Understanding your predicted times
The table shows Riegel projections from two example race results. Predictions spanning a small distance gap (10K to half marathon) are generally more reliable than large jumps (5K to marathon).
| From race | Predicted 5K | Predicted 10K | Predicted half | Predicted marathon |
|---|---|---|---|---|
| 10K in 50:00 | 23:59 | — | 1:50:19 | 3:50:01 |
| 10K in 60:00 | 28:47 | — | 2:12:23 | 4:36:01 |
| Half marathon in 2:00:00 | 26:05 | 54:23 | — | 4:10:12 |
| 5K in 25:00 | — | 52:07 | 1:55:00 | 3:59:47 |
- The formula assumes equivalent training for both distances. A 5K result reflects speed; a marathon additionally demands endurance built through sustained training volume — predictions up in distance are optimistic without it.
- Vickers & Vertosick (2016) found the 1.06 exponent underestimates marathon times for most recreational runners; many coaches treat Riegel marathon predictions as a best-case scenario.
- Course profile, weather, altitude and race-day execution can each shift real times by minutes at the marathon distance.
- Predictions from a recent race are more meaningful than from an old personal best, since they reflect current fitness.
- No formula guarantees a race outcome; individual variation between runners of identical shorter-distance ability is large.
What is a race time predictor?
A race time predictor estimates the finish time a runner could achieve at one distance based on a performance at another. The underlying observation, formalized by engineer Peter Riegel in a 1981 American Scientist article, is that average speed declines predictably as race distance increases: plotting record times against distance on logarithmic axes yields a nearly straight line with a slope of about 1.06 across a wide range of running distances.
The exponent 1.06 means that doubling the race distance multiplies the time by about 2.085 rather than exactly 2 — the extra 4–5% reflects the slower sustainable pace at longer distances. Riegel found this 'fatigue factor' fit men's and women's running records well for durations from roughly 3.5 minutes to 230 minutes.
Predictions are honest only within the formula's assumptions. The formula assumes the runner is equally trained and prepared for both distances. In practice, a fast 5K does not guarantee the endurance base a marathon requires: research on recreational runners (Vickers & Vertosick, 2016) found that the standard Riegel exponent tends to underestimate marathon times for typical non-elite runners, often by several minutes or more, with predictions improving when training volume is taken into account.
How to use this marathon time predictor
- Select the distance of a recent race — 5 km, 10 km, half marathon or marathon. A recent, maximal-effort race gives the most meaningful input.
- Enter your finish time as minutes:seconds (for example 50:00) or hours:minutes:seconds (for example 1:45:30).
- Read the predicted times for the other three distances — the calculator shows predictions only for distances different from the one you entered.
- Treat marathon predictions from short races as optimistic scenarios that assume full marathon-specific training.
The formula behind the prediction
The Riegel formula scales a known time T1 at distance D1 to a predicted time T2 at distance D2 using a power law with exponent 1.06. The exponent encodes how much average pace slows as distance grows.
Worked example: a 50:00 10K predicts a marathon of 50:00 × (42.195/10)^1.06 ≈ 3:50:01, a half marathon of about 1:50:19, and a 5K of about 23:59. Note the marathon prediction is more than four times the 10K time even though the distance ratio is 4.22 — the exponent adds the endurance penalty.
The formula is symmetric: it predicts down in distance as well as up. Downward predictions (marathon time to 5K) carry the mirror-image caveat — they assume the speed that shorter racing requires, which high-mileage marathoners do not always retain.
Common mistakes
- Predicting a marathon from a 5K and treating the result as a promise — the jump assumes marathon-specific endurance the 5K cannot demonstrate.
- Using an old personal best instead of a recent race, so the prediction reflects past rather than current fitness.
- Entering a training-run time rather than a maximal race effort, which skews all predictions slow.
- Ignoring conditions: predictions transfer poorly between a flat, cool race and a hilly or hot one.
- Setting a race pace exactly at the predicted time with no margin, leaving no room for the formula's known optimism at longer distances.
Questions fréquentes
How accurate is the Riegel formula for marathon prediction?
Riegel's 1.06 exponent fits record-level performances well across distances, but a 2016 study of recreational runners by Vickers and Vertosick found it tends to underestimate real marathon times for typical non-elite runners — the shortfall grows when weekly training volume is low. Predictions from a half marathon are generally closer than predictions from a 5K or 10K.
What marathon time does a 50-minute 10K predict?
Using the Riegel formula, 50:00 × (42.195 ÷ 10)^1.06 ≈ 3:50:01. This assumes marathon-appropriate endurance training; for many recreational runners the realistic time is slower, which is why coaches often treat Riegel marathon projections as a best-case scenario.
Why is the exponent 1.06?
Peter Riegel plotted record times against distance on logarithmic axes and found running performances fall on a nearly straight line with slope of about 1.06 for durations between roughly 3.5 and 230 minutes. The exponent captures the empirical rate at which sustainable pace declines as distance increases; an exponent of exactly 1.0 would mean pace never slows with distance.
Which race distance gives the best marathon prediction?
The half marathon, in general. It is close enough to the marathon that the pace-endurance relationship transfers reasonably well, while a 5K mostly measures speed. The Vickers & Vertosick analysis of recreational runners likewise found predictions improve as the input race gets closer to the target distance and when training volume is accounted for.
Can the predictor work in reverse, from marathon to 5K?
Mathematically yes — the formula scales in both directions, and this calculator predicts all distances other than the one entered. A 4:00:00 marathon corresponds to about a 25:01 5K under the formula. The reverse caveat applies: high-mileage marathon training does not always preserve the top-end speed a 5K demands.
Why doesn't the calculator show a prediction for the distance I entered?
The entered race is your actual result, not a prediction, so the calculator displays only the other three distances. Enter a 10K time and you receive predicted 5K, half-marathon and marathon times.
Références
- Riegel PS. Athletic records and human endurance. American Scientist 1981; 69(3): 285–290.
- Vickers AJ, Vertosick EA. An empirical study of race times in recreational endurance runners. BMC Sports Science, Medicine and Rehabilitation 2016; 8: 26.
- Tucker R, Lambert MI, Noakes TD. An analysis of pacing strategies during men's world-record performances in track athletics. International Journal of Sports Physiology and Performance 2006; 1(3): 233–245.
- World Athletics. Competition and technical rules — certified road-race distances (half marathon 21.0975 km, marathon 42.195 km).
- American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription, 11th edition. Wolters Kluwer, 2021.