Race Time Predictor

Overview

A race-time predictor uses one known race result to estimate finish times across other distances. It is the runner's equivalent of a units converter, and it answers questions like "if I just ran 21:30 for a 5K, what is a realistic goal pace for a half marathon in eight weeks?" The same tool also works for cycling, rowing, swimming, and any endurance sport where pace is a power function of distance.

The prediction is most useful for setting target paces in training, picking a race-day strategy, and benchmarking progress between events. It assumes that you are equally well-trained for the predicted distance — extrapolating a 5K time to a marathon for a runner who has never gone past 10 km will systematically over-predict.

How it works

The classic model is Pete Riegel's formula, published in 1977: T2 = T1 × (D2 / D1) ^ 1.06. T1 is your known time over distance D1, T2 is the predicted time over the new distance D2, and the exponent 1.06 captures empirically how pace slows as distance grows. The exponent works well across roughly 1,500 m up to the marathon for runners; for ultra distances and for novice athletes it should be raised slightly (1.07–1.08).

The 1.06 exponent reflects the average pace fade observed across thousands of road race results. A linear scaling (T2 = T1 × D2 / D1) would predict identical paces at every distance, which contradicts physiology — fuel limits, fatigue, and biomechanical efficiency all degrade with duration.

Examples

• A 5K in 22:00 → 10K predicted as 22 × (10 / 5) ^ 1.06 ≈ 45:50.
• A 10K in 45:00 → half marathon 45 × (21.0975 / 10) ^ 1.06 ≈ 1:39:55.
• A half marathon in 1:40:00 → marathon 100 × (42.195 / 21.0975) ^ 1.06 minutes ≈ 3:28:00.
• A mile in 6:00 → 5K 6 × (5 / 1.609) ^ 1.06 ≈ 20:23.

FAQ

Is Riegel always accurate?
It is a reasonable first estimate. Real marathon times often run 1–3% slower than Riegel predicts from shorter races for under-trained marathoners.

Can I use this for cycling?
Yes, though the exponent for cycling time trials is slightly lower (around 1.04–1.05) because drafting and aerodynamics differ from running.

Why does the exponent matter so much?
Tiny changes magnify across long distances. Moving from 1.06 to 1.07 lengthens a marathon prediction by several minutes.

Should beginners trust the marathon prediction?
Only if the long-run training is solid. Otherwise, add a 5–10% buffer to the predicted time.

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