Race Time Predictor

Planning your next race? Enter a recent race time and distance, then select your target race distance to get a predicted finish time. This calculator uses the Riegel formula — the industry standard used by running coaches, race organisers, and training apps — along with your required average pace.

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Known Race Distance

Known Race Time

Hours

Minutes

Seconds

Target Race Distance

About This Calculator

This tool uses the Riegel formula (T2 = T1 x (D2/D1)^1.06) to predict race times. The formula is widely used by running coaches and is most accurate when predicting between adjacent distances. Use a recent race result (within 2-3 months) run at full effort for the best accuracy. Predictions assume proper training for the target distance.

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Frequently Asked Questions

How does the race prediction formula work?

The Riegel formula (1977) is: T2 = T1 × (D2/D1)^1.06, where T1 is your known time, D1 is the known distance, D2 is the target distance, and 1.06 is the fatigue factor. This accounts for the natural slowdown over longer distances.

How accurate are race predictions?

Predictions are most accurate between adjacent distances (e.g. 10K to half marathon). Predicting from 5K to marathon has more uncertainty. Accuracy depends on proper training for the longer distance, race-day conditions, pacing strategy, and nutrition.

Why do I slow down at longer distances?

The fatigue exponent (1.06 in Riegel's formula) reflects that as distance increases, pace naturally slows due to glycogen depletion, accumulated fatigue, and the shift from anaerobic to aerobic energy systems. You cannot maintain your 5K pace for a marathon.

How recent should my reference race be?

Use a race result from the last 2-3 months for the most accurate prediction. Your fitness level changes over time, so older results may not reflect your current ability. Also ensure you ran the reference race at full effort.

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