VO₂ max calculator
Estimate aerobic fitness from a completed Cooper run or Rockport walk test.
Race time predictor
Use two recent races to estimate how your pace changes with distance, then project another race.
Race time predictor
Example valuesUse two recent races to estimate how your pace changes with distance, then project another race.
No account needed. Calculations happen in your browser; your entries are not saved or sent to Century.
Example · Predicted finish time
1:54:26
A projection from two races; terrain, weather and preparation can change the finish.
- Fitted exponent
- 1.057
- Target distance
- 21.10 km
How it’s calculated
This tool fits the Riegel power law T₂ = T₁ × (D₂ ÷ D₁)^b. Rather than assuming the population exponent 1.06, it estimates b = ln(T₂ ÷ T₁) ÷ ln(D₂ ÷ D₁) from two races. The races must differ enough in distance and produce a plausible exponent. This is an extrapolation from past performances, not a guarantee for terrain, weather or preparation.
Reference: Stanford: evaluating race-time prediction models
For general wellness information. This tool does not diagnose health conditions or replace medical advice.
Related tools
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Race time predictor: use your own data
What to enter
Use two recent races to estimate how your pace changes with distance, then project another race.
The equation can extrapolate that far, but the uncertainty grows with distance. Marathon-specific endurance, fueling, course and weather are not measured by short races.
Read the result in context
The fitted exponent is about 1.057. Project the half-marathon using the personal exponent, not a constant 5:00/km pace.
Races too close in distance make the fitted exponent unstable. An exponent outside a plausible running range may mean one race was not representative or the inputs need checking.
Where this estimate stops
The pace calculator assumes constant pace. This predictor models the slowdown typically seen across different race distances.
Put your race preparation and recovery in context with Century.
Frequently asked questions
Why use two races instead of one?
Two results let the calculator estimate how your own finish time changes with distance. A one-race predictor must assume a fixed fatigue exponent for everyone.
Can I predict a marathon from a 5K and 10K?
The equation can extrapolate that far, but the uncertainty grows with distance. Marathon-specific endurance, fueling, course and weather are not measured by short races.
Why are some pairs rejected?
Races too close in distance make the fitted exponent unstable. An exponent outside a plausible running range may mean one race was not representative or the inputs need checking.
Is this the same as the pace calculator?
No. The pace calculator assumes constant pace. This predictor models the slowdown typically seen across different race distances.