Race Time Predictor
Turn a recent race result into a realistic goal for any other distance. This predictor uses Riegel's formula — the same maths behind most race predictors — but with one honest difference: the fatigue factor is yours to adjust, because the standard value assumes you're fully trained for the longer distance, and real life is rarely that tidy.
How sensitive is the prediction?
Small changes to the fatigue factor move the prediction a lot over long distances — which is exactly why one fixed value misleads. Here is your prediction at three common values:
| Fatigue factor | Runner profile | Predicted time |
|---|
How Riegel's formula works
In 1981 the engineer and runner Peter Riegel published a deceptively simple observation: race times across distances follow a power law.
T₂ = T₁ × (D₂ ÷ D₁)b
Your predicted time T₂ equals your known time T₁ multiplied by the ratio of the distances raised to a fatigue exponent b. With b = 1 you'd hold the same pace forever; every notch above 1 encodes how much you slow as distance grows. Riegel measured b ≈ 1.06 across trained club runners, and that value is baked into most predictors you'll find online.
What the fatigue factor really means
- 1.00–1.04 — high-mileage endurance runners. Big weekly volume, long runs done religiously. Elite marathoners sit near the bottom of this band.
- 1.05–1.07 — typical trained club runner. Riegel's classic 1.06 lives here. A reasonable default if your training genuinely matches the target distance.
- 1.08–1.15 — speed-biased or lower mileage. If your long run tops out at an hour, or you're predicting a marathon from a 10K, be honest and slide right. The prediction gets slower — and much more likely to be true.
When the prediction breaks down
Riegel's formula assumes you are equally well prepared for both distances and racing in similar conditions. That assumption fails more often than it holds. A real example, from the runner who built this site: in 2015 I ran the Leeds Half Marathon in 2:24. At the standard b = 1.06, that predicts a marathon of almost exactly five hours — 5:00:14 on the tool above. My actual marathons: 5:20 at York 2017 (an effective b of about 1.15) and 5:47:44 at London 2018 — run in the hottest London Marathon on record at the time — an effective b of about 1.27. The formula wasn't wrong; my preparation and a 24 °C day simply weren't what it assumes.
- Heat and wind — hot days alone can add 20–30 minutes to a 4–6 hour marathon.
- Hills — compare course profiles, not just distances.
- Distance jumps beyond ~2× — predicting a marathon from a 5K stacks assumptions on assumptions.
- Missing long runs — the formula can't know your longest run was 90 minutes.
Use the slider honestly, treat the result as a planning number, then print the pace band and let even splits do the rest.
Frequently asked questions
How accurate is the Riegel predictor?
Good when you're equally trained for both distances and conditions match; least reliable when stepping up to a distance you haven't trained for. Most first-time marathoners run slower than their half-marathon time predicts.
What fatigue factor should I use?
1.06 suits most trained runners. High-mileage endurance runners: 1.03–1.05. Low mileage or speed-biased: 1.08–1.15.
Why was my marathon slower than predicted?
Usually heat, hills, short long-runs, or a too-fast first half. The formula assumes equally good preparation and conditions for both races.