Race Time Predictor
Predict race times from one resultRuns locally · nothing uploaded
| Distance | Predicted time | Pace /km |
|---|---|---|
| 1500 m | 0:06:59 | 4:39 |
| 1 mile | 0:07:31 | 4:40 |
| 3K | 0:14:33 | 4:51 |
| 5K | 0:25:00 | 5:00 |
| 10K | 0:52:07 | 5:13 |
| 15K | 1:20:07 | 5:20 |
| Half marathon | 1:55:00 | 5:27 |
| 30K | 2:47:01 | 5:34 |
| Marathon | 3:59:47 | 5:41 |
| 50K ultra | 4:47:02 | 5:44 |
Estimates from general formulas and population averages; individual results vary. For training reference only.
How it works
When to use it
- Set a realistic half or full marathon goal from a recent 5K or 10K.
- Compare distances to see whether you lean toward speed or endurance.
Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06Riegel's formula; 1.06 is the fatigue factor.
Worked example
Given: 5K in 25:00, predicting a half marathon
- T₂ = 1500 s × (21.0975 ÷ 5)^1.06 ≈ 6900 s.
- Pace = 6900 ÷ 21.0975 ≈ 327 s per km (5:27/km).
Result: About 1:55:00 for the half; about 3:59:47 for the marathon.
Limitations
- Best for races of about 3.5 minutes to 4 hours.
- Heat, hills and fueling problems make real times slower.
Sources
- Riegel PS. Athletic Records and Human Endurance. American Scientist. 1981;69(3):285–290.
How to use
- 1Choose a distance you've raced and enter your time.
- 2Pick the distance to predict and read the time, pace and speed.
- 3Adjust the fatigue factor if needed: lower for endurance runners, higher for speedsters.
FAQ
How accurate is it?
For races from about 3.5 minutes to 4 hours, Riegel is usually within a few percent. Predictions from a distance closer to your goal—e.g. half to full marathon—are more reliable.
Why was my marathon slower than predicted?
The formula assumes you've trained for the distance. Low mileage, poor fueling or heat often cause a big slowdown late in a marathon.
What does a sub-4 marathon take?
With the default factor, roughly a 52-minute 10K or a 1:55 half marathon; average pace about 5:41 per km (9:09 per mile).