Running
Race Time Predictor
Updated 9 Nov 2026 · Source: Riegel (1981) ↗
Choose the race you ran, enter your finish time, and the predictor estimates your time at the other common distances from the mile to the marathon.
Worked example
- Riegel (1981): T2 = T1 × (D2 ÷ D1)^1.06
- Marathon: 55:00 × (42,195 ÷ 10,000)^1.06 = 4:13:01
Predicted times from a 10K
| 10K | Mile | 5K | Half | Marathon |
|---|---|---|---|---|
| 40:00 | 5:46 | 19:11 | 1:28:15 | 3:04:00 |
| 45:00 | 6:29 | 21:35 | 1:39:17 | 3:27:01 |
| 50:00 | 7:13 | 23:59 | 1:50:19 | 3:50:01 |
| 55:00 | 7:56 | 26:23 | 2:01:21 | 4:13:01 |
| 1:00:00 | 8:39 | 28:47 | 2:12:23 | 4:36:01 |
| 1:05:00 | 9:22 | 31:11 | 2:23:25 | 4:59:01 |
| 1:10:00 | 10:06 | 33:34 | 2:34:27 | 5:22:01 |
| 1:15:00 | 10:49 | 35:58 | 2:45:29 | 5:45:01 |
| 1:20:00 | 11:32 | 38:22 | 2:56:31 | 6:08:01 |
| 1:25:00 | 12:16 | 40:46 | 3:07:33 | 6:31:01 |
| 1:30:00 | 12:59 | 43:10 | 3:18:35 | 6:54:01 |
The formula behind the prediction
The tool uses the time and distance power law from Peter Riegel's work (1977 and 1981): predicted time = known time x (new distance / known distance) raised to 1.06. The exponent 1.06, as commonly used, means you slow down a little as the race gets longer.
With the default 10K in 55:00, the formula gives 26:23 for a 5K, 2:01:21 for a half marathon and 4:13:01 for a marathon. Riegel described this pattern for efforts lasting roughly 3.5 to 230 minutes.
Pick the right race to start from
- Recent: a result from the last two or three months reflects your current fitness.
- All out: a race you ran hard, not a training run or a run with a friend.
- Close in distance: a half marathon predicts a marathon better than a mile does.
- Fair conditions: a hot day or a hilly course makes every prediction slower than your real ability.
Where predictions run too fast
Critics of the 1.06 exponent point out that it can be too optimistic for long races, the marathon most of all. The formula assumes you trained for the longer distance just as well as for the shorter one. A runner with a strong 10K and few long runs will usually fade in the last miles of a marathon.
For a first marathon, treat the prediction as a best case. Starting at a slightly slower pace and speeding up later is safer than chasing the number from mile one.
Using the result to set paces
Once you have a target time, turn it into a pace per mile with the marathon pace chart or the half marathon pace chart. Then plan your splits mile by mile with the split calculator.
Predictions toward shorter races are also handy. If you want to know your likely mile from a 5K, the mile time calculator runs the same formula in that direction.
Reading the table
The table shows predictions from 10K times between 40 and 90 minutes. Each 5 minutes on the 10K adds about 23 minutes to the predicted marathon, which shows how small gains at short distances grow over 26.2 miles.
Related pages
Mile Time Calculator, Treadmill Incline Calculator, Split Calculator, Pace Calculator
Questions
How accurate is the Riegel formula?
It is a fair estimate between distances you have trained for. It tends to be too fast for the marathon when long runs are missing from your training.
Can I use a training run instead of a race?
You can, but the prediction will be slower than your real ability unless the run was a true all-out effort on a measured course.
Why 1.06 and not another number?
It is the value most calculators use, based on Riegel's work. It describes how runners slow down on average, so your own fade over a longer race can be larger or smaller.
How we calculate: every formula, with its author and year, is on our method page; data sources and their licenses are on sources.
gymnical.com gives estimates from published formulas and public data. It is not medical advice and not a training prescription. Last reviewed November 2026.