About this tool
The Race Time Predictor is a free, in-browser tool that projects your finish time at other standard distances from one recent race result, using Pete Riegel's endurance formula T2 = T1 × (D2 ÷ D1)^1.06.
Pick the distance you raced, type your time, and the table instantly shows equivalent performances for 5K, 10K, half marathon (21.0975 km) and marathon (42.195 km) with the matching pace per kilometre. Everything runs locally in your browser.
Frequently asked questions
How does the Riegel formula work?
It models how pace slows as distance grows: T2 = T1 × (D2/D1)^1.06. The 1.06 exponent is the fatigue factor Riegel derived from elite and club runners across running distances.
How accurate are the predictions?
They assume equivalent training for the target distance. Predictions one step up or down (10K from a 5K) are usually close; marathon times from a 5K tend to be optimistic unless you have real endurance mileage.
What time formats can I type?
Use mm:ss or h:mm:ss, for example 22:30 or 1:45:00. Minutes and seconds must be below 60; the prediction updates as you type.
Which exact distances are used?
5 km, 10 km, 21.0975 km for the half marathon and 42.195 km for the marathon — the officially certified road-race distances.