Race Time Predictor
Enter a recent race result and see what it predicts across every distance from a mile to the marathon.
Equivalent times at every distance
| Distance | Predicted time | Pace /mi |
|---|---|---|
| 1 mile | 6:29 | 6:29 |
| 5K | 21:35 | 6:57 |
| 10Kyour race | 45:00 | 7:15 |
| 15K | 1:09:10 | 7:25 |
| 10 miles | 1:14:31 | 7:27 |
| Half marathon | 1:39:17 | 7:34 |
| Marathon | 3:27:01 | 7:54 |
Predictions assume you are equally well trained for both distances. Stepping up to a marathon without the long runs behind you will run slower than the number here.
How the race predictor works
This calculator uses Riegel's formula, the standard method for converting a result at one distance into an equivalent result at another. It multiplies your known time by the ratio between the two distances, raised to a fatigue exponent of 1.06. That exponent is what encodes the obvious truth that you cannot hold your 5K pace for a marathon: as distance grows, pace drops in a predictable way. Enter your recent race, pick a target, and the table shows equivalents at every common distance at once.
Reading the prediction honestly
The formula assumes you are equally well trained for both distances, and that assumption is where predictions go wrong. It is dependable from the mile up to the half marathon. Pushing it out to a marathon off a short race will flatter you, often by several minutes, because the marathon punishes missing endurance in a way a 10K never does. Use the fatigue factor to correct for this: 1.02 if you have a deep base and high mileage, 1.06 as the default, 1.10 if your mileage is modest or you are stepping a long way up in distance.
Once you have a target time, turn it into per-mile numbers with the running pace calculator, compare paces side by side on the running pace chart, and set the intensity of your sessions with the heart rate zone calculator.
A prediction is a snapshot of the fitness you already have. Closing the gap between that and the time you want is the training, and that is what Vaast builds for you.
Frequently asked questions
- How does a race time predictor work?
- This calculator uses the Riegel formula: predicted time equals your known time multiplied by the ratio of the two distances raised to a fatigue exponent, normally 1.06. In plain terms, it works out how much you slow down as the distance grows and applies that to your recent result.
- How accurate is it for the marathon?
- Least accurate there, and it almost always predicts too fast. The formula assumes you are equally well trained for both distances, which is rarely true when you predict a marathon from a 5K or 10K. Without the long runs and weekly volume behind you, expect to run slower than the prediction. Raising the fatigue factor to 1.10 gives a more realistic number for lower-mileage runners.
- Can I predict my half marathon from a 10K?
- Yes, and this is where the formula is at its most reliable. Riegel holds up well between about 1500m and the half marathon, so a recent 10K is a good basis for a half marathon goal. Pick 10K as your recent race, enter your time, and set the prediction to half marathon.
- What is the fatigue factor?
- It is the exponent in the formula, and it controls how much you are assumed to slow down over longer distances. Use 1.06 as the standard. Drop toward 1.02 if you have a deep endurance base and high weekly mileage, and raise it toward 1.10 if your mileage is modest or you are stepping up a long way in distance.
- How do I actually train to hit the predicted time?
- A prediction tells you what your current fitness is worth, not how to improve it. Vaast takes your goal race and date, reads your real training data, and builds a periodized plan with the specific sessions to close the gap, then adapts it as your fitness changes.
Related tools
- Running Pace CalculatorWork out your pace, time, or distance from any two of the three, with splits and mi/km support.
- Heart Rate Zone CalculatorFind your five training zones from your max heart rate, heart rate reserve, or threshold.
- Running Pace ChartEquivalent min/km and min/mile paces, speed, and 5K to marathon finish times in one chart.