Race Time Predictor
Predict your finish times at other distances from one recent race result using Riegel's proven formula.
By Jonas Lindemann · Updated · How we build our calculators
Using Riegel's formula: T₂ = T₁ × (D₂ / D₁)^1.06
| Distance | Time | Pace /km |
|---|---|---|
| 1 Mile | 7:13 | 4:29 |
| 5K | 23:59 | 4:48 |
| 10K | 50:00 | 5:00 |
| 15K | 1:16:51 | 5:07 |
| 10 Mile | 1:22:48 | 5:09 |
| Half Marathon | 1:50:19 | 5:14 |
| Marathon | 3:50:01 | 5:27 |
Note: Riegel's formula assumes consistent training and is most accurate for predictions within roughly 2× of your input distance. Extrapolating from a 5K to a marathon overestimates fitness unless you have the endurance base.
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About the Race Time Predictor
Learn more about the calculator and its creator
Jonas
I'm a software developer from Hamburg and have been running since 2016 alongside cycling and triathlon. I built this calculator to help runners like you and me with training and racing.
The race time predictor takes one recent race result — a distance and the time you ran it in — and estimates what you could run at other distances. It uses Riegel's formula, a power-law relationship between distance and time that has held up well across decades of race data.
Riegel's Formula
Pete Riegel proposed (first in Runner's World in 1977, then in American Scientist in 1981) that a runner's time scales with distance raised to the power 1.06:
Here T1 and D1 are your known time and distance, and T2 is the predicted time at the new distance D2. The exponent 1.06 (slightly above 1.0) captures the fact that you slow down as distance grows — if you held the exact same pace, the exponent would be 1.0.
Worked Example
Suppose you run a 10K in 50:00 (3000 seconds) and want a marathon prediction. D2/D1 = 42.195/10 = 4.2195:
Prediction Table from a 50:00 10K
| Distance | Predicted Time | Pace /km |
|---|---|---|
| 5K | 24:00 | 4:48 |
| 10K | 50:00 | 5:00 |
| 15K | 1:16:38 | 5:06 |
| Half | 1:50:34 | 5:14 |
| Marathon | 3:49:15 | 5:26 |
When the Prediction Is Reliable
Riegel's model works best when the predicted distance is within roughly 2× the input distance and when your training matches the target race. Predicting a marathon from a 5K will flatter you — the formula assumes you have the endurance base to sustain effort for hours, which a fast 5K alone does not prove.
- Most accurate: 10K → half marathon, or half → marathon, with matching training.
- Optimistic: short race → long race when weekly mileage is low.
- Conservative: long race → short race, since the formula does not capture pure speed.
Note: A predicted time is a fitness estimate, not a promise. Actual results depend on course, weather, fueling, and race-day execution. This tool is informational only and not a replacement for a coach.
Frequently Asked Questions
Riegel's formula is most accurate when the predicted distance is within about 2× the input distance and your training matches the target race. Predicting a marathon from a 5K tends to be optimistic.
Riegel's formula is T2 = T1 × (D2/D1)^1.06, where the 1.06 exponent captures the way runners slow as distance increases.
Yes, and this is one of the more reliable predictions, especially if you have the endurance base and matching training.