Race time predictor

One recent race result in - predicted times for 5K, 10K, half and marathon out, with the pace each one needs.

h / m / s

Riegel formula (exponent 1.06). Predictions assume you train appropriately for the target distance - a 5K result predicts a marathon you could run after marathon training, not next weekend.

How the prediction works

This uses Peter Riegel's 1977 formula: T2 = T1 × (D2 ÷ D1)1.06. Your time scales with distance raised to the power 1.06 - that exponent captures how everyone's pace naturally decays as the distance grows. It has held up remarkably well across five decades and underpins most predictor tools, including this one.

Reading the predictions honestly

  • Neighbouring distances are reliable. 10K → half is solid; 5K → marathon is a best-case scenario.
  • The formula assumes the training is done. It predicts what your engine could deliver over the distance with appropriate preparation - it doesn't check whether you've done the long runs.
  • Recent and flat beats old and hilly. Predict from an all-out effort in the last couple of months on a fair course.
  • Marathon reality check: most club runners run 5-10 minutes slower than their half-based prediction on the day. Fuelling, weather and pacing discipline are the difference.

FAQs

How accurate is the Riegel formula?

Within about 1-2% between neighbouring distances for trained runners. The gap grows as the distances diverge - short race to marathon is where it flatters most.

Can I use a parkrun time?

Yes - if it was a genuine all-out effort. A social parkrun under-predicts you everywhere; a max-effort one is a perfectly good 5K input.

Why is my marathon always slower than predicted?

The marathon punishes missing endurance and fuelling in a way no shorter race does. Predict from a half, do the long-run block, practise race fuelling, and the gap closes.