Pace Calculator

How to use the pace calculator

Enter the distance you covered and the time it took, and the tool reports the same effort three ways: pace per kilometre, pace per mile, and speed. Start with the distance and set the km or mi toggle to match how the run was actually measured, because the toggle changes what your typed number means rather than converting it.

A track session or a metric race goes in as kilometres; a US road race or a treadmill set in miles goes in as miles. The defaults, 5 km in 25 minutes, are there so you can watch the arithmetic work before you replace them. Whichever unit you choose on input, the results come back in both, so you never have to convert a pace by hand to compare notes with someone who thinks in the other system.

The time goes in across three boxes, hours, minutes and seconds, exactly as your stopwatch showed it. A 25-minute 5K is 0 hours, 25 minutes, 0 seconds; a two-hour-ten half marathon is 2, 10, 0.

The three boxes are simply added into one total, so 0 h 90 m 0 s and 1 h 30 m 0 s mean the same thing to the calculator and there is no need to tidy ninety minutes into an hour and a half first.

Read the pace as a time per unit of distance: the minutes and seconds it takes to cover one kilometre or one mile. On the defaults that is 5:00 per km and 8:03 per mile, and the mile figure is the bigger of the two purely because a mile is the longer unit, 1.609344 kilometres of it.

Speed is the same effort turned upside down: how much ground you cover in an hour rather than how long one unit takes. The 25-minute 5K works out to 12 km/h, which is 7.46 mph.

Neither number is more correct than the other; they are the same rate described from opposite ends, and which one you want depends entirely on the machine or the person in front of you. Runners on the road quote pace to each other, and pace is what you hold with your legs.

Will the pace hold as the distance grows?

One thing the tool cannot do is promise that a pace holds as the distance grows. Peter Riegel published a time-versus-distance equation in American Scientist in 1981, fitted to world records from 100 metres to 100 miles, in which the finish time scales as the distance ratio raised to the power 1.06.

That exponent means each doubling of race distance costs roughly 4.2 percent in per-kilometre pace, so a 5K pace will not survive a marathon and was never meant to. Use the table above to see what a pace you can hold produces at each of the four standard road distances, and treat those figures as a projection built on a published curve rather than a forecast of your own race.

Within a single race, even splits or a slight negative split are the usual advice, because going out hard spends energy that does not come back.

The formula, worked line by line

Pace and speed are one effort read from opposite ends. Pace is time over distance, the seconds it takes to cover one unit; speed is distance over time, the units you cover in one hour. Each is the reciprocal of the other, which is why a calculator that knows one always knows the other, and why the two numbers can never disagree about how fast you actually ran.

Converting between per-kilometre and per-mile pace uses the same exact factor as any other distance conversion, because the hour on both sides of the expression is the same hour. The mile has been defined as exactly 1,609.344 metres since the 1959 international yard and pound agreement, so the conversion carries no measurement error at all; the only imprecision in a pace figure comes from rounding the display to whole seconds.

pace  = total time / distance
speed = distance / total time
sec per mile = sec per km x 1.609344
Riegel (1981): T2 = T1 x (D2 / D1)^1.06

The fourth line is the one that stops a pace table from lying to you. Peter Riegel fitted world-record times from 100 metres to 100 miles to a power curve in American Scientist in 1981 and found an endurance exponent of 1.06, meaning finish time grows slightly faster than distance.

Feed the default 25:00 5K into it and the equation projects 52:07 for 10K, 1:55:00 for the half marathon and 3:59:46 for the marathon. Compare those with the flat arithmetic in the table above, where holding 5:00 per km all the way would give 50:00, 1:45:29 and 3:30:59, and the gap between the two sets of numbers is the price of distance.

Riegel is an average of record performances, not a prediction about you, and it assumes a comparable level of training and specific preparation across the distances. Marathon results in particular scatter far more widely than the curve suggests, because fuelling, heat, course profile and the last 10 km of a marathon introduce failure modes that a 5K does not have.

Use the equation as a sanity check on an ambitious goal rather than a target, and within a single race aim for even splits or a slightly faster second half, since going out hard spends energy that cannot be recovered later in the same run.

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