Riegel Race Time Predictor

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.