Running Pace & Race Predictor
Pace, even splits, and Riegel race-time predictions flagged by how far the extrapolation is stretching.
- 01Pick a distance and enter your finish time as mm:ss or h:mm:ss.
- 02Use the splits table to plan pacing on race day.
- 03Check the confidence flag on each prediction before trusting it.
What the terms mean
- Pace
- Time per unit distance, e.g. 5:00 per kilometre.
- Splits
- Cumulative times at intermediate distances during a race.
- Riegel formula
- A race-time prediction scaling time by distance raised to the power 1.06.
- Even splits
- Running each segment at the same pace throughout.
Enter as mm:ss or h:mm:ss — for example 50:00 or 1:45:30.
10 km in 50:00 — about 12.0 km/h.
Method & sources
Predictions use the Riegel formula, which scales time by the distance ratio raised to the power 1.06. The exponent encodes the fact that pace slows as distance grows, and it was fitted to race data across a range of events.
Its central assumption is that you have trained appropriately for the target distance. It cannot know whether you have done the long runs a marathon requires, which is why extrapolating from a 5K to a marathon is optimistic and flagged as rough here. Predictions between adjacent distances are considerably more trustworthy.
Riegel PS, Athletic records and human endurance, American Scientist 1981
What these numbers
actually tell you.
Predictions assume you trained for the target distance
This is the formula’s central and largest assumption. It cannot know whether you have done the long runs a marathon requires. Predicting a marathon from a 5K is therefore optimistic, and the tool flags such predictions as rough. Adjacent distances — 10K to half marathon — are considerably more trustworthy.
Even splits are a plan, not a law
Elite performances are frequently run with a slightly faster second half. Terrain, heat and wind all justify deviating. Use the splits as a pacing anchor for the early kilometres, where over-enthusiasm does the most damage.
What a prediction does not account for
Fuelling, heat acclimatisation, course elevation and race-day nerves are all absent from the arithmetic. Treat the predicted time as what your current aerobic fitness permits under good conditions with appropriate preparation.
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