FREE RUNNING TOOLS
Race time predictor
Use a recent race to explore equivalent times at other distances. Keep the estimate alongside your actual preparation.
Calculations happen in your browser. Your inputs are not uploaded or added to the URL.
Example result
5 km · 25:00
| Race distance | Estimated time |
|---|---|
| 5 km | 25:00 |
| 10 km | 52:07 |
| Half marathon | 1:55:00 |
| Marathon | 3:59:47 |
A mathematical estimate, not a race target or guarantee. Longer-distance estimates, especially the marathon, can be optimistic when preparation is insufficient.
How it works
This calculator uses T₂ = T₁ × (D₂ / D₁)^1.06, with both distances in the same unit. The exponent represents a modeled slowdown as distance increases. It does not use training history, course conditions or endurance-specific evidence.
Research in recreational runners found that Riegel-style predictions could be too optimistic for marathon performance. This simple web calculator is separate from Runome’s app prediction engine, which considers additional performance evidence.
Read the race-prediction studyTry this example
A 25:00 5K projects to about 52:07 for 10K. This is a mathematical extrapolation. Use a recent representative race effort, not an easy run, as the starting point.
Common questions
Can I use a normal training run?
An easy run is not evidence of your fastest race performance. Start with a recent race or representative hard effort and treat the result as an estimate.
Does this predict my marathon readiness?
No. An extrapolated time does not measure your training volume, long runs, health or ability to finish. The marathon estimate can be especially optimistic when the starting race is much shorter.
Is this the same prediction as in the Runome app?
No. This public calculator uses one race and a fixed exponent. The app has separate rules for qualifying performance evidence and presenting predictions.