Marathon Time Predictor
Predict your marathon or half-marathon finish time from a recent race result using the Riegel formula.
Did this tool work for you?
How to use this calculator
The Riegel formula predicts finish time (T2) for a target distance (D2) from a known time (T1) over a known distance (D1). The exponent 1.06 accounts for the physiological fatigue that accumulates at longer distances.
- 1
Select the distance of a recent race you have results for.
- 2
Enter your finish time for that race in total minutes.
- 3
Select the target distance you want to predict (half or full marathon).
- 4
The Riegel formula predicts your finish time and pace at the longer distance.
Frequently asked questions
What formula does the predictor use?
T2 = T1 × (D2 / D1)^1.06, the Riegel formula. T1 is your known time in seconds, D1 the known distance and D2 the target distance. The distances used are 5 km, 10 km, 21.0975 km and 42.195 km. Pace per km and per mile are the predicted time divided by the target distance, with miles at 1.60934 km.
How do I enter my known time?
As total minutes in a single box. A 45:30 10K is 45.5, a 1:40:00 half is 100. Decimals are allowed, but hours and seconds have no separate fields. A time of 0, which is also what an emptied box becomes, or a negative time returns a dash.
Which distances can I predict between?
Known distance is 5K, 10K or half marathon, and the target is half marathon or marathon. Nothing stops you picking a known half marathon and a half marathon target, in which case you simply get your time back.
Does it know how much I train?
No. The formula takes a time and a distance and nothing else, so weekly mileage, long runs, age, weather and course are not inputs. A runner who has not trained for 42 km gets the same prediction as one who has, which is why the number is best read as an estimate from your speed alone, not a forecast of how your training will carry you.
How big is the slowdown from 10K to marathon?
It is baked into the 1.06 exponent. A 10K in 45:00 is 4:30 per km, and the tool predicts a 3:27:01 marathon at 4:54 per km. The distance ratio is 4.2195, and raising it to 1.06 makes the time grow faster than distance, so the per-km pace slows.
Is the prediction reliable for ultras or very short races?
The tool does not offer them. Its distances stop at the marathon, and the exponent is one fixed constant for every runner. Anything beyond that is outside what the code supports, so use other methods for it.
Which recent race should I use?
Use one run at full effort on a fair course, and as close in distance to your target as you have. Predictions from a half marathon are generally seen as more telling for a marathon than a 5K, because the efforts are more alike, but the formula itself has no way to weigh that.
Marathon time predictor — Riegel formula for half and full marathons
What the Riegel formula does
Peter Riegel published the equation in American Scientist in 1981. It multiplies a known time by the ratio of target distance to known distance, raised to the power 1.06. Because 1.06 is above 1, the predicted time grows a little faster than distance, so your pace per kilometre is slower for the longer race. The calculator applies this to one input time and returns finish time, pace per km and pace per mile. It does this in one line of arithmetic, so the same inputs always give the same output, and the only judgement left to you is which recent race to trust.
Predictions from the calculator
Each row was run through the tool and rounded to whole seconds.
| Known race | Target | Predicted time | Pace /km | Pace /mile |
|---|---|---|---|---|
| 5K in 25:00 | Half marathon | 1:55:00 | 5:27 | 8:46 |
| 5K in 25:00 | Marathon | 3:59:47 | 5:41 | 9:09 |
| 10K in 45:00 | Marathon | 3:27:01 | 4:54 | 7:54 |
| 10K in 50:00 | Marathon | 3:50:01 | 5:27 | 8:46 |
| Half in 1:40:00 | Marathon | 3:28:30 | 4:56 | 7:57 |
Reading two of the rows
A 10K in 45:00 averages 4:30 per km. The predictor puts the marathon at 3:27:01, or 4:54 per km, so the race pace is slower than your 10K pace. A half marathon in 1:40:00 is 4:44 per km, and the predicted marathon of 3:28:30 at 4:56 per km is a smaller slowdown, because the half marathon is closer in distance to the target. The shorter the source race, the larger the extrapolation.
Using a prediction to set a marathon goal
The output is a reference for what your current speed implies if your endurance matches your speed work. Pair it with the Running Split Calculator to turn the finish time into a pace you can follow. Many runners pick a goal slightly slower than the prediction when they are unsure about long-run fitness, which is a personal choice the tool does not make for you.
What the number ignores
Training load, fuelling, course profile, temperature, age and experience with the distance are not inputs, and the exponent is the same for everyone. It is general running information, not coaching or medical advice. If you are new to marathons or returning from injury, build the distance gradually and consider a plan from a qualified coach. A prediction is a number on a screen; the race will still be decided by the training behind it.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →