Running Pace Calculator

About this tool

Pace, time and distance are three views of the same run. Once you know any two of them the third follows, and this calculator solves for whichever one you leave out — pick pace, time or distance and fill in the other two.

Pace is time divided by distance, written in minutes per kilometer or per mile. Speed is the same relationship turned around: distance divided by time, in kilometers or miles per hour. Runners tend to hold a pace because it stays steady over hills and into the wind, while treadmills and bike computers often prefer speed.

Everything runs locally in your browser, with nothing sent anywhere. The mile used here is the international mile of exactly 1.609344 kilometers, so converted results line up with official race distances instead of drifting away from a rounded figure.

Why 1:70 is rejected instead of read as 2:10

The minutes and seconds fields must stay between 0 and 59. 1:70 is treated as a typing error, not as two minutes ten seconds, because reading it as a valid time would hand you a result nobody typed on purpose.

Carrying silently is the tempting shortcut, and it is the wrong one: 1:70 in a time field is a mistake, and the runner who typed it does not know whether they meant 1:70 as written or 2:10. The tool refuses and says the format is invalid rather than inventing the carry for you.

A single number with no colon is the exception. 300 is read as 300 seconds, not as five minutes and change, because with only one field there is no carry to be ambiguous about.

Times under an hour are written the way a stopwatch reads — 5:00, not 05:00 — so the hours digit stays unpadded while minutes and seconds are each two digits.

The distance field accepts a comma as a decimal point

A half-width comma in the distance is read as a decimal separator, so "42,195" is understood as 42.195 kilometres. That follows the European convention and lets a marathon distance be typed exactly as written.

The comma is not a thousands separator here, because the field holds a single distance rather than a list of numbers. Turning it into a decimal point is the same treatment the body-metric tools use, and it means a European runner can enter the distance the way they write it at home.

The sign and range rules still apply first: a distance of zero or below is reported as out of range, while something that is not a number at all is reported separately. The two errors ask for different corrections — one is a value to change, the other a format to fix — so they are not merged into a single message.

Both paces come from the clock, not from each other

The per-kilometre and per-mile paces are each recalculated from the time and the distance, not converted one from the other. That way the two figures never disagree by a rounding step.

A naive approach converts minutes per kilometre into minutes per mile with one multiplication. Each such step carries its own rounding, and the two paces can end up inconsistent with the time and distance actually entered. Here both come straight from the same division.

The speed figure follows the same discipline: it is the distance in kilometres divided by the time in seconds, scaled to an hour, so it does not depend on whichever pace happened to be on screen. Whichever of the three quantities you ask the tool to solve, the derived numbers converge on the same point.

The mile is 1.609344 km, and the last three digits move the finish line

Converting with 1.609 instead of 1.609344 shifts a marathon by about nine metres, which is enough to disagree with an official split.

The exact figure is 42.195 divided by 1.609344, which gives 26.2188 miles, the number every race organiser and every mile-based training plan uses. With the rounded 1.609 the same marathon comes out at 26.2244 miles, a difference of 0.0056 miles, or roughly nine metres.

Nine metres sounds like nothing until it is expressed in time. At a three-hour marathon pace of 4:16 per kilometre, nine metres is about 2.2 seconds. That is inside the noise of a GPS watch but well outside the tolerance of a certified course, and it is exactly the size of error that makes two people argue about whether a split was on pace.

The half marathon drifts the same way: 13.1094 miles exact against 13.1122 rounded. The constant is stored at full precision rather than rounded to three decimals because a conversion factor is not a measurement, it is a definition, and a definition has no reason to be approximate.