Race Time Predictor
Enter one recent result, predict the rest — 5K, 10K, half marathon, marathon.
How does race time prediction work?
The Riegel formula extrapolates a finish time at one distance to a predicted time at another, using the relationship T2 = T1 × (D2/D1)^1.06 — the 1.06 exponent (rather than exactly 1.0) reflects that endurance drops off slightly faster than pace alone as distance increases, which is why your marathon pace is meaningfully slower than your 5K pace, not just proportionally slower. It works best when the target distance isn't wildly different from your reference race — predicting a 10K from a 5K is more reliable than predicting a marathon from a 5K, where training-specific endurance starts to matter more than the maths captures.
Frequently asked questions
What is the Riegel formula?
A widely used race-prediction formula: T2 = T1 × (D2/D1)^1.06, where T1 and D1 are your known time and distance, and T2 is the predicted time for a new distance D2. The 1.06 exponent accounts for endurance dropping off slightly faster than pace alone would suggest over longer distances.
How accurate is a race time predictor?
Most accurate when predicting a distance reasonably close to your reference race — predicting a marathon from a recent 5K is a bigger extrapolation than predicting a 10K from a recent 5K, and endurance training, pacing and race-day conditions all affect the real result.
Does this account for hills or weather?
No — it's a pure time-distance extrapolation. Hilly courses, heat, and wind will all affect your actual result regardless of what this predicts.
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