Predict your race times from one recent result
This race time predictor estimates what you could run over another distance from a recent race or hard time trial. Pick the race you ran, type your time, choose the distance to predict, and read the answer along with a full table of equivalent times from 1,500 m to the marathon, each with pace per kilometre and per mile.
It uses the formula published by Peter Riegel, and shows it so you can check the arithmetic. It is a planning estimate, not a promise: it cannot know your training, the course or the weather on the day.
How to use the Race Time Predictor
- Choose the race you ran, or select Custom distance and enter kilometres.
- Type your finishing time as minutes:seconds or hours:minutes:seconds.
- Choose Predict for to set the headline distance. The table below lists every distance at once.
- Leave the exponent at 1.06 unless you want to test another value. Read the warnings under the result.
Race Time Predictor features
Riegel formula in the open
Every number comes from T₂ = T₁ × (D₂ ÷ D₁)^1.06; there is no hidden model.
Editable exponent
Try 1.04 or 1.08 to see how sensitive the prediction is. An exponent of 1 gives your straight pace.
Nine race distances
1,500 m, mile, 3K, 5K, 10K, 15K, 10 miles, half marathon and marathon, plus any custom distance.
Honest warnings
Flags results from very short or very long efforts and predictions far beyond the distance you raced.
How the prediction works
Riegel fitted record times across distances and found that endurance falls off in a predictable way: T₂ = T₁ × (D₂ ÷ D₁)^1.06. Doubling the distance multiplies the time by 2^1.06, about 2.085, so you slow by roughly 4% per doubling. The model was published in American Scientist (1981, vol. 69, issue 3, pages 285–290), after earlier magazine articles.
Worked example: a 5K in 25:00 is 1,500 seconds. A half marathon is 21.0975 ÷ 5 = 4.2195 times as far, and 4.2195^1.06 is about 4.60, so the predicted time is 1,500 × 4.60 = 6,900 seconds, or 1:55:00. The same 5K predicts a 10K of 52:07.
| Recent race | Predicted 10K | Half marathon | Marathon |
|---|---|---|---|
| 5K in 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 5K in 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 5K in 30:00 | 1:02:33 | 2:18:00 | 4:47:44 |
| 10K in 50:00 | 50:00 | 1:50:19 | 3:50:01 |
Limits you should know
- The formula is less accurate beyond the marathon and for efforts under about 3½ minutes, and it is built on the performances of trained runners.
- Untrained or newer runners usually slow more than 1.06 allows when they step up to the half or full marathon, so a marathon predicted from a 5K is often optimistic. Without long runs, add a margin.
- Treat the result as a goal range. Heat, hills, wind, altitude, a hard taper and how you fuel all move real times by minutes.
- It predicts from one result. For a steadier picture, use a recent race rather than a training run, and compare with the VO2 max calculator.
Using the prediction
Set goal pace for race day with the marathon pace chart, turn paces into treadmill settings with the treadmill pace calculator, or solve pace, time and distance with the pace calculator. This tool gives general training information, not coaching or medical advice.
Race Time Predictor FAQ
What is the Riegel formula?
T₂ = T₁ × (D₂ ÷ D₁)^1.06, where T₁ is a time over distance D₁ and T₂ the predicted time over D₂. Peter Riegel published it in American Scientist in 1981.
How accurate is a race time predictor?
Within a few percent for runners who are equally trained for both distances, mostly between 5K and the marathon. Training, course and weather change real results, so treat it as an estimate.
Can I predict a marathon from a 5K?
The tool will calculate it, but 5K to marathon is more than eight times the distance and usually gives a time that is too fast unless your weekly mileage supports it. A 10K or half marathon result is a better base.
Why does the exponent matter?
It sets how fast you slow with distance. A higher exponent means more fade. Riegel’s own fits clustered near 1.06; individual runners differ.
Is this the same as the Garmin race predictor?
No. Watch predictors use your recent training data and heart rate. This tool uses only the one race you enter and the published formula, so the numbers will differ.
Does it work in miles?
Yes. Pick the Mile, 10 miles or half marathon option, and every row shows pace per km and per mile.