Marathon From Half
Calculator
Results
- Predicted marathon time (minutes)
- 208.493152
- Marathon pace (seconds per km)
- 296.470888
- Slowdown versus double the half (%)
- 4.246576
Endurance training results
| Predicted marathon time (minutes) | 208.493152 |
| Marathon pace (seconds per km) | 296.470888 |
| Slowdown versus double the half (%) | 4.246576 |
formula-map diagram
- Predicted marathon time (minutes)
- 208.493152
- Marathon pace (seconds per km)
- 296.470888
- Slowdown versus double the half (%)
- 4.246576
Formula breakdown
Formula
T_marathon = T_half × (42.195 ÷ 21.0975)^1.06= 208.49315216822
Note
Simplified model: these are standard endurance-training formulas (Riegel, ACSM, Banister TRIMP, ACWR, Minetti gradient cost and published fuelling rates) applied to the numbers you enter. They assume even effort, typical conditions and average physiology, and ignore terrain, heat, wind, sleep, individual sweat and carbohydrate tolerance, and training history, so results are indicative planning figures, not medical or coaching advice.
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See all →Frequently asked questions
How does the calculator predict a marathon time from a half marathon?+
It typically applies a scaling factor, commonly around 2.1 to 2.2 times your half marathon time rather than a straight doubling, since fatigue and pacing demands increase disproportionately over the extra distance. This is often based on Riegel's endurance formula, which models how race times scale nonlinearly with distance.
Why isn't the marathon time just double the half marathon time?+
Doubling would ignore the added physiological demand of running twice the distance, including glycogen depletion, muscular fatigue, and pacing discipline, all of which cause most runners to slow down per-mile in the marathon's second half. The extra 13.1 miles are meaningfully harder than the first 13.1.
How accurate is this prediction in practice?+
It's a reasonable starting estimate for runners with solid endurance training, but accuracy drops for anyone who hasn't done adequate long runs or marathon-specific training, since the formula assumes your aerobic endurance scales predictably. Race-day factors like weather, pacing strategy, and fueling can shift the actual result by several minutes.
Does this prediction assume I've trained specifically for the marathon?+
It assumes a baseline of endurance fitness reflected in your half marathon performance, but it doesn't know whether you've built up the long runs and weekly mileage a marathon specifically requires. Without marathon-specific training, most runners underperform this prediction due to hitting the wall late in the race.
Should I use this predicted time as my race-day goal pace?+
It's a useful ballpark for setting a training target, but it's safer to start slightly conservative and negative-split rather than committing fully to the predicted pace from the gun. Many runners who blow up in marathons started at or faster than their predicted pace without accounting for late-race fatigue.