The formula
How to calculate race time predictor
A recent race result carries useful information about a new, untested distance. The Riegel formula takes a known time over a known distance and scales it to predict a finish time at a different distance, using an exponent derived from how endurance performance typically degrades over longer efforts.
The 1.06 exponent is the heart of the formula: if performance simply scaled with distance, doubling the distance would double the time and the exponent would be 1. The extra 0.06 models the real-world slowdown that comes from extending an effort, capturing endurance decay without needing any physiological detail about the runner.
Fill in the following:
- Known race distance
- Known time, hours (h)
- Known time, minutes (min)
- Known time, seconds (s)
- Target race distance
No submit button: type and the answer moves. Your inputs end up in the link, so the page can be shared already filled in.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why race time predictor matters
Most people who look up a race time predictor calculation already have a specific number in mind — a quote, an offer, a target — and want to check it rather than learn the theory behind it. This page is built for that: enter your own figures, see the result immediately, and change any field to see how the answer moves without redoing the arithmetic from scratch each time.
This tends to come up when comparing two concrete alternatives — two lenders, two savings products, two ways of structuring the same decision — rather than in the abstract. Run both scenarios through the same calculator with the same assumptions and the comparison becomes fair, because the only thing changing between the two results is the number you are actually trying to test.
It is also worth being clear about what a single figure like this can and cannot settle on its own. It answers the specific question the formula was built to answer, and nothing more — a favourable race time predictor result does not automatically mean a decision is a good one overall, since plenty of other factors that a formula cannot capture, from personal circumstances to how comfortable a commitment feels, usually matter just as much as the arithmetic. Use the number as one solid input among several rather than the whole of the decision.
In practice, most people arrive at a page like this one having already tried a version of the calculation by hand or in a spreadsheet, and use the calculator here to confirm it rather than replace it. That is a reasonable way to use it — the two should agree to the last decimal place if the same inputs and the same formula are used, and if they do not, the formula shown above is the one to check your own working against first.
Worked example
Work through the defaults on this page:
- Known race distance: 10k
- Known time, hours: 0 h
- Known time, minutes: 48 min
- Known time, seconds: 0 s
- Target race distance: marathon
That gives:
- Predicted finish time: 220.81 min
- Required pace: 5.23 min/km
- Average speed: 11.47 km/h
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
Treat the prediction as a reasonable target rather than a guarantee. It assumes the training behind the known result also supports the new distance — a runner who trained specifically for a fast 5K without any long runs will typically undershoot a marathon prediction built from that result.
Where this goes wrong. The formula gets less reliable the further apart the two distances are. Predicting a marathon time from a 5K result stretches the model far beyond where it was validated, since marathon performance depends heavily on fuelling and long-run endurance that a 5K result says nothing about; predicting a half marathon from a 10K result, by contrast, is a much smaller extrapolation and tends to be considerably more accurate.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
It tends to be reasonably accurate between similar distances — 10K to half marathon, for example — but increasingly optimistic for well-trained marathoners predicting from short races, since it does not account for glycogen depletion and fuelling, which become the limiting factor beyond about 90 minutes of running.
An exponent of 1 would mean pace stays exactly constant regardless of distance, which does not match observed race results. Riegel derived 1.06 empirically from a large set of world-record and age-group performances across distances.
The answer it gives you is predicted finish time. With 0 h known time, hours, 48 min known time, minutes and 0 s known time, seconds, that comes to 220.81 min. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.