Race Time Predictor
Enter one recent race result and we'll predict your equivalent time at other standard distances, using Riegel's formula (T2 = T1 × (D2/D1)^1.06).
Only 1 mile to marathon is listed: race predictions aren't reliable from results below 1 mile or above marathon distance.
| Distance | Predicted time | Pace /km |
|---|---|---|
| 1 Mile | 7:31 | 4:40 |
| 5K | 25:00 | 5:00 |
| 10K | 52:07 | 5:13 |
| 15K | 1:20:07 | 5:20 |
| 10 Mile | 1:26:19 | 5:22 |
| Half Marathon | 1:55:00 | 5:27 |
| Marathon | 3:59:47 | 5:41 |
This estimate uses a generic formula. Connect your Strava/Whoop/Garmin data for a version personalised to your actual training.
Get early accessWhat a race time prediction is
A race time prediction takes one result you have already run and estimates your equivalent finish time at a different distance. It answers a common question: given a recent 10K, what marathon time is realistic right now.
How it is calculated
This calculator uses Riegel's formula: T2 = T1 × (D2/D1)^1.06. T1 and D1 are the time and distance of your known result, D2 is the target distance, and T2 is the predicted time. The 1.06 exponent accounts for the fact that pace slows as distance increases, so the prediction is not a simple linear scaling of pace. Predictions are generated for seven standard distances: 1 mile, 5K, 10K, 15K, 10 mile, half marathon, and marathon.
Why runners use it
It gives a grounded starting point for a goal time at a distance you have not raced recently, or at all, based on fitness you have already demonstrated at another distance.
Sources and limitations: see our methodology.
Worked example
Take a runner who just raced a 5K in 25:00 and wants a realistic half marathon target.
Riegel's formula scales the 25:00 5K up to a predicted 1:55:00 half marathon. That is a starting target, not a guarantee: holding it on race day also depends on pacing discipline and endurance-specific fitness that a single 5K result cannot capture, which matters more the further apart the two distances are.
FAQ
How do I predict my marathon time from a 10K?
Enter your 10K distance and time into this calculator. It applies Riegel's formula, T2 = T1 × (D2/D1)^1.06, using your 10K as T1/D1 and the marathon distance (42.195 km) as D2, to estimate a realistic marathon finish time based on your current fitness.
What is Riegel's formula for race time prediction?
Riegel's formula is T2 = T1 × (D2/D1)^1.06, where T1 and D1 are the time and distance of a race you have already run, D2 is the target distance, and T2 is the predicted time at that distance. The 1.06 exponent accounts for pace naturally slowing as race distance increases.
Are race time predictors accurate?
Riegel's formula gives a reasonable estimate for well-trained runners predicting between similar distances, such as 10K to half marathon. It gets less reliable the further apart the two distances are: predicting a marathon time from a 1-mile result, for example, tends to be optimistic, since it does not account for endurance-specific fitness, fueling, or pacing discipline.
Can I predict a 5K time from a marathon result?
Yes, Riegel's formula works in either direction, scaling a known result up or down to a target distance. Predictions from a longer race down to a shorter one are generally less reliable than the reverse, since raw endurance does not translate predictably into shorter, faster-paced efforts.
What 10K time can I expect from my 5K?
Riegel's formula predicts T2 = T1 × 2^1.06, so a 25:00 5K predicts a 10K of about 52:07. That is a close-distance prediction, which Riegel's formula handles well, but your actual 10K will still depend on pacing and how your endurance holds up over the extra 5 km.