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An independent guide to UK running events. Not the official website of any race, organiser or timing company, and not an entry or results service. About this site

Race time predictor

Enter a recent race or time-trial result and the predictor will estimate what you could run at the other common distances if you train properly for them.

How the prediction works

The predictor uses the Riegel formula, published by the engineer and runner Peter Riegel in the 1970s. It says that as a race gets longer your time grows slightly faster than the distance does:

predicted time = known time × (new distance ÷ known distance)1.06

The exponent of 1.06 is the "fatigue factor". It means that doubling the distance takes a little more than double the time: about 2.08 times as long. So a 50-minute 10K points to roughly a 1:50 half marathon, not 1:45.

What it does not tell you

  • It assumes you train for the new distance. A fast 5K does not give you a marathon unless you have done the long runs. Predictions going up in distance are a ceiling, not a promise.
  • It is most reliable between neighbouring distances. 10K to half marathon is a good guide. 5K to marathon is a rough one, and tends to flatter newer runners.
  • It knows nothing about the course. Hills, heat, wind and a congested start can easily cost several minutes over a half marathon.
  • Use a recent, all-out result. A time from last year, or from a run where you held back, will give a misleading answer.

Turning a prediction into a plan

Take the predicted time for your race distance to the pace calculator to find the pace per mile, then practise that pace in training: a few miles at target pace inside a longer run tells you quickly whether the target is realistic. For a first attempt at a longer distance, add five per cent to the prediction and aim for that. Our training plans show where race-pace running fits in the week, and the pace band generator will give you splits for the day.