Speed of Light Calculator

How fast is the speed of light?

In a vacuum, light covers 299,792,458 metres every second. That single number is the speed limit of the universe: it is the fastest that light, radio, gravity or any other information-carrying signal can travel, and nothing with mass can reach it at all. Physicists write it as c, and it turns up far beyond optics — it is the c in E = mc².

The table below is the same constant expressed in the units people actually search for. Every row is computed from the exact metres-per-second figure at page load, not copied from another site — so the miles-per-hour value is exactly what the definition implies, to the digit.

That last row is the one hardware engineers keep in their heads: light travels very nearly one foot per nanosecond. Grace Hopper famously handed out 11.8-inch lengths of wire to make the point — that is how far a signal gets while a single nanosecond elapses, and it is why the physical length of a circuit board trace becomes a real design constraint at gigahertz clock speeds.

Why 299,792,458 m/s is exact — and the metre is the thing being defined

Most physical constants carry an uncertainty: a measured value, plus or minus something. The speed of light does not, and the reason is the part almost every page gets backwards.

Until 1983, the metre had its own independent definition and the speed of light was measured against it — each new experiment shaved the error bars a little. Then the General Conference on Weights and Measures inverted the relationship. It fixed c at exactly 299,792,458 m/s by decree and redefined the metre as the distance light travels in 1/299,792,458 of a second. Since a second was already defined by caesium atomic transitions, that made length a derived quantity.

The consequence is genuinely strange and worth stating plainly: it is now impossible to measure the speed of light more accurately, because it is not a measurement. Point a better laser interferometer at the problem and you are no longer refining c — you are refining your knowledge of how long a metre is. The uncertainty moved out of the constant and into the unit. Today c is one of the seven defining constants that the entire SI is built on, alongside the caesium frequency and the Planck constant.

Primary sources: NIST — speed of light in vacuum, BIPM — the seven SI defining constants, and NIST — the metre.

How far does light travel in a second, a minute, a year?

Turn the speed around and you get a family of distance units. A light-second is just how far light gets in one second — and it is a surprisingly human-scale number, about seven and a half laps of the equator.

That last row is the light-year — a distance, never a duration, which is the single most common misunderstanding of the term. Astronomers define it against the Julian year of exactly 365.25 days, or 31,557,600 seconds, rather than the calendar year. Using the 365.2425-day Gregorian year instead would shorten the answer by about 194 million km, which sounds enormous until you notice it is a difference of 0.0021% — but the convention exists so that everyone's figures agree exactly. One light-year also works out at 63,241 astronomical units, which is a more useful way to picture it: Neptune orbits at about 30 AU, so a light-year is roughly two thousand times the distance from the Sun out to Neptune.

Light delay across the solar system

Because c is finite, every astronomical image is a picture of the past, and every conversation with a spacecraft has a built-in lag. Set the calculator above to Distance → travel time and the preset buttons will load each of these.

Mars deserves the honest answer rather than the single number most pages quote. Both planets are moving, so the distance between them swings enormously over a 26-month cycle: from roughly 54.6 million km at the closest oppositions out to about 401 million km when Mars is on the far side of the Sun. Light delay to Mars is therefore not one figure but a range — roughly 3 to 22 minutes each way, and 6 to 44 minutes for a command and its acknowledgement. That is exactly why rovers have to drive themselves: by the time a human saw the hazard, the rover would already be in it.

The Moon figure is the reason Apollo transmissions have that distinctive pause, and the Proxima Centauri figure sets a hard floor on interstellar conversation. Even at light speed, a question sent to our nearest stellar neighbour and its answer take 8.5 years to complete the round trip.

The speed of light in water, glass and fibre — v = c ÷ n

c is the speed of light in a vacuum. Inside any transparent material light effectively slows down, and the amount is captured by a single number: the refractive index, n. The relationship is as simple as it looks — v = c ÷ n — and it is what the medium selector in the calculator applies.

Two things fall straight out of that table. The first is why things bend and sparkle: a beam crossing from one medium into another changes speed at the boundary, and changing speed at an angle means changing direction. Diamond slows light to 41% of c, the largest jump of any common material, which is precisely why a cut diamond throws light around the way it does.

The second is a number network engineers budget with. Long-haul fibre runs at n ≈ 1.468, so signals crawl along at about 204,190 km/s — roughly 4.9 milliseconds of latency for every 1,000 km of cable. A New York to London link of about 5,570 km therefore costs around 27.3 ms one way and 54.6 ms round trip before a single router touches the packet, against 37.2 ms if the same path could be flown through vacuum. That ~17 ms gap is real money: it is the entire commercial case behind microwave and hollow-core-fibre trading links between financial exchanges. Set the medium selector to the fibre-optic option and the calculator will work out any route you like.

One caveat worth stating, because popular articles routinely garble it: individual photons never travel at anything other than c. What slows is the wave as a whole, as light is repeatedly absorbed and re-emitted by the atoms of the material. The practical effect is exactly as the formula says, but the underlying picture is subtler than "photons move slower in glass".

Speed of light in miles per hour, and why the mile matters

The mph figure is where derived numbers most often drift between websites, so it is worth showing the working. The international mile has been exactly 1,609.344 metres since the 1959 agreement between the English-speaking national standards bodies. Because both the mile and c are exact, the conversion is exact too:

299,792,458 ÷ 1,609.344 = 186,282.397 mi/s  →  × 3,600 = 670,616,629.38 mph

Round it however you like — 670 million mph, or the familiar "186,000 miles per second" — but the full value is fixed, not approximate. The same logic gives 1,079,252,848.8 km/h and 983,571,056.43 ft/s. If another calculator disagrees with these in the third significant figure, it has almost certainly rounded c before converting rather than after.

Frequently asked questions

How fast is the speed of light?

In a vacuum, light travels 299,792,458 metres every second — that is 299,792.458 km/s, about 186,282 miles per second, or roughly 670,616,629 mph. It is the fastest anything carrying information can move, and nothing with mass can ever reach it.

Why is the speed of light an exact number with no uncertainty?

Because it is no longer measured. In 1983 the metre was redefined as the distance light travels in 1/299,792,458 of a second, which fixed c at exactly 299,792,458 m/s by definition. Every later experiment that looks like it is "measuring the speed of light" is really measuring a length. The exactness is in the definition, not in our instruments.

How far does light travel in one second?

One light-second is 299,792.458 km, or about 186,282 miles — roughly seven and a half times around the Earth’s equator. Light from the Moon reaches you in about 1.28 seconds; light from the Sun takes about 8 minutes 19 seconds.

How long is a light year in kilometres?

A light-year is the distance light covers in one Julian year of 365.25 days, which is 31,557,600 seconds. Multiplying that by c gives 9,460,730,472,580.8 km — about 9.46 trillion km, 5.88 trillion miles, or 63,241 astronomical units. It is a unit of distance, never of time.

Does light slow down in water, glass or fibre-optic cable?

Yes. In any transparent material light travels at v = c ÷ n, where n is the refractive index. Water (n ≈ 1.333) slows light to about 75% of c, ordinary glass (n ≈ 1.5) to about 67%, and a fibre-optic core (n ≈ 1.468) to about 68% — which is why signals in long-haul fibre cost roughly 4.9 ms of latency for every 1,000 km travelled.

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