How this calculator works
Pick pace converter or race time predictor. In pace converter mode, enter a pace in minutes per mile or per km and the calculator applies the exact mile-to-km ratio to convert it in either direction. In race time predictor mode, enter a known finish time and distance, choose a target distance, and the calculator applies the Riegel formula, a power-law relationship between distance and endurance, to estimate your finish time at the new distance. Results are shown in a readable minutes:seconds clock format.
The formula
Pace conversion: minPerKm = minPerMile / 1.609344 minPerMile = minPerKm x 1.609344 Race time prediction (Riegel formula): T2 = T1 x (D2 / D1) ^ 1.06 T1 = known finish time, D1 = known distance, D2 = target distance, T2 = predicted finish time
Worked example
Predicting a 10K time from a known 5K result of 25:00:
- Distance ratio: 10 / 5 = 2
- Apply the exponent: 2 ^ 1.06 = 2.084931…
- Predicted time: 25 x 2.084931… = 52.12 minutes, which is 52:07
Frequently asked questions (FAQ)
How do I convert my pace from minutes per mile to minutes per km?
Divide your minutes-per-mile pace by 1.609344, the exact number of kilometers in a mile. For example, an 8:00 min/mile pace divides to 4.97 min/km, which is 4:58 per km. To go the other way, multiply your min/km pace by 1.609344.
What is the Riegel formula and where does it come from?
It is a race-time prediction equation published by Pete Riegel in "Athletic Records and Human Endurance," American Scientist, 1981. It estimates a finish time at one distance from a known finish time at another, using the ratio of the two distances raised to the power of 1.06.
Why does the Riegel formula use an exponent of 1.06 instead of 1.0?
An exponent of 1.0 would assume you hold the exact same pace no matter how far you race, which is not realistic. Riegel found that endurance performance declines slightly as distance grows, and an exponent of 1.06 fits that pattern across a wide range of race distances and abilities.
How accurate is a Riegel race time prediction?
It is an estimate, not a guarantee. It tends to work best for well-trained runners predicting between moderate distance ranges, such as a 5K to a 10K or half marathon. Predictions across very different distances, especially into the marathon, can be less reliable because fueling and pacing strategy matter more.
Can the Riegel formula predict a shorter race from a longer one, or only the other way around?
Either direction works. The same formula predicts a faster 10K time from a known marathon result just as it predicts a slower marathon time from a known 10K result. The distance ratio and exponent apply the same way regardless of whether the target distance is shorter or longer.
Does this calculator account for terrain, weather, or fitness changes between races?
No. The Riegel formula assumes similar conditions and fitness at both distances, based only on the distance ratio. Hills, heat, wind, and changes in your training between now and race day are not factored in, so treat the predicted time as a starting estimate, not a certainty.