Race Time Predictor Calculator
Predict your finish time at any race distance from a recent result using Riegel’s formula — 5K to 10K, 10K to half, mile to marathon, any pair.
How it works
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Riegel’s 1977 formula captures how pace fades as distance grows: the 1.06 exponent means each doubling of distance slows your pace by roughly 6%. Unlike a marathon-specific predictor, this works between any two distances — predict a 10K from a 5K, a half from a 10K, or a mile from a 5K. Accuracy is best when the two distances are near each other and you have actually trained the longer one.
Worked example
A 22:00 (1,320-second) 5K predicting 10K: T₂ = 1,320 × (10 ÷ 5)^1.06 = 1,320 × 2.085 ≈ 2,752 s, about 45:52 — a 4:35 per km pace. Predict the marathon from the same 5K and the number balloons, which is exactly why longer predictions need long-run training to hold.
Frequently asked questions
How is this different from the marathon time predictor?
The marathon predictor is fixed to the 42.195 km target. This one goes any distance to any distance, so you can plan a 10K off a parkrun 5K or a half off a recent 10K.
Why did I run slower than predicted at a longer distance?
The formula assumes proportionate endurance. Jump far beyond your training distance — a 5K predicting a marathon — and you will fade late, where fitness the formula cannot see runs out. Predict from a race close to your target for the best odds.
Can I predict a shorter race from a longer one?
Yes — the math runs both ways. Predicting a 5K from a half is usually a touch pessimistic, because your fresh top-end speed often beats what a long race implies. Trained speedwork will have you faster than the formula says.
Related calculators
Estimates for general fitness and education — individual results vary with terrain, fitness, technique and measurement error. Not medical advice.