Pace Calculator
Compute running pace per km and per mile from any distance and time, then predict race times with Peter Riegel's 1.06 exponent formula.
This tool runs entirely in your browser. Your data never leaves your device — it is not uploaded to any server.
How to use
- 1Pick a race distance preset, or choose Custom and enter any distance in km or miles.
- 2Type your finish time in hours, minutes and seconds.
- 3Read pace per km, pace per mile and both speeds — or switch to Race predictor for projected times at every distance.
Pace Calculator
Enter any distance — 5K, 10K, half marathon, marathon or a custom one in kilometers or miles — plus your finish time, and see your pace per kilometer, pace per mile, and speed in km/h and mph at once. A half-marathon in 2:00 shows as 5:41/km, 9:10/mile and 10.6 km/h without any mental math.
The race predictor applies Peter Riegel's endurance formula T₂ = T₁ × (D₂/D₁)^1.06: give it one honest race result and it projects your expected finish for every standard distance, from the 5K to the marathon.
Frequently asked questions
Where does the race prediction formula come from?
From Peter Riegel, a research engineer who published T₂ = T₁ × (D₂/D₁)^1.06 in Runner's World in 1977. The 1.06 exponent captures how pace fades with distance: doubling the race distance takes about 9% longer than twice the time. It assumes you trained specifically for the target distance — it will not predict a marathon from a single fast 5K if you never ran long.
Why is my predicted pace per km slower for longer races?
That is the formula doing its job. The 1.06 exponent means each doubling of distance costs roughly 6% extra time beyond the proportional amount, reflecting glycogen depletion, fueling and fatigue. Shorter predictions (10K from a half marathon) come out faster per km for the same reason.
Does the calculator handle miles as well as kilometers?
Yes. Custom distances accept either unit, and results are always shown both ways: pace per km and per mile, speed in km/h and mph. The conversion uses the exact international mile of 1.609344 km.
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