Estimate your 5K, 10K, half and full marathon times from a recent performance using the Riegel formula.
Predict your 5K, 10K, half and full marathon times from a recent race (Riegel).
Predict your 5K, 10K, half and full marathon times from a recent race with the Riegel formula.
Riegel formula: T₂ = T₁ × (D₂/D₁)^e. Valid for 3–240 min efforts on nearby distances.
Published by Peter Riegel in 1977, this formula estimates your race time over one distance from a performance over another. It is written: T2 = T1 × (D2 ÷ D1) ^ 1.06.
The 1.06 exponent captures something every runner knows from experience: you cannot hold the same pace when the distance doubles. A runner who covers 10K in 50 minutes will not run a marathon in 3:20, but closer to 3:50. That exponent puts a number on the slowdown.
The 1.06 default is an average drawn from trained runners. It does not fit everyone, and it is the single biggest source of error in race predictions.
Riegel predicts what your current fitness allows, not what your training block will deliver. A marathon projection from a 5K assumes you have actually done the long runs: without them, the wall at mile 20 makes the prediction meaningless.
The formula also ignores elevation, heat, wind and course quality. Treat the output as a starting point for building a pacing strategy, not as a finish-line guarantee.
It is reliable between neighbouring distances: predicting a 10K from a 5K gives very usable numbers. The error grows with big jumps, typically 5K to marathon, where distance-specific preparation matters more than raw speed. Use a recent result over a distance as close as possible to your target race.
Garmin, Polar and Coros race predictors use your estimated VO2 max and training history, which makes them more contextual. They still tend to be optimistic for runners short on long runs. Cross-checking a watch estimate against a Riegel calculation gives you a more honest range than either source alone.
The one closest to your goal distance, and the most recent. To plan a marathon, an all-out half marathon run within the last six weeks is the best input you can give. Anything older than three months no longer reflects your current fitness.
This is the most common complaint, and it almost always comes down to the exponent. If you are quick over 5K or 10K but your weekly mileage is modest, raise the exponent towards 1.10 and the projection becomes realistic. The marathon punishes a lack of miles far more than a lack of speed.