Race Predictor Calculator
Predict a race time at one distance from a result at another.
About Race Predictor Calculator
Riegel’s formula raises the distance ratio to a power slightly above one, which captures the pace degradation over longer races. It is accurate within about two per cent for distances within a factor of four. Predicting a marathon from a 5K stretches the formula beyond its reliable range, because marathon performance depends on endurance training the shorter race never tests.
Formula
Race Predictor
Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06
- fatigue exponent
- Riegel’s 1.06 reflects how pace degrades with distance
- equivalent performance
- The time representing the same fitness at another distance
Worked example
With the default values below, Race Predictor Calculator returns Predicted Time of 3:22:25. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Known race distanceminimum 0.001 km
- 10 km
- Known race timeminimum 0.01 min
- 44 min
- Target race distanceminimum 0.001 km
- 42.195 km
- Fatigue exponentaccepts 1–1.3
- 1.06
Results
- Predicted Time
- 3:22:25
- Predicted Time
- 202.41 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 Predictor
- 3Predicted Time= 3:22:25
- 4Predicted Time= 202.41 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 estimates based on population-average formulas and are intended for training and recreational planning only. They are not medical advice. Consult a qualified health professional before starting or changing an exercise programme, particularly if you have an existing 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 estimates based on population-average formulas and are intended for training and recreational planning only. They are not medical advice. Consult a qualified health professional before starting or changing an exercise programme, particularly if you have an existing medical condition.
Frequently Asked Questions
How is Race Predictor calculated?
Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06 Riegel’s formula raises the distance ratio to a power slightly above one, which captures the pace degradation over longer races. It is accurate within about two per cent for distances within a factor of four.
Why does Race Predictor matter?
Predicting a marathon from a 5K stretches the formula beyond its reliable range, because marathon performance depends on endurance training the shorter race never tests.
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.