Race Time Predictor
Predict your finish time at a new distance from a recent race using Riegel's formula.
Predicted time52.12min
Predicted pace5.21min/km
Riegel's formula predicts a race time as t₂ = t₁ × (d₂ ÷ d₁)^1.06, where the 1.06 exponent reflects the natural slow-down over longer distances. A 5 km in 25 minutes predicts a 10 km of ≈ 25 × (10/5)^1.06 ≈ 52.12 minutes — a slightly slower pace than the 5 km. It is most reliable when the two distances are within a factor of about four and you are well-trained for the longer one.
Questions
How does the race time predictor work?
It uses Riegel's formula: predicted time = known time × (new distance ÷ known distance) raised to the power 1.06. The 1.06 exponent accounts for the inevitable slowing over longer races.
How accurate is the prediction?
It is most accurate when the two distances are reasonably close (within about a four-fold range) and you have trained for the longer one. Extrapolating a 5 km straight to a marathon will tend to be optimistic.