Race Time Predictor

How this is calculated

This uses Riegel's formula, published by Peter Riegel in Runner's World in 1977: predicted time = known time x (new distance / known distance) raised to the power 1.06. The 1.06 exponent was derived empirically from race data and encodes a simple fact - longer races are proportionally harder, so pace slows as distance grows. If the exponent were exactly 1 your pace would be identical at every distance, which no runner experiences. At 1.06, DOUBLING the distance multiplies your time by 2 to the power 1.06, which is about 2.085 rather than 2 - so your pace slows by roughly 4.25 percent each time the distance doubles. Because the calculation depends only on the RATIO of the two distances, the units cancel. Kilometres and miles give identical answers provided both boxes use the same one. ⚠️ This assumes you are equally well trained for both distances, which is the assumption that breaks most often. A 5K time predicts a marathon on the basis that you have done marathon training - if you have not, the prediction is describing a runner you are not yet. Riegel's formula is a statement about how race performance scales, not about whether you have done the long runs. Accuracy is best over SMALL jumps. A 5K predicting a 10K is reliable; a 5K predicting a marathon is a rough guide at best, and published assessments put these formulas at around 80 percent accuracy overall - meaning roughly one runner in five misses meaningfully. The exponent is also an average: stronger endurance runners slow less than 1.06 predicts, and newer runners typically slow more. The formula holds for race durations between roughly 3.5 and 230 minutes. Beyond the marathon it becomes unreliable for reasons that are not physiological scaling at all - terrain, elevation, nutrition, sleep and simply being awake for a very long time.

Common questions

Why 1.06 and not just 1?

Because an exponent of exactly 1 would mean your pace is identical at every distance, which no runner experiences. Riegel derived 1.06 empirically from race data, and it encodes how much performance decays as races get longer. The practical consequence: doubling the distance multiplies your time by about 2.085 rather than 2, so your pace slows roughly 4.25 percent with each doubling. That single number is the entire content of the formula.

How accurate is it?

Best over small jumps and shaky over large ones. A 5K predicting a 10K is reliable; a 5K predicting a marathon is a rough guide. Published assessments put these formulas around 80 percent accuracy, which means roughly one runner in five misses meaningfully. The exponent is an average too - stronger endurance runners slow less than 1.06 predicts, and newer runners typically slow more, so your own history across two distances is better evidence than the formula.

It says I can run a marathon much faster than I think.

Almost certainly because it is assuming you are trained for one. That is the assumption this formula makes and the one that breaks most often - it describes how race performance SCALES with distance, not whether you have done the long runs, practised fuelling, or ever been on your feet for three hours. A 5K time predicts what a marathon-trained version of you could do. If that version does not exist yet, neither does the time.

Do I have to use kilometres?

No - use whatever you like, as long as BOTH boxes use the same unit. The calculation depends only on the ratio between the two distances, so the units cancel out entirely and miles give exactly the same answer as kilometres. Mixing them is the only way to get it wrong, and it will not warn you.

Can I predict a shorter race from a longer one?

Yes, and the formula runs backwards perfectly well - the pace-change figure simply comes out negative, meaning you would expect to run faster per unit. It tends to be less useful in that direction, though, because a marathon time reflects endurance and fuelling more than the raw speed a 5K asks for. Predicting down usually flatters you less than predicting up flatters you.

Does it work for ultras?

Not really. The formula holds for race durations of roughly 3.5 to 230 minutes, and past the marathon the limits stop being about physiological scaling at all - terrain, elevation, nutrition, sleep deprivation and simply being awake a very long time dominate. Those are not things an exponent can describe.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Race Time Predictor at https://www.bfcbrilliance.com/tools/race-time-predictor.

What I entered:
- Distance you have raced: ___
- Your time — hours (h): ___
- — minutes (min): ___
- — seconds (s): ___
- Distance you are aiming at: ___

What it calculated:
- Predicted finish: ___
- In seconds: ___
- Target pace — minutes: ___
- — and seconds: ___
- Your pace is expected to slow by: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

More running & walking tools