Race Time Predictor Calculator

A race time prediction takes a result you have already run and scales it to a new distance. The idea is that your average speed drops by a fairly steady amount as the distance grows. This calculator uses two published methods.

What a Race Time Prediction Measures

It answers one question: if you were as fit, and raced as well, at the new distance as you did in your recent race, what time would you run? It does not measure fitness directly, and it does not know your training, the course or the weather. A pace converter, by contrast, only turns a time into a pace for the same distance.

The Formula

Predicted time = your time x (new distance / your distance) ^ 1.06

Peter Riegel described this kind of power law in 1981, after fitting it to world records, and called the exponent a "fatigue factor"¹. He found about 1.08 for world-class runners and 1.05 to 1.06 for men aged 40 to 70, and his fit covered efforts of about 3.5 to 230 minutes¹. This calculator uses 1.06 as its main method. The second method is the VDOT system, published by Jack Daniels and Jimmy Gilbert in 1979². It turns your race into a score from the oxygen cost of your speed and the share of your top oxygen use you can hold for that long. It then finds the time that gives the same score at the new distance.

How This Calculator Computes Your Result

Choose your recent race (1 mile, 5K, 10K, 15K, 10 mile, half marathon, marathon, or a custom distance from 1.5 to 50 km), enter your time in hours, minutes and seconds, then choose a target distance. The calculator converts everything to meters and seconds, applies the Riegel formula and rounds to the nearest second. It shows your predicted time, pace per mile and per km, and a table for all standard distances, with the VDOT time beside the Riegel time. It warns you when the target is far from your race or is a marathon, and it rejects times that work out to a pace faster than 2:00 or slower than 15:00 per km.

What Your Result Means

There is no good or bad prediction. What matters is how far your target is from your race. The calculator reads the gap like this. The 4 and 8 times cut-offs are our own design choices, not published limits.

For example, a 25:00 5K gives these predictions:

The two methods land within about two minutes of each other everywhere. That is common when both distances are 5K or longer (our calculation). Agreement does not make either one right for the marathon, as the next two sections show.

How Accurate Race Predictions Are

Vickers and Vertosick surveyed 2,303 recreational runners, with a median of 30 miles a week³. A Riegel-style prediction (exponent 1.07) was well calibrated for the 10K and the half marathon³. For the marathon, it was at least 10 minutes too fast for about half of runners³. The middle half of runners finished between 2:48 and 19:47 slower than predicted, and 5% were more than 36 minutes slower³.

A 2026 study tested Daniels' VDOT tables on 7,663 runners who ran both the Valencia half marathon and marathon⁴. Predicting the marathon from the half marathon, the average absolute error was 7.92%⁴. It was 1.11% in the sub-2:30 finish group, 5.96% in the sub-4:00 group and 10.43% in the sub-5:00 group⁴. On a finish near 4 hours, 6% is about 14 minutes (our arithmetic).

Why Marathon Predictions Run Too Fast

A shorter race does not test what the marathon tests. Weekly mileage is one example. In the same survey, running 50 miles a week instead of 30 meant a marathon 25:32 faster for men and 31:41 faster for women³. This calculator has no mileage input, so it cannot account for that.

Weather matters too. In seven large marathons, the fastest male finishers ran 1.7% slower than the course record in the coolest weather band and 4.5% slower in the warmest, and slower runners lost more time⁵. If your long runs are short or race day is warm, plan for a time slower than the prediction.