Race Time Predictor Calculator
Predict your 5K, 10K, half marathon, marathon or any race time from one recent result, with the pace each one needs.
Result…
How it's calculated
T₂ = T₁ × (D₂ ÷ D₁)^1.06 Pace = time ÷ distance 1 mile = 1.609344 km; half marathon = 21.0975 km; marathon = 42.195 km
Peter Riegel noticed that, across world records in running, swimming and walking, time grows slightly faster than distance: double the distance and the time rises by about 2^1.06, a little over double. Given one honest race result, the formula scales it to any other distance in the endurance range he studied, about 3.5 minutes to just under 4 hours. The exponent is editable: a higher value means slowing more over longer races. For an even-pace breakdown of a single race, use the pace calculator.
Your numbers, step by step
About the Race Time Predictor calculator
This race time predictor takes one recent race result and estimates what you could run at other distances: 1 mile, 5K, 10K, half marathon, marathon and a target distance of your choice in kilometres or miles. It uses Peter Riegel's formula, T₂ = T₁ × (D₂ ÷ D₁)^1.06, described in American Scientist in 1981, which allows for the way runners slow down as races get longer. Each prediction comes with the pace it needs per kilometre and per mile, so you can set a goal and pace a training run. Runners use it to choose a target for a first half marathon or marathon, or to check whether a goal is realistic. The exponent is editable if you want a more cautious prediction. The working is shown step by step.
How to use it
- Pick “Metric” or “Imperial” for the target distance.
- Choose your “Recent race distance”, or “Other” and enter it.
- Enter your finish time in “Hours”, “Minutes” and “Seconds”.
- Enter “Your target distance”, and leave the “Fatigue exponent” at 1.06 or raise it for a cautious estimate.
- Read the predicted times and the pace each one needs.
Worked example
From a 10K in 50:00, the marathon prediction is 50:00 × (42.195 ÷ 10)^1.06 = 3:50:01, a pace of 5:27 per km or 8:46 per mile. The same result predicts 23:59 for 5K and 1:50:19 for a half marathon. For an even-pace split of any one race, use the pace calculator.
Updated
Questions people ask
How do I predict my marathon time from a half marathon?
Multiply the half marathon time by 2^1.06, about 2.085. A 1:45:00 half predicts a 3:38:55 marathon with the standard exponent, or 3:41:58 with 1.08.
What is Riegel's formula?
T₂ = T₁ × (D₂ ÷ D₁)^1.06, where T₁ is a time over distance D₁ and T₂ the predicted time over D₂. The 1.06 means doubling the distance a little more than doubles the time.
Why is my marathon prediction too fast?
Predictions from short races assume you have done the long-run training for the longer distance. Many runners without it slow more, so try an exponent of 1.07 to 1.10, or predict from a longer race.
Which race result should I use?
A recent, all-out effort on a flat course, as close as possible to the target distance. A result from months ago or a hilly, hot race will skew every prediction.
How reliable are these predictions?
They are estimates. Riegel fitted the formula to efforts of roughly 3.5 to 230 minutes, and weather, course and pacing all change real results. A coach can give a plan for your own fitness, and check with a doctor before stepping up training.