Free tools
Race time predictor: 5K, 10K, half marathon and marathon
One honest race result says a lot about the others. Enter a recent race or time trial and this calculator predicts your 5K, 10K, half marathon and marathon times with the formula most race predictors are built on, Peter Riegel’s. Read the predictions as what your current fitness allows if you have trained for the distance, not as a promise.
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Short answer
Riegel’s formula predicts a time T2 for distance D2 from a race of time T1 over D1: T2 = T1 × (D2 ÷ D1)1.06. A 10K in 50:00 predicts about 23:59 for 5K, 1:50:19 for a half marathon and 3:50:01 for a marathon. The longer the jump from the race you entered, the less certain the prediction.
How the prediction works
Pace slows as races get longer, and it slows in a surprisingly regular way. Peter Riegel, an engineer and runner, described it in 1977 and in more detail in a 1981 paper: double the distance and your time grows by a little more than double, by a factor of about 2.08. The exponent of 1.06 captures that fatigue. It fits trained runners well from about 3 minutes to a few hours of racing.
What the numbers assume
- You trained for the distance. A marathon prediction from a 5K assumes the long runs and weekly volume a marathon needs. Without them, most runners finish slower than predicted.
- The race was a real effort on a fair course, in reasonable weather, and recent, ideally within the last six weeks.
- The jump is not too big. Predictions more than about four times the input distance are marked with an asterisk. A half marathon is the best input for a marathon prediction.
Many runners find the marathon prediction from a short race too optimistic, while 5K and 10K predictions from each other are usually close. If your predictions from two recent races disagree, believe the one closer to your target distance.
Turning a prediction into a plan
A prediction tells you where to start, not where to finish. Use it to set training paces and a realistic first goal, then check again after the next race. Keho Coach uses your races and personal bests the same way: its pace targets come from your recent results, or from your VO2max estimate when there is no race, and the plan adapts as your fitness and recovery change. See how a plan is built in our half marathon and marathon guides, and turn any time into a pace with the pace calculator.
Questions
Can I use a training run instead of a race?
Only a hard, evenly paced effort, such as a solo time trial. An easy run, or a run with stops, predicts far slower times than you can actually race.
Does it work for trail races?
No. Climbing, descending and terrain change the time for a given distance too much. The formula is meant for road and track races.
How does this compare with VO2max predictions?
A race result already includes your running economy and your ability to hold a pace, so it usually predicts better than a VO2max estimate. You can go the other way with the VO2max calculator, which estimates VO2max from a race.