Race Time Predictor
Predict your race time at any distance.
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Just raced a 10K and wondering what that means for your half marathon? This tool applies the well-known Riegel formula to predict your finishing time at a longer or shorter distance, along with the pace you'd need to hold.
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Race Time Prediction — The Quick Answer
The Riegel formula estimates a new time from an old one: T₂ = T₁ × (D₂ ÷ D₁)^1.06. The 1.06 exponent reflects that you slow down as distance increases.
Shortcut
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Race distances used
| Race | Distance (km) |
|---|---|
| 5K | 5.00 |
| 10K | 10.00 |
| 15K | 15.00 |
| Half marathon | 21.10 |
| Marathon | 42.20 |
The 1.06 exponent comes from Peter Riegel's 1977 analysis of world running records. It's an approximation — your own endurance profile may differ, especially between very short and very long races.
How It Works
The Formula
Riegel's endurance equation
Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Variables
Known time and distance
A recent race result you trust, ideally run at a hard but sustained effort.
Predicted time and distance
The race you're planning for. The formula scales your time to the new distance.
Riegel exponent
The fatigue factor. A higher exponent predicts a bigger slowdown over longer distances; 1.06 is the standard value.
Note: Predictions are most reliable between distances that are within roughly 2–4× of each other. Going from a mile to a marathon stretches the model well beyond its comfort zone.
Worked Example: Predicting a Half Marathon from a 50-Minute 10K
Applying the Riegel formula
Take your recent race
10K (D₁ = 10) in 50:00 (T₁)
Divide target distance by known distance
21.1 ÷ 10 = 2.11
Raise to the power 1.06
2.11^1.06 ≈ 2.21
Multiply by your known time
50:00 × 2.21 ≈ 1:50
Reference Guide
| Target race | Predicted from a 50:00 10K |
|---|---|
| 5K | ~24:00 |
| 10K | 50:00 (known) |
| Half marathon | ~1:50 |
| Marathon | ~3:50 |
How Much to Trust the Prediction
Where Riegel is reliable — and where it isn't
When the two distances are within a few times of each other and you're trained for both, predictions are often within a couple of percent — e.g. 5K→10K or 10K→half. That's remarkably good for one tidy equation.
Best for: Predicting a nearby race distance
A 10K is only about a quarter of a marathon, so the formula extrapolates a long way and quietly assumes you've done the long-run training a marathon demands. A half marathon is a far better base for a marathon prediction.
Best for: Treating long extrapolations as rough ballparks
The standard 1.06 suits most runners. Those with less endurance tend to fade more over distance (a slightly higher exponent fits), while highly trained endurance athletes may fit a lower one. Adjust it if your own race history suggests a different fade. Not medical advice.
Best for: Fitting the model to your own results
What Is the Riegel Race Prediction Formula?
The Riegel formula predicts your finishing time at one distance from a known time at another: T₂ = T₁ × (D₂ ÷ D₁)^1.06. Published by Peter Riegel in the late 1970s from an analysis of running records, the 1.06 exponent — slightly above 1 — captures the fact that you run slower as the distance grows, so doubling the distance more than doubles the time. It's the most widely used race-equivalency method, and it works best between distances that aren't too far apart and when you're genuinely trained for both. This tool applies it across the standard race distances and lets you adjust the exponent.
Key Features
💡 Pro Tips
- →Base your prediction on the longest recent race you've run hard — it reflects your endurance better than a short time trial.
- →For a marathon goal, predict from a half marathon rather than a 10K; the shorter the base, the more uncertain the extrapolation.
- →If you consistently fade more than predicted over distance, nudge the exponent up slightly to match your history.
- →Treat the prediction as your potential given appropriate training — it can't conjure endurance you haven't built.
Common Mistakes
Predicting a marathon from a short race
A mile or 5K says little about your ability to fuel and endure for hours. The formula will flatter you, because it assumes marathon-appropriate training you may not have done.
Expecting the prediction to be a guarantee
It estimates potential under good conditions and proper training. Weather, course, fuelling, and your training all move the real result, so treat it as an informed estimate.
Ignoring that the exponent can be tuned
1.06 is an average. Using it blindly can misjudge runners who fade much more or less over distance; adjusting it to your own results improves accuracy.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
Riegel, P. S. (1981). Athletic Records and Human Endurance. American Scientist, 69(3).
The original basis of the Riegel race-prediction formula.
View source - [2]
Daniels, J. (2013). Daniels' Running Formula (3rd ed.) — race-equivalency tables. Human Kinetics.
Alternative race-equivalency approach for comparison.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 7, 2026
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Frequently Asked Questions
How accurate is the Riegel formula?
It's a good rule of thumb for distances within a few times of each other — most runners find it lands within a few percent for nearby distances. It's less reliable across very large gaps, and it assumes you're equally well trained for both races.
What distance should I base my prediction on?
Use the longest race you've run recently at a strong, sustained effort — ideally within a couple of months. That reflects your current fitness and endurance better than an older or much shorter result.
How do I predict my marathon time from a 10K?
Enter your 10K time and select marathon as the target. The formula scales it with the 1.06 exponent. Just remember that a 10K is a small fraction of a marathon, so the prediction carries more uncertainty than one from a half marathon.
Can I change the exponent?
Yes. The standard is 1.06, but some coaches use slightly higher values for beginners (who slow more over distance) and lower values for highly trained endurance runners. Adjust it if it fits your own results better.