a rood in square feet?

A rood is 10,890 square feet, which is 1,210 square yards or about 1,011.71 square metres. It is a quarter of an acre, and an acre is 43,560 square feet. A rood is roughly the size of a large building plot.

Why is it written as 15a 3r 35p?

That is the shorthand clerks used, with a for acres, r for roods and p for perches. You will also see it as 15.3.35 with full stops, or written out as "15 acres, 3 roods and 35 perches". They all mean the same thing. Note that 15.3.35 is not a decimal number — the middle figure can only be 0 to 3, and the last only 0 to 39.

Is the acre in an old deed the same as an acre today?

Usually yes, but not always, and this is the mistake worth guarding against. The statute acre of 4,840 square yards has been standard for centuries and is what this converter uses. But some regions kept their own larger "customary" acre well into the 1800s — the Irish acre was about 1.62 statute acres and the Cheshire acre about 2.12. If a document gives a total that does not match the sum of its parts, a customary acre is a likely reason.

Where do these units come from?

From the rod, a surveying stick 16.5 feet long, also called a perch or a pole. A square perch is that stick squared. Forty of them make a rood, and a rood is one furlong long by one rod wide — the strip a team of oxen could plough. The numbers look arbitrary now, but every one of them came from how the land was actually worked and measured.

Related tools

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• Square perches to square metres
• Acres to hectares
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