Marathon Time Predictor Calculator
A recent race time tells you a lot about what you can run over a longer distance, and this calculator turns that into a marathon prediction using the well-known Riegel formula. The method scales a known result up to the marathon by multiplying your known time by the ratio of the two distances raised to the power 1.06. That exponent is the key: if it were exactly 1 you would hold the same pace at any distance, but real runners slow down as the distance grows, and an exponent of about 1.06 captures that decline across a wide range of athletes. Enter a recent race distance and your finish time, choose the marathon as the target, and the calculator estimates your marathon finish. A half marathon makes the best basis because it is close to the marathon in demand, while a 10K is reasonable and a 5K tends to overstate marathon fitness. Treat the result as a realistic target rather than a promise, because marathons expose fueling and endurance limits that shorter races do not. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator defaults so you can set your race plan with confidence.
The Riegel formula scales a race up: T2 = T1 x (D2 / D1)^1.06. A 1:45:00 half marathon predicts a marathon of about 3:38:55 (13,135 seconds).
Riegel marathon formula
T2 = T1 x (D2 / D1) ^ 1.06
T1 = your known race time
D1 = known distance, D2 = marathon (42.195 km)
1.06 = endurance slowdown exponent
The distance ratio raised to 1.06 scales your known time up to the marathon, adding the expected slowdown over the longer distance. An exponent above 1 means pace eases as the race lengthens.
Worked example
Predict a marathon from a 1 hour 45 minute half marathon (21.0975 km).
- Known time = 1:45:00 = 6,300 seconds
- Distance ratio = 42.195 / 21.0975 = 2.0
- 2.0 ^ 1.06 = 2.084932
- Predicted = 6,300 x 2.084932 = 13,135 seconds = 3:38:55
The predicted marathon is about 3:38:55. These are the calculator's default inputs, so the result above matches the widget exactly.
Half marathon to marathon predictions
Riegel predictions from common half marathon times.
| Half marathon | Predicted marathon |
|---|---|
| 1:30:00 | 3:07:39 |
| 1:45:00 | 3:38:55 |
| 2:00:00 | 4:10:12 |
| 2:15:00 | 4:41:28 |
Physical activity and health reference: US Centers for Disease Control and Prevention (CDC).
Marathon time predictor calculator: frequently asked questions
How does the marathon predictor work?
It uses the Riegel formula, which scales a known race time up to a longer distance: the predicted time equals your known time multiplied by the ratio of the two distances raised to the power 1.06. The exponent above 1 reflects that you slow down as distance grows. A 1 hour 45 minute half marathon predicts a marathon of about 3 hours 39 minutes.
Why the exponent 1.06?
Peter Riegel found across many runners that endurance performance declines in a predictable way as distance increases, and an exponent of about 1.06 fits the data well. If the exponent were 1, you would hold the same pace at any distance, which is unrealistic; 1.06 builds in the expected slowdown over longer events.
How accurate is the prediction?
It is a guide, not a guarantee. The formula assumes similar conditions and, crucially, that you have trained appropriately for the longer distance. Marathons expose endurance and fueling limits that a half does not, so an undertrained runner may finish slower than predicted. Use the estimate to set a realistic target, then adjust for your training and race-day conditions.
What race should I base it on?
A recent race at a distance not too far from the marathon gives the best prediction, so a half marathon is ideal, and a 10K is reasonable. Very short races like a 5K can overstate marathon fitness because they lean more on speed than endurance. Use your most recent honest race effort.
What is the Riegel formula?
Predicted time equals the known time multiplied by (target distance divided by known distance) raised to the power 1.06. Distances must use the same units, and times the same units, here seconds.
Official sources
- Physical activity, exercise and health reference: US Centers for Disease Control and Prevention (CDC). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.