Race Time Predictor — Riegel Formula
Predict your 5k to marathon times from one recent race, using Riegel's endurance formula — assumptions stated.
- Recovery
- 2 cited sources
- Runs in your browser
Riegel predictions
| Distance | Predicted time |
|---|---|
| 5 km | 23:59 |
| Half marathon | 1:50:19 |
| Marathon | 3:50:01 |
Assumes you’re as trained for the target distance as the known one — marathon predictions in particular are a best case without marathon-specific mileage.
Methodology
Predictions use Peter Riegel's endurance formula, published in American Scientist in 1981:
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Riegel fitted the 1.06 exponent to record performances across running, swimming and other endurance sports, capturing how sustainable pace declines as distance grows. Half a century on, it remains the standard first-order race predictor because it keeps working for trained runners between adjacent distances.
Interpreting your result
Read predictions as "what your engine could deliver, appropriately trained for that distance". From a fresh 10k, the half-marathon prediction is usually realistic if you're running the weekly mileage a half demands; the marathon prediction assumes marathon training you may not have done. Empirical studies of recreational runners (Vickers & Vertosick, 2016) found actual marathon times average several minutes slower than Riegel predicts, for exactly this reason.
The best use is target-setting: predict from a recent race, train specifically for the target distance, and treat the prediction as your optimistic goal pace.
Limitations
- Accuracy degrades with distance gaps — predicting a marathon from a 5k compounds every assumption.
- The formula knows nothing about heat, hills, fuelling or race-day execution, which dominate marathon outcomes.
- Both directions matter: predicting downwards (marathon → 5k) assumes speed work you may equally not have done.
Sources
Pull the receipts — 2 cited sources
Frequently asked questions
- How accurate are race time predictions?
- Good within neighbouring distances (10k from a 5k, half from a 10k) for runners with appropriate training. Marathon predictions from short races are systematically optimistic unless you have genuine endurance volume behind you.
- What is the Riegel formula?
- T₂ = T₁ × (D₂ ÷ D₁)^1.06 — published by Peter Riegel in 1981 from analysis of record performances across endurance sports. The 1.06 exponent captures how pace naturally slows as distance doubles.
- Why is my marathon prediction too fast?
- The formula assumes you're as trained for the target distance as the known one. Without marathon-specific mileage, glycogen limits and fatigue bite harder than the exponent expects — treat marathon predictions as a best case.
- Which race should I base predictions on?
- Your most recent all-out effort, ideally within the last couple of months and at a distance adjacent to the target. A fresh 10k predicts a half marathon far better than a year-old 5k.
Written and reviewed by Mathew Beale — MSc Biotechnology, University of Reading.
Last reviewed:
Related tools
- Running Pace Calculator — Pace, Speed & Splits
Free running pace calculator: min/km and min/mile pace, speed, and kilometre splits for any distance from 400 m to ultra distances.
- Heart Rate Zone Calculator — Tanaka & Karvonen
Free heart rate zone calculator using the Tanaka equation (208 − 0.7 × age) and the Karvonen heart-rate-reserve method, with a full five-zone training table.