Race Time Predictor
Predict your finish time at another distance from a recent race with Riegel's formula, T2 = T1 × (D2 ÷ D1)^1.06.
What your 10 km time says about your marathon — and the other way round.
How the race time predictor works
Peter Riegel noticed in 1981 that race times scale with distance to the power of about 1.06 across every distance from a mile to the marathon: doubling the distance takes a little more than double the time, because pace fades as the effort lengthens. The calculator applies T2 = T1 × (D2 ÷ D1)^1.06 to your recent result.
The prediction assumes you are as well trained for the target distance as for the one you raced. Predicting a marathon from a 5 km is optimistic unless you have the long runs behind you; Detailed mode lets you raize the exponent (1.07–1.10) for that case.
Formula: T2 = T1 × (D2 ÷ D1)^1.06
Worked examples
| Inputs | Predicted time | Note |
|---|---|---|
| 10 km in 48:30 → marathon | 3:43:07 | about 3:44 |
| 5 km in 22:00 → 10 km | 45:52 | 45:52 |
| Half in 1:45:00 → marathon | 3:38:55 | the classic half-to-full step |
| 10 km in 48:30 → marathon, exponent 1.08 | 3:49:38 | a more cautious prediction |
FAQFrequently asked questions
How accurate is the Riegel formula?
Good for neighbouring distances (5 km to 10 km, half to full) in runners trained for both; it is optimistic when jumping from a short race to a marathon without long-run preparation.
Why is the exponent 1.06?
Riegel fitted it to world and age-group records across running, swimming and other sports; 1.06 means doubling the distance takes 2^1.06 ≈ 2.085 times as long.
Why does doubling my 10 km time not give my half-marathon time?
Because pace fades with distance. A 50-minute 10 km predicts a 1:50 half, not 1:45, and a 3:50 marathon rather than 3:30.
Can I use it the other way — marathon to 5 km?
Yes, but predictions from long to short assume you have the speed, which marathon training does not always build. Treat it as a ceiling.
Where these figures come from
- Riegel PS, "Athletic records and human endurance", American Scientist 1981 — T2 = T1 × (D2 ÷ D1)^1.06
- Cooper KH, "A means of assessing maximal oxygen intake", JAMA 1968 — the 12-minute run test
- Ainsworth BE et al., Compendium of Physical Activities (2024 update) — MET values by activity
- OpenPowerlifting — DOTS coefficients (Tim Konertz, 2019) — the DOTS polynomial used for scoring
- Naismith's rule (1892) as used by UK and Australian walking bodies — 1 hour per 5 km plus 1 hour per 600 m of ascent
- American College of Sports Medicine — exercize-science guidelines
Last checked: September 2026. Formulas are fixed by their published sources; the MET values come from the Compendium of Physical Activities.