Race Time Predictor
Predict your 10K, half or marathon time from a recent race, using Riegel's endurance formula.
- Predicted finish time3:59:47
- Predicted pace per km5:41
Inputs used
- Recent race5K
- Hours0
- Minutes25
- Seconds0
- Predict time forMarathon
About this calculator
Pete Riegel's 1977 formula still powers most race predictors: T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent captures how everyone slows as distance grows — double the distance and your time slightly more than doubles.
From a 25-minute 5K it predicts a 52-minute 10K, a 1:55 half and just under 4 hours for the marathon. The predictions assume you're equally trained for the target distance: a fast 5K on 30 km-a-week training won't cash into the marathon prediction without the long runs to back it.
Predictions from nearby distances (5K → 10K) are tighter than long extrapolations (5K → marathon), where fuelling and endurance limits bite harder than the formula knows.
Quick reference
Marathon predictions from 5K times
Riegel prediction (h:mm:ss) for the marathon from an all-out 5K.
| Minutes | Predicted finish time |
|---|---|
| 18 | 2:52:38 |
| 20 | 3:11:49 |
| 22 | 3:31:00 |
| 25 | 3:59:47 |
| 28 | 4:28:33 |
| 30 | 4:47:44 |
| 35 | 5:35:41 |
Frequently asked questions
How accurate is the Riegel formula?
Within a few percent when the target distance is no more than about double the source race and you're trained for it. Marathon predictions from short races tend to be optimistic — the formula doesn't know about glycogen depletion at 32 km.
What marathon time does a 25-minute 5K predict?
About 3:59:50 — 25:00 × (42.195 ÷ 5)^1.06. The same run predicts a 52:07 10K and a 1:55:07 half marathon.
Which recent race gives the best prediction?
The longest recent one you raced flat-out. A half marathon predicts a marathon far better than a 5K does, because it already includes much of the endurance component.
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Results are informational only — not financial, medical or legal advice.