Running pace calculator with race time predictions
Enter a distance and a finish time to get your pace per kilometre and mile, your speed, and predicted times for every standard race distance.
Runs in your browser — your input is not uploaded to ToolsNow.
Leave hours empty for anything under an hour.
Predictions use Riegel’s formula (T₂ = T₁ × (D₂/D₁)^1.06), which is reasonable between about 1500 m and the marathon. It assumes equivalent training for each distance, so it flatters you at the long end — a marathon prediction from a 5K time is optimistic unless you have actually done the endurance work.
How to use
- Choose your unit or a race. Kilometres or miles, or pick a standard distance to fill it in for you.
- Enter the distance and time. Leave the hours box empty for anything under an hour.
- Read your pace and speed. Per kilometre and per mile, plus km/h and mph.
- Check the predictions. Equivalent times at 1 km through marathon, based on the same fitness level.
Questions
What is a good running pace?
It depends entirely on distance and experience. A common recreational 5K pace is 6:00–7:00 per km; a sub-25-minute 5K (5:00/km) is a solid club-runner benchmark. Comparing yourself to your own previous times is far more useful than to averages.
How are the race predictions calculated?
Riegel’s formula: T₂ = T₁ × (D₂/D₁)^1.06. The 1.06 exponent reflects that you inevitably slow as distance grows. It is well regarded between about 1500 m and the marathon.
Why is my marathon prediction too optimistic?
Because Riegel assumes you are equally well trained for every distance. Predicting a marathon from a 5K time assumes endurance you may not have built. Marathons also introduce fuelling and glycogen limits that shorter races never test.
How do I convert pace to speed?
Speed in km/h is 60 divided by your pace in minutes per km. A 5:00/km pace is 60 ÷ 5 = 12 km/h.
Methodology & sources
How this is calculated
Pace, speed and distance are related by one identity — pace = time ÷ distance — so any two of them give the third exactly; there is no estimation involved in those figures. Predicted times for other distances use Riegel’s formula, T₂ = T₁ × (D₂ ÷ D₁)^1.06, where the 1.06 exponent reflects the observed slowing of pace as race distance increases.
Assumptions
- Riegel’s exponent was derived from race results across a range of distances and assumes a runner with training appropriate to the target distance.
- Pace and speed conversions are exact arithmetic and carry no assumptions beyond the units entered.
Limitations
- Race prediction assumes adequate endurance training for the longer distance. A well-trained 5 km runner who has not built distance will not achieve the predicted marathon time, and the error grows with the gap between the two distances.
- The formula makes no allowance for terrain, heat, humidity, altitude, or the individual variation in how well a runner holds pace as distance grows.
- It is a planning estimate, not a performance target, and it says nothing about whether attempting a given distance is advisable for you.
Sources
- Riegel PS. “Athletic Records and Human Endurance.” American Scientist 1981;69(3):285–290 — the origin of the endurance exponent used for race-time prediction American Scientist (Sigma Xi)
Sources last checked 30 July 2026. Estimates for general information only — not medical advice, and no substitute for a qualified clinician.
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