Race time predictor
Put in one recent race and get equivalent times for every classic distance, using Peter Riegel’s endurance formula.
Exponent k = 1.06 (adjust it in the explainer below).
| Distance | Predicted | Pace /km | Pace /mi |
|---|---|---|---|
| 1 mile | 7:13 | 4:29 | 7:13 |
| 5K | 23:59 | 4:48 | 7:43 |
| 10K | 50:00 | 5:00 | 8:03 |
| Half marathon | 1:50:19 | 5:14 | 8:25 |
| Marathon | 3:50:01 | 5:27 | 8:46 |
Why pace fades with distance
If you could hold one pace forever, time would rise in a straight line with distance (k = 1.00, the dashed line). Real runners slow down as the distance grows, so the curve bends upward. Drag the exponent and watch the marathon estimate swing.
With k = 1.06, doubling the distance multiplies your time by 2.085, so your pace slows by 4.2% each time the distance doubles.
How the prediction works
Riegel noticed that across running, swimming and cycling, time and distance follow a power law: T2 = T1 × (D2 ÷ D1)^k, with k close to 1.06 for trained runners. A 50:00 10K therefore predicts about 1:50:19 for a half marathon and 3:50:01 for a marathon.
The formula assumes you’re equally trained for both distances. That holds well from 5K to half marathon. For the marathon, it assumes the long runs and weekly mileage of an experienced marathoner, which is why many first-timers finish slower than predicted. If you’re new to the distance, set k to 1.08 for a safer target, then turn it into even splits with the pace calculator. The VDOT calculator uses a different, physiology-based model and gives similar numbers plus training paces.
Questions runners ask
How accurate is a race time predictor?
Riegel’s formula is usually within a few percent between neighbouring distances such as 5K to 10K. Accuracy drops for big jumps, especially predicting a marathon from a 5K, because the marathon also depends on endurance training and fuelling. Treat long-range predictions as the best case for a well-trained runner.
What is the Riegel formula?
Peter Riegel, an engineer, published T2 = T1 × (D2 ÷ D1)^1.06 in 1977 (expanded in American Scientist, 1981). It says that doubling the distance makes you take slightly more than twice as long, because pace fades as distance grows.
Why is my marathon slower than predicted?
Most recreational runners run marathons slower than the 1.06 exponent predicts. Low weekly mileage, long runs under 30 km and poor fuelling all push the real exponent toward 1.08–1.10. Try the higher exponent in the explainer below for a more cautious estimate.
Which race should I base the prediction on?
Use a recent, all-out race on a certified course, ideally the distance closest to your target. A 10K predicts a half marathon better than a 5K does.