Race predictor
What could you run, from what you just ran?
Your watch predicts races from its VO2max estimate, which the training-load guide explains is only as good as the heart-rate data behind it. This page uses a stopwatch instead: one recent result, Riegel’s formula, and the one assumption it makes shown where you can change it.
A recent race
Distance
Finish time
50:00 for 10 km is 5:00 /km.
Pace per
Predicted finishes
| Distance | Time | Pace |
|---|---|---|
| Mile | 7:13 | 4:29 /km |
| 5 km | 23:59 | 4:48 /km |
| 10 kmyour result | 50:00 | 5:00 /km |
| Half marathon | 1:50:19 | 5:14 /km |
| Marathon | 3:50:01 | 5:27 /km |
T₂ = T₁ × (D₂ ÷ D₁)1.06
Training paces
- Easy and long runs
- 6:32 /km – 7:15 /km
- Marathon pace
- 5:27 /km
- Threshold about an hour’s effort
- 5:03 /km
- Intervals about 3 km race pace
- 4:39 /km
Set these as pace alerts or a workout on the watch and it will hold you to them; the heart-rate guide is the place to start if you would rather train by effort.
How the sum works
- Riegel’s law. Time over a new distance is the old time multiplied by the ratio of distances raised to a power, k. Riegel measured k = 1.06 across a range of runners in 1981 and it has held up since as a rule of thumb, which is all it claims to be.
- The exponent is the assumption. A runner with a big aerobic base slows less over distance than 1.06 predicts; a runner new to long races slows more, especially at the marathon. Hence the three settings. If your predicted marathon looks optimistic, it usually is: the formula knows your speed, not your mileage.
- Threshold pace is what the same formula says you could hold for an hour — the effort most coaching systems put threshold work at. Interval pace is the predicted pace for a 3 km race. Easy pace is 20–33% slower than predicted marathon pace, the band where conversation stays possible.
- Where the watch differs. Its predictor starts from a VO2max estimate that moves with heart-rate quality, heat and how recently you raced. Yours is a stopwatch result. When the two disagree, the stopwatch was there.
Predictions assume a similar course and day: a hilly, hot or windy race will not follow a flat, cool one, in either direction.
Find a watch that holds these pacesWill it last the race? Battery planner