Race Time Predictor

Frequently Asked Questions

How does the Riegel formula work?

T2 = T1 × (D2/D1)^1.06. The exponent 1.06 reflects the typical pace fade across longer distances. Pete Riegel proposed it in 1981 from analysis of race times across distances.

How accurate are race time predictions?

Most accurate within ~2× the source distance. Predicting marathon time from a 5K is typically optimistic by 5–15% unless you have built specific endurance; from a half-marathon it is much closer. This estimate is for general guidance only - work with a qualified coach or sports medicine professional before pushing limits or recovering from injury.

Why is my marathon slower than predicted?

Beyond ~90 minutes of running, glycogen depletion, fueling, heat, and pacing dominate. Long-run preparation, race-day nutrition, and weather often matter more than VO2 max for marathon results.

Should I race tune-up events at full effort?

Going hard within 2–3 weeks of a goal race risks fatigue or injury. Most coaches recommend tune-ups at 90–95% effort or shorter than goal distance. This estimate is for general guidance only - work with a qualified coach or sports medicine professional before pushing limits or recovering from injury.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

This tool predicts your finish time at every standard race distance - 1 mile, 5K, 10K, half marathon, marathon and 50K - from a single recent result. You enter the distance you raced and your finish time in hours, minutes and seconds, and the calculator applies Pete Riegel's endurance model to extrapolate the others. It also reports your input pace per kilometre and per mile so you can sanity-check the source effort. The key physiological insight built into the math is that sustainable pace slows as distance grows: nobody holds 5K pace for a marathon, so a naive "same pace" projection would be wildly optimistic over longer distances.

Formula

The Riegel formula, published in 1977, is T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known time over distance D1 and T2 is the predicted time over the target distance D2. The exponent 1.06 is the fatigue factor: it inflates the predicted time slightly faster than distance alone, capturing the gradual loss of pace endurance athletes experience. Pace per kilometre for each prediction is simply T2 ÷ D2, and pace per mile multiplies that by 1.60934. The model is most reliable when D2 is within roughly half-to-double of D1; far outside that range, training specificity matters more than the equation can capture.

Worked Example

Suppose you run a 5K (D1 = 5 km) in 22:30, which is 1350 seconds. To predict your 10K time: T2 = 1350 × (10 ÷ 5)^1.06 = 1350 × 2^1.06 ≈ 1350 × 2.085 ≈ 2815 seconds, about 46:55. For a marathon (42.195 km): T2 = 1350 × (42.195 ÷ 5)^1.06 ≈ 1350 × 9.93 ≈ 13,400 seconds, roughly 3:43:30. Notice the marathon pace (≈ 5:18/km) is markedly slower than the 5K pace (4:30/km) - exactly the fatigue effect the exponent encodes.

Common Mistakes

Using a time-trial or training run instead of an all-out race effort - the prediction inherits whatever effort level you fed it.

Predicting a marathon from a 1-mile result - the distance gap is too large and the model will under-predict your real marathon time unless you have the endurance base.

Ignoring course and weather - the formula assumes a flat course and cool conditions; heat, humidity, hills and altitude all slow real finishes.

Forgetting that a proper 2-3 week taper can improve race-day performance 2-4% beyond a fatigued training result.

Entering minutes in the seconds box or vice versa - double-check the h : mm : ss fields.

Tips & FAQ

How accurate is it? Within a few percent for trained runners predicting nearby distances; treat longer extrapolations as ballpark targets. Should I adjust the exponent? Some coaches use 1.07-1.08 for ultramarathons where cumulative fatigue dominates; this tool uses the classic 1.06. What about altitude? Racing above 1,500 m can slow times 1-3% per 1,000 m of elevation gain - subtract that mentally from your prediction. How do I use it for training paces? Your tempo pace is roughly your one-hour race effort, easy runs sit 1-2 min/km slower than 5K pace, and VO2 max intervals run near 3K-5K race pace.

Race Time Predictor

Frequently Asked Questions

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