Race Time Predictor

Enter a recent race result and get a predicted time for any other distance, with the paces that go with it. Everything runs in your browser.

Recent race
An all-out result works best, not a training run. Enter the distance and finish time.
Target race
Pick the distance you want a prediction for.
Predicted time
Riegel's formula scales your known effort by the distance ratio raised to the 1.06 power.

1:50:19

Pace per km
5:14
Pace per mile
8:25

At the same effort

5K
23:59
Half marathon
1:50:19
Marathon
3:50:01

The longer targets lean optimistic. The formula assumes your training matched the distance, so treat a marathon prediction built from a 5K as a stretch goal.

How to use

  1. Enter a recent race: pick one of the common distances or a custom distance in kilometers, then type your finish time.
  2. Pick the distance you want a prediction for.
  3. Read the predicted time and the per-kilometer and per-mile paces that come with it.
  4. Check the list of the other common distances for the same level of effort, and temper the long ones with the caveats below.

How the prediction works

The tool applies the Riegel formula: T2 = T1 x (D2 / D1)^1.06, where T1 is your known time over distance D1 and D2 is the target distance. Peter Riegel derived the 1.06 exponent from a log-log fit over recorded performances ('Athletic Records and Human Endurance', Experientia, 1981), and it remains the standard closed-form race model.

The exponent assumes training that matches the target distance. Predictions for distances well above the known race, a marathon predicted from a 5K being the classic case, run optimistic for most runners. Use them as upper bounds to build toward, and let long runs and race experience set your real pacing.

Frequently asked questions

Is any of my race data uploaded?

No. The prediction runs in your browser tab. Distances and times you enter never leave your device, nothing is stored, and the page sets no cookies. Ad slots on the page are served by third-party networks, which follow their own privacy policies.

Which formula does the predictor use?

Riegel's formula: the predicted time equals your known time multiplied by (target distance / known distance) raised to the power 1.06. Peter Riegel published it in 1981 in the journal Experientia as 'Athletic Records and Human Endurance', fitting world records and quality performances across distances. The exponent 1.06 captures how pace decays as races get longer.

Why do long-distance predictions run optimistic?

The formula assumes your training matched the target distance. A marathon prediction built from a 5K ignores the specific endurance that only long runs build, so the predicted time is hard to hold. The shorter the jump and the bigger your weekly mileage, the closer the prediction lands. Treat a first marathon estimate from a 5K as a stretch goal, not a plan.

What result should I enter?

An all-out race from the last few weeks on a course with terrain similar to your target. Training runs, charity fun runs, and personal bests from years ago all skew the prediction. If you must choose between a slightly shorter all-out effort and a longer comfortable one, the all-out effort predicts better.

Does it account for hills, heat, or altitude?

No. The formula compares distances under neutral conditions. A hilly or hot target race will run slower than the prediction, and a fast, flat, cool course can run faster. Adjust by feel and by course history.