Riegel Race Time Predictor
This race time predictor uses the classic Riegel formula — T2 = T1 × (D2 ÷ D1)^1.06 — to turn one recent race into predicted times at 800m through the marathon, with pace per mile and per kilometer. Unlike black-box race equivalency calculators, the formula is shown on the page and the fatigue exponent is adjustable (1.02–1.12), so speed-type and endurance-type runners can tune predictions to match how they actually race.
Recent Race Distance
Fatigue Exponent: 1.06
T2 = T1 × (D2 ÷ D1)^1.06
(Riegel, 1981 — every number below comes from this one formula)Enter a recent race above and every distance from 800m to the marathon appears here instantly. Or start from an example:
The Riegel formula assumes you are equally trained for both distances. It tends to overestimate marathon performance for many runners — treat long predictions as a best-case ceiling, not a race plan, unless mileage backs them up.
How coaches use the riegel race time predictor
Use this race time predictor when you want a 5K to 10K conversion, a marathon prediction from a half, or to know what your 5K should be based on your mile — with the Riegel formula and exponent visible instead of hidden.
- Convert a recent 5K into 10K, half marathon, and marathon predictions before setting season goals.
- Answer 'what would my 5K time be based on my mile?' for athletes moving up in distance.
- Tune the exponent (1.05 speed-type, 1.07–1.08 endurance-type) to match how an athlete actually races.
- Set realistic entry-standard targets for athletes racing an unfamiliar distance.
Riegel Race Time Predictor FAQ
It tends to be optimistic. Riegel assumes you are equally trained for both distances, and most runners are not marathon-trained relative to their 5K fitness. Treat marathon predictions as a best-case ceiling and adjust with a higher exponent (1.07–1.08) or real long-run data.
1.06 is the standard published value and works for most trained runners. Use around 1.05 if you are speed-type (your short races are relatively better) and 1.07–1.08 if you are endurance-type or predicting well beyond your training distance.
Usually aerobic development lags speed. If the predictor says your mile is 'worth' a much faster 5K than you run, that gap is the training opportunity — more threshold and volume, not more speed work.
Same idea, but transparent. The exact formula and exponent are shown on the page, so you can see why the prediction is what it is and adjust it instead of trusting a black box.
Yes — that is the classic use. A 20:00 5K predicts about 41:42 for 10K at the standard 1.06 exponent (1200 × 2^1.06 seconds), assuming you are trained for the longer race.
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