Race Time Predictor
One race, four distances.
Race Time Predictor
Use one race result to estimate an equivalent time at another distance with the Riegel formula.
Set the target to
The prediction assumes endurance pacing and roughly equivalent training. It is least reliable when the target is more than twice the known distance, or for runners whose speed and endurance are unusually skewed.
1:50:19
5.23 min/km · 21.1 km
| Race | Time | Pace |
|---|---|---|
| 5K | 23:59 | 4.8 /km |
| 10K | 50:00 | 5 /km |
| Half marathon | 1:50:19 | 5.23 /km |
| Marathon | 3:50:01 | 5.45 /km |
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What This Calculator Does
The Race Time Predictor uses a recent race result to estimate what you could run at another distance. It applies Riegel’s endurance formula, then shows the equivalent times for 5K, 10K, half marathon, and marathon so you can see the whole progression rather than a single number.
How It Works
The formula scales your time by the ratio of the distances raised to an endurance exponent: T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent above one reflects the fact that longer races are run at a slower average pace, because fatigue accumulates. The predictor also converts the result into a pace per kilometre.
Example
A 50-minute 10K is 3,000 seconds. For a half marathon, the distance ratio is 2.10975, and 2.10975^1.06 is about 2.207, giving roughly 6,620 seconds — 1:50:20, or 5:14 per kilometre. The same effort predicts about 3:49 for a marathon.
Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06pace = T₂ ÷ D₂1.06An empirical endurance constant. It fits road races reasonably well and drifts for very short or ultra distances.
Reading the Results
Predicted finish
The time the model expects at the target distance if your training and endurance are broadly typical for the result you entered. It is an estimate, not a guarantee.
Pace
The average pace the prediction implies. Compare it with your known race pace to see how much slowdown the model expects over the longer distance.
Sources and References
Riegel, P. S. — Athletic Records and Human Endurance (American Scientist, 1981)
The original endurance scaling relationship
The source of the distance-to-time power law used here.
World Athletics — Race distance definitions
Official road race distances
Confirms the 21.0975 km half marathon and 42.195 km marathon distances.
American College of Sports Medicine — Guidelines for Exercise Testing and Prescription
Endurance performance and pacing context
Background on why average pace falls as distance rises.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How accurate is the Riegel formula?
It is usually within a few minutes for road races between 5K and the marathon, but it is an average model. Runners with strong endurance tend to beat it at long distances, while speed-oriented runners tend to fall short.
Why does the prediction slow down as distance increases?
Because the exponent is greater than one, the time grows slightly faster than the distance. That captures the fatigue, fuelling limits, and pacing compromises that make a marathon pace slower than a 10K pace.
Can I use a training run instead of a race?
You can, but a race is more reliable because it is a maximal effort with tapering, competition, and accurate measurement. A training run usually understates your capability unless it was a time trial.
Should I trust the marathon prediction from a 5K?
Treat it cautiously. Extrapolating more than about four times the known distance assumes endurance you may not have trained. Use a half marathon or a long tempo run as the input for a marathon prediction.