Race Time Predictor β 5K, 10K, Half & Marathon (Riegel)
Predict your 5K, 10K, half-marathon and marathon times from any recent race result using the Riegel formula, with target paces per km.
π Runs entirely in your browser. Your input and output are never sent to a server or stored β fully private.
Riegel formula (exponent 1.06). Assumes comparable training for the target distance; predictions get less reliable for big jumps in distance (e.g. 5K β marathon). Not coaching or medical advice.
About this tool
This race time predictor estimates your times for 5K, 10K, half marathon and full marathon from a single race result, using Riegel's formula (with the standard 1.06 fatigue exponent). Enter a distance you have raced and your finish time, and it projects the others along with the pace per kilometre for each. It is a quick way to set a realistic goal, plan even splits, or see what a recent 5K suggests you could run over 42 km. Everything is calculated in your browser β nothing is sent anywhere.
Frequently asked questions
How accurate are the predictions?
Riegel's formula assumes you are equally well trained for the target distance β which is rarely true across a big jump. A 5K predicting a marathon will usually be optimistic unless you have done the long-run endurance work, because marathon time depends heavily on fuelling, heat and the wall. Predictions between nearby distances (5Kβ10K) are quite reliable; the further you extrapolate, the more you should treat the result as a ceiling, not a promise.
What is the exponent in the formula?
Riegel's model is T2 = T1 Γ (D2 Γ· D1)^1.06. The 1.06 exponent captures how pace slows as distance grows: if you doubled the distance at the same pace the exponent would be 1.0, so 1.06 adds a realistic slowdown. Elite, well-trained runners sometimes fit a slightly lower exponent (they fade less), while under-trained runners fade more β another reason to treat long extrapolations as approximate.
Is my data kept private?
Yes. The prediction is arithmetic run by JavaScript in your browser, so the distance and time you enter are never uploaded or stored. It also works offline once the page has loaded, which is handy for checking splits at a race. This is guidance for planning, not coaching or medical advice β build up distance gradually and listen to your body.