Race Time Predictor
Enter one recent race result and this race time predictor uses Riegel's formula to estimate your finish time at any other distance — from a 5K to a full marathon.
How it works
The predictor is built on Pete Riegel's endurance formula, first published in 1977 and still the most widely used model for cross-distance race prediction:
t₂ = t₁ × (d₂ ÷ d₁)1.06
Here t₁ is your known finish time over distance d₁, and t₂ is the predicted time over distance d₂. The exponent 1.06 is a fatigue factor: if it were exactly 1, you would hold the same pace at every distance, but real runners fade slightly as the race gets longer, so the exponent is a touch above 1. The prediction is most reliable when your training and the race-day conditions are similar across both distances.
Worked example
Suppose you ran a 20:00 5K (1,200 seconds). To predict a 10K, scale by the distance ratio raised to 1.06:
1200 × (10 ÷ 5)1.06 ≈ 2,502 s ≈ 41:41
Push the same effort all the way to the marathon (42.195 km) and the formula returns 3:11:49. That marathon figure is a best-case ceiling — it assumes you are as well trained for 42 km as you are for 5 km, which is rarely true.
Predictions from a 20:00 5K
What a 20-minute 5K predicts at every standard road distance via Riegel's formula:
| Distance | Predicted time | Pace (per km) |
|---|---|---|
| 5K | 20:00 | 4:00 |
| 10K | 41:41 | 4:10 |
| Half marathon | 1:32:00 | 4:22 |
| Marathon | 3:11:49 | 4:33 |
Notice how predicted pace slows as distance grows — that fade is exactly what the 1.06 exponent encodes.
Pace and other tools
Once you have a target time, turn it into per-kilometre or per-mile splits with the Pace Calculator, or plan race-day strategy with the Marathon Pace Calculator. To gauge the aerobic fitness behind your times, see the VO2 Max Calculator.
Frequently asked questions
How does a race time predictor work?
- It takes one recent race result — a distance and finish time — and scales it to another distance, assuming your endurance fades at a predictable rate as the distance grows. The standard model is Riegel's formula.
What is Riegel's formula?
- Pete Riegel's formula is t2 = t1 × (d2 ÷ d1)^1.06, where t1 is your known time over distance d1 and t2 is the predicted time over distance d2. The 1.06 exponent is a fatigue factor: pace slows slightly each time the distance increases.
How accurate are race predictions?
- For distances within a few-fold of your input race, Riegel's formula is usually accurate to within a couple of percent — provided conditions and training are similar. Predictions for very different distances drift further from reality.
Why does the marathon prediction feel too fast?
- The 1.06 exponent assumes you are equally trained for both distances. The marathon adds fueling, glycogen depletion and many more hours on your feet, so an under-trained runner will be slower than a 5K-based prediction suggests. Treat it as a best-case ceiling.
Can I predict a 5K from a marathon?
- Yes — the formula runs in either direction. Predicting a short race from a long one tends to be conservative, because most runners have more speed than a marathon time implies if they have done any short-distance work.
Related calculators
Disclaimer. Predictions assume similar course conditions and that you are adequately trained for the target distance; actual results vary with terrain, weather, fueling and fatigue. Not medical advice.