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Race Time Predictor

Predict your race time at a target distance using the Riegel formula. Enter a recent race or workout (distance + time) and a target distance — get the predicted time and pace, plus a warning when the extrapolation crosses into territory where Riegel becomes unreliable.

Units
Examples

Doubles the distance. Riegel predicts ≈ 52:07 over 10 km.

Predicted target time
0:52:07
Predicted average pace
5:13 /mi

Target is within ~2× the known distance — Riegel's reliable zone.

Estimate only. Actual race times depend on training, pacing, weather, and course. Train safely and consult a coach for race-day strategy.

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Examples

How It Works

Formula

t2=t1⋅(d2d1)1.06t_2 = t_1 \cdot \left(\frac{d_2}{d_1}\right)^{1.06}

Variables

t1t_1

Known time(seconds)

d1d_1

Known distance(km or mi)

t2t_2

Predicted time(seconds)

d2d_2

Target distance(km or mi)

Enter a known race time at a known distance, then the target distance. The calculator applies the Riegel formula t2 = t1 · (d2/d1)^1.06 to estimate the time at the target distance, and divides by the target distance to give the average pace.

Frequently Asked Questions

01What does the Riegel formula actually do?
The Riegel formula is an empirical fit to race-distance scaling: t2 = t1 · (d2/d1)^1.06. The exponent 1.06 was published by Pete Riegel in 1977 from athletic data. Above 1 it means time grows slightly faster than distance — doubling the distance more than doubles the time. It is a curve fit to averages, not a physiological model.
02When does it overestimate or underestimate?
Riegel tends to over-predict (give a slower time than you would actually run) when the target distance is much longer than the known one and the runner has good endurance — because the constant 1.06 is averaged across a population whose long-distance fade varies a lot. It under-predicts when the known race was a sprint and the target is also short, because anaerobic effort scales differently from aerobic.
03Why is the formula different for sprints vs. endurance?
Sprints (≤ 800 m) are limited by anaerobic power and biomechanics; endurance races (≥ 5 km) by aerobic capacity, fuel, and pacing discipline. Riegel was fit on endurance data and is reliable from roughly 1500 m up to a half-marathon. Outside that band the underlying physiology stops looking like a single power-law, and the prediction degrades.
04Why are marathon predictions from a 5K time often wrong?
A 5K is a 15–25 minute effort dominated by VO₂max; a marathon is a 3–5 hour effort dominated by glycogen, hydration, pacing, and the wall. The factor between the two distances is ≈ 8.4×, well past Riegel's reliable range. Most 5K → marathon Riegel predictions are 5–15 % too fast for runners who haven't built the endurance base — which is why the calculator flags long extrapolations as unreliable.
05How should I use this prediction in training?
Treat it as a ceiling, not a target. If Riegel says 1:50 for your half-marathon goal, train at the paces that support that effort — but plan to race by feel and split, not by chasing the predicted time blindly. The prediction assumes you have done the specific endurance work for the target distance.

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