About this calculator
This is a (beta) race time predictor which takes into account your sex and whether you are best at short or long races. It starts with the Riegel formula and then weights your result.
The Riegel formula is a widely used method for predicting race times across different distances, but it has limitations. It assumes a consistent endurance profile and a fixed fatigue factor, which may not reflect individual variations in training, pacing strategy, or physiological strengths. As a result, it tends to be less accurate for very short races (e.g., 5 km) where speed rather than endurance dominates, for ultra-distances where cumulative fatigue plays a larger role, or for athletes whose strengths are highly distance-specific.
How to use it
Choose a recent race you ran well and enter the distance and time.
Enter your target race distance
Select your sex
Choose if you are better at longer races or shorter ones. If you can beat your mate in a 5km but not a 10km, or you tend to slow down over time choose shorter.
Predict time
You get both the Riegel Formula result and a weighted result for your sex and race type preference. Over time you can see which is closer and understand which formula fits you best.
How do your choices affect the calculation?
Women tend to be stronger over longer races (in relation to their own race PRs). So for example a woman with a 1:28 half marathon has a slightly higher chance of a 3 hour marathon than a man with the same PR. But if a woman and a man both have a 1:28 half marathon time and hope to break 19 minutes for the 5km, the man has the higher chance of doing so.
Is this better than Riegel?
Riegel devised his formula in 1977 when fewer women were running, so it may be weighted slightly in favour of men. This formula considers the fact that many more people are now running - and doing races of all distances rather than specialising.
Like this? There are nine more free calculators on my website, helping you with pacing, hydration, fuel, caffeine planning and more check them out
Better or Beta?
This is a beta tool, I’d love to know how your predicted time fares against your race outcome! Contact me