Marathon Time Predictor
Predict your marathon finish time from a recent 5K, 10K or half marathon using Riegel’s formula.
How it works
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Riegel’s formula, published in 1977, models how pace decays as race distance grows: doubling the distance slows your pace by about 6%, not just your time. It is remarkably accurate between well-trained race efforts from 5K up through the marathon. The shorter the race you predict from, the more the marathon number depends on training you haven’t necessarily done yet.
Worked example
A 24:30 5K is 1,470 seconds. Marathon = 1,470 × (42.195 ÷ 5)^1.06 = 1,470 × 9.59 ≈ 14,099 seconds — about 3:54:59, an 8:58/mile pace. The same run projects a 51:05 10K and a 1:52:42 half.
Frequently asked questions
Why did I miss my predicted marathon time by 20 minutes?
The prediction assumes proportionate endurance training — weekly mileage and long runs appropriate to the marathon, not just 5K fitness. Most first-timers who undertrain the long stuff fade after mile 20, exactly where the formula can’t see.
Which race predicts the marathon best?
The half marathon, by far — it is closest in energy systems and pacing. A 5K-based marathon prediction is a best-case ceiling; treat it as a stretch goal.
Is the 1.06 exponent right for everyone?
It fits trained runners on average. Speed-leaning runners fade more (their exponent runs 1.07–1.10); high-mileage endurance types can beat it slightly. After a race, back out your own exponent and use that.
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This calculator is for educational purposes. Double-check important results before acting on them.