Predicting a race from another race
A recent race is the best guide to what you can run at a different distance. The challenge is that pace slows as distance grows: nobody holds their 5K pace for a marathon. In 1977, engineer Peter Riegel noticed that, for many endurance sports, time rises a little faster than distance, by a constant power. His formula is still the most widely used race predictor.
Riegel’s formula
- T2 = T1 × (D2 ÷ D1)^1.06
- T1 and D1 are your recent race time and distance, D2 is the distance you want to predict
- The exponent 1.06 is the fatigue factor: 1.00 would mean you hold the same pace at any distance
- Target pace = predicted time ÷ distance
Worked example
You ran a 10K in 50:00 and want to plan your first marathon.
- Distance ratio: 42.195 ÷ 10 = 4.22.
- 4.22^1.06 = 4.60.
- Predicted time: 50 min × 4.60 = 3 hours 50 minutes.
- Target pace: about 8:46 per mile (5:27 per km).
The same 10K predicts a 5K in about 24:00 and a half marathon in about 1:50:20.
Equivalent performances
With the exponent 1.06:
| 5K | 10K | Half marathon | Marathon |
|---|---|---|---|
| 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 22:30 | 46:55 | 1:43:30 | 3:35:48 |
| 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 27:30 | 57:20 | 2:06:30 | 4:23:45 |
| 30:00 | 1:02:33 | 2:18:00 | 4:47:44 |
Choosing the exponent
The 1.06 value fits runners who are well trained for the target distance. Studies of recreational marathoners suggest that many fade more, with exponents closer to 1.07 or 1.08. If you run fewer than about 30 miles a week, or have never run longer than 15 miles, use a larger exponent: the table in the calculator shows the effect.
Getting a reliable prediction
- Use a recent, all-out race on a flat course, not a training run.
- Predict from the closest distance you have raced: a half marathon predicts a marathon better than a 5K.
- Heat, hills and wind slow you down on race day; adjust your target accordingly.
- Long runs matter. The prediction assumes your endurance matches your speed. For a marathon, that usually means several runs of 16 to 20 miles in the build-up.
- Start a little slower than the predicted pace and speed up in the second half if you feel good.