Race Time Predictor
Predict a race time at a new distance from a recent result.
About Race Time Predictor
The exponent slightly above one means pace slows as distance grows. Predictions are reliable within a factor of about four in distance and become optimistic beyond that. Predicting a marathon from a 5K assumes endurance the shorter race never tests, which is why such predictions are consistently too fast.
Formula
Race Time Predictor
Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06
- fatigue exponent
- Riegel's 1.06 captures how pace degrades with distance
- equivalent performance
- The time representing the same fitness at another distance
Worked example
With the default values below, Race Time Predictor returns Predicted Time of 1:41:12. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Known race distanceminimum 0.01 km
- 5 km
- Known race timeminimum 0.01 min
- 22 min
- Target race distanceminimum 0.01 km
- 21.0975 km
- Fatigue exponentaccepts 1–1.3
- 1.06
Results
- Predicted Time
- 1:41:12
- Predicted Time
- 101.20 min
- Predicted Pace
- 4.7970 min/km
- Known Race Pace
- 4.4000 min/km
- Pace Slowdown at the Longer Distance
- 9.02%
Step by step
- 1Values used
- 2Race Time Predictor
- 3Predicted Time= 1:41:12
- 4Predicted Time= 101.20 min
- 5Predicted Pace= 4.7970 min/km
- 6Known Race Pace= 4.4000 min/km
- 7Pace Slowdown at the Longer Distance= 9.02
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- These figures are population-average estimates for training and recreational planning only and are not medical advice. Individual responses vary widely. Consult a qualified health professional before starting or significantly changing an exercise, supplement or nutrition programme, particularly if you have any medical condition.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
These figures are population-average estimates for training and recreational planning only and are not medical advice. Individual responses vary widely. Consult a qualified health professional before starting or significantly changing an exercise, supplement or nutrition programme, particularly if you have any medical condition.
Frequently Asked Questions
How is Race Time Predictor calculated?
Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06 The exponent slightly above one means pace slows as distance grows. Predictions are reliable within a factor of about four in distance and become optimistic beyond that.
Why does Race Time Predictor matter?
Predicting a marathon from a 5K assumes endurance the shorter race never tests, which is why such predictions are consistently too fast.
What values do I need to enter?
This calculator takes 4 inputs: Known race distance, Known race time, Target race distance, Fatigue exponent. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.