Race Time Predictor
Enter one recent race and get predicted times at every distance from the mile to 50K, using the Riegel formula and the Daniels VDOT model side by side.
Your VDOT
40.0
Daniels fitness score from this race
Race pace
8:03 /mi
5:00 /km
| Distance | Riegel | VDOT | Pace /mi (VDOT) |
|---|---|---|---|
| 1 mile | 7:13 | 7:07 | 7:07 |
| 5K | 23:59 | 24:06 | 7:45 |
| 10K(your race) | 50:00 | 50:00 | 8:03 |
| 15K | 1:16:51 | 1:17:01 | 8:16 |
| 10 miles | 1:22:48 | 1:23:02 | 8:18 |
| Half marathon | 1:50:19 | 1:50:52 | 8:27 |
| Marathon | 3:50:01 | 3:49:34 | 8:45 |
| 50K | 4:35:21 | 4:33:12 | 8:48 |
Both models assume you are equally trained for the target distance. Predicting a marathon from a 5K usually flatters you; predicting a 5K from a marathon usually undersells you.
Riegel vs VDOT for common race results
Marathon predictions from each model. The gap between them is a fair estimate of the uncertainty in any prediction from that distance.
| Your race | VDOT | 10K (Riegel / VDOT) | Half (Riegel / VDOT) | Marathon (Riegel / VDOT) |
|---|---|---|---|---|
| 5K 20:00 | 49.8 | 41:42 / 41:28 | 1:32:00 / 1:31:50 | 3:11:49 / 3:11:17 |
| 5K 25:00 | 38.3 | 52:07 / 51:53 | 1:55:00 / 1:55:04 | 3:59:47 / 3:57:55 |
| 10K 45:00 | 45.3 | 45:00 / 45:00 | 1:39:17 / 1:39:44 | 3:27:01 / 3:27:15 |
| 10K 55:00 | 35.8 | 55:00 / 55:00 | 2:01:21 / 2:01:56 | 4:13:01 / 4:11:36 |
| Half 1:45:00 | 42.6 | 47:35 / 47:22 | 1:45:00 / 1:45:00 | 3:38:55 / 3:37:50 |
| Half 2:00:00 | 36.5 | 54:23 / 54:07 | 2:00:00 / 2:00:00 | 4:10:12 / 4:07:45 |
Worked example: 10K in 45:00
- Riegel to the marathon: 42.195 ÷ 10 = 4.2195; 4.2195^1.06 = 4.600; × 45:00 = 3:27:01.
- VDOT: 10,000 m in 45 min is 222 m/min. Oxygen cost at that speed is −4.6 + 0.182258 × 222 + 0.000104 × 222² = 41.0 ml/kg/min. A 45-minute effort can be held at 90.7% of VO2max, so VDOT = 45.3.
- VDOT to the marathon: find the time at which running 42,195 m gives that same VDOT -- 3:27:15.
The two disagree by a couple of minutes here. For an experienced marathoner on good mileage the VDOT figure is usually the better guess; for a first marathon, Riegel plus a few percent.
What makes predictions miss
- Mileage. The models describe runners training 40+ miles a week. Below that, endurance drops off faster than the exponent assumes, and long races come in slower than predicted.
- Course and weather. The input race was on some course in some weather; the target will be on another. Hills and heat can be worth 3-5% each.
- Distance specialisation. Fast-twitch runners outperform predictions going down in distance; endurance types outperform going up. If your 5K predicts a slower marathon than you have actually run, believe the marathon.
- Stale results. A race from more than 6-8 weeks ago describes a different runner.
Frequently Asked Questions
How does the Riegel formula work?
T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 encodes that pace slows about 4% every time the distance doubles -- so a runner who does 10K in 50:00 is predicted at 1:50:16 for a half (2.11× the distance, 2.20× the time). Pete Riegel published it in 1981 from a fit to world-class times across many endurance sports.
Why does the VDOT prediction differ from Riegel?
Riegel is one exponent for everyone; VDOT is a physiological model with a curve for how large a fraction of VO2max can be held for a given duration. They agree closely from 5K to half marathon and diverge at the marathon, where VDOT is usually a few minutes more optimistic for well-trained runners and Riegel is closer for everyone else.
Which race is the best predictor?
The one closest in distance to the target. A half marathon predicts a marathon far better than a 5K does, because it tests the same aerobic and fuelling limits. Predicting up more than about 4× in distance is a guess dressed as a number.
Why is my actual marathon slower than every predictor?
Because both formulas assume you are as well trained for 26.2 as for the race you entered, and most people are not. Under-mileage runners typically miss the prediction by 5-10%. A conservative rule for a first marathon: take the Riegel prediction and add 5%.
These calculators use published exercise-science formulas and are for training guidance. They are estimates, not medical advice: if you have a heart condition or are starting exercise after a long break, talk to a doctor before training by heart rate or pace targets.