How do you predict your marathon time?
Convert a recent half marathon with a power-law model, sanity-check it against your weekly volume and long runs, then subtract some optimism.
The single most informative input is a recent half marathon, because it is long enough to test the same aerobic and durability qualities the marathon does. A power-law conversion — Riegel with an exponent around 1.08 for marathon predictions — turns that into a marathon estimate in one step.
A 10 km will do at a push, and a 5 km is nearly useless on its own. The shorter the input race, the more the prediction extrapolates and the more it flatters you: the marathon's limiters are fuelling, glycogen and the ability to hold form on tired legs, and a 5 km measures none of them.
The second check is training, not arithmetic. A prediction that implies a marathon pace you have never sustained for more than 10 km in training, on weekly volume that does not support the distance, is a number rather than a plan. If your long runs have not comfortably reached 2 to 2.5 hours, the prediction is describing a runner you are not yet.
The third adjustment is the course and the day. Elevation, heat and field congestion all cost time, and this site's course pages compute the grade-adjusted split table for your specific race so the prediction lands on the course you are actually running rather than an idealised flat one.
Key numbers
| Best input | recent half marathon | |
|---|---|---|
| Exponent used | ~1.08 | for marathon predictions |
| Sanity check | long runs of 2–2.5 h |
Half marathon to marathon
| Half marathon | Predicted marathon | Pace /km | Pace /mi |
|---|---|---|---|
| 1:20:00 | 2:49:00 | 4:00/km | 6:27/mi |
| 1:30:00 | 3:10:00 | 4:31/km | 7:15/mi |
| 1:40:00 | 3:31:00 | 5:01/km | 8:04/mi |
| 1:45:00 | 3:42:00 | 5:16/km | 8:28/mi |
| 1:50:00 | 3:53:00 | 5:31/km | 8:52/mi |
| 2:00:00 | 4:14:00 | 6:01/km | 9:41/mi |
| 2:10:00 | 4:35:00 | 6:31/km | 10:29/mi |
| 2:20:00 | 4:56:00 | 7:01/km | 11:17/mi |
| 2:30:00 | 5:17:00 | 7:31/km | 12:06/mi |
| 2:45:00 | 5:49:00 | 8:16/km | 13:18/mi |
Riegel with the 1.08 exponent the race predictor uses, rounded to the minute. Marathon pace comes out about 6% slower than half-marathon pace. These assume marathon-ready training; on low weekly mileage, or for a first marathon, expect to finish slower.
Work it out for yourself
Race time predictor — Predict your marathon and see how each physiological factor moves the estimate.
Frequently asked questions
How do I predict my marathon time from my half marathon time?
Multiply the half by about 2.1 — that is what Riegel's formula with a 1.08 exponent works out to, so a 1:45 half predicts roughly 3:42. Then sanity-check it against your long runs and weekly volume: the conversion assumes you have the endurance to hold the pace for twice the distance.
How much slower is marathon pace than half marathon pace?
About 6% by the same formula — roughly 15–30 seconds per kilometre across most of the field. Plenty of recreational runners, especially in a first marathon, slow by more than that, which is why the prediction is best treated as a ceiling.
Can I predict a marathon from a 5 km?
You can compute a number, but it will usually be optimistic by a wide margin. The 5 km tests aerobic power; the marathon tests fuelling, durability and pacing. Use a half marathon where you possibly can.
Why did I run so much slower than my predicted marathon time?
The usual suspects, in order: insufficient weekly volume to support the predicted pace, going out too fast, under-fuelling, and racing a hillier or hotter course than the prediction assumed. All four are addressable, and all four are decided before the start line.
Sources
- Riegel (1981). Athletic records and human endurance. American Scientist.
- Vickers & Vertosick (2016). An empirical study of race times in recreational endurance runners. BMC Sports Science, Medicine and Rehabilitation.
Related explainers
Glossary: Riegel Model, Joyner Model