🏃 Race Time Predictor
By ToolNimba Health Team · Reviewed by ToolNimba Editorial Review, endurance training content · Updated 2026-06-19
This predictor gives a mathematical estimate, not a guarantee. Your real race time depends on training, the course profile, weather, fueling, pacing and how rested you are on the day. The Riegel model assumes you are equally trained for the new distance, which is rarely exactly true. Treat the result as a planning guide, and consult a coach or your doctor before starting hard training or racing a much longer distance.
| Distance | Predicted time | Pace / km |
|---|
This race time predictor estimates how fast you could run a different distance based on a result you already have. Enter a recent race or time trial, choose the distance you want to predict, and it applies the Riegel formula to give you a target finish time plus your pace per kilometre and per mile. It is a quick way to set a realistic goal, plan even pacing, and see how a 5K effort translates up to a 10K, half marathon or full marathon.
What is the Race Time Predictor?
The predictor uses Pete Riegel's endurance formula, published in 1977 and still the most widely used way to project race times. The rule is T2 = T1 x (D2 / D1)^1.06, where T1 is your known time over distance D1 and T2 is the predicted time over the new distance D2. The exponent 1.06 (called the fatigue factor) captures a simple truth of distance running: you cannot hold the same pace forever, so each time you roughly double the distance you slow down by a predictable amount.
The reason the exponent is greater than 1 is fatigue. If a runner could hold one pace at any distance the exponent would be exactly 1, and a 25-minute 5K would scale to a 50-minute 10K. In reality the same runner finishes a touch slower per kilometre as the distance grows, so 1.06 nudges the prediction up. The factor was fitted to real race data and works best for trained runners over distances from about 1500 metres up to the marathon.
Prediction quality depends on how far you extrapolate and how specifically you have trained. Predicting a 10K from a 5K is usually close, because the distances and the energy systems are similar. Predicting a marathon from a 5K is far less reliable: the marathon is limited by endurance, fueling and the wall, none of which a fast 5K measures. As a rule of thumb, the closer your known distance is to your target distance, and the more long-run training you have done, the more you can trust the number.
When to use it
- Setting a realistic goal time for an upcoming 10K, half marathon or marathon from a recent shorter race.
- Working out an even target pace per km or per mile to follow on race day.
- Checking whether a goal time is sensible given your current fitness, instead of guessing.
- Comparing two recent results (say a 5K and a 10K) to see which suggests stronger endurance.
How to use the Race Time Predictor
- Choose the distance of a recent race or time trial you trust.
- Enter that finish time in hours, minutes and seconds.
- Pick the target distance you want to predict.
- Read the predicted finish time, plus your pace per kilometre and per mile, and the full distance table below.
Formula & method
Worked examples
You ran a 5K in 25:00 and want your predicted 10K time.
- Convert the known time to seconds: 25:00 = 1500 s
- Distance ratio D2 / D1 = 10 / 5 = 2
- Raise to the fatigue power: 2^1.06 = 2.0849
- T2 = 1500 x 2.0849 = 3127 s
- Convert back to time: 3127 s = 52:07
- Pace per km = 3127 / 10 = 312.7 s = 5:13 /km
Result: Predicted 10K time about 52:07, roughly 5:13 per km
You ran a 5K in 25:00 and want your predicted marathon time (42.195 km).
- Known time 25:00 = 1500 s
- Distance ratio = 42.195 / 5 = 8.439
- Raise to the fatigue power: 8.439^1.06 = 9.591
- T2 = 1500 x 9.591 = 14387 s
- Convert back: 14387 s = 3:59:47
- Note: predicting a marathon from a 5K usually reads optimistic unless you have marathon-specific endurance.
Result: Predicted marathon about 3:59:47, but treat long-range jumps with caution
Predicted times from a 25:00 5K using the Riegel formula
| Target distance | Predicted time | Pace per km |
|---|---|---|
| 5K | 25:00 | 5:00 |
| 10K | 52:07 | 5:13 |
| Half marathon (21.0975 km) | 1:55:00 | 5:27 |
| Marathon (42.195 km) | 3:59:47 | 5:41 |
Common race distances in kilometres and miles
| Race | Kilometres | Miles |
|---|---|---|
| 1 mile | 1.60934 | 1.000 |
| 5K | 5.000 | 3.107 |
| 10K | 10.000 | 6.214 |
| Half marathon | 21.0975 | 13.109 |
| Marathon | 42.195 | 26.219 |
Common mistakes to avoid
- Predicting a marathon from a single short race. The Riegel model assumes equal training for the new distance. A fast 5K does not prove marathon endurance, so a marathon predicted from a 5K is usually optimistic. Use a recent long race or a half marathon as your input when targeting the full distance.
- Using an all-out sprint as the known result. The formula is fitted to endurance distances from roughly 1500 metres upward. Feeding it a 400 metre or 800 metre time produces unrealistic long-distance predictions, because short sprints rely on different energy systems.
- Ignoring course, weather and conditions. A hilly course, heat, humidity or a strong headwind can add minutes the formula never sees. Treat the prediction as a flat, fair-weather best case and adjust your goal for the actual conditions.
- Treating the prediction as a training plan. The number tells you what your current fitness suggests, not how to get there. Hitting a longer-distance prediction still requires the appropriate long runs, fueling practice and pacing discipline.
Glossary
- Riegel formula
- An endurance prediction rule, T2 = T1 x (D2 / D1)^1.06, that scales a known race time to a new distance.
- Fatigue factor
- The exponent 1.06 in the formula, which reflects how runners slow slightly as distance increases.
- Pace
- Time taken to cover one unit of distance, here shown as minutes and seconds per kilometre or per mile.
- Time trial
- A solo, maximal-effort run over a set distance, used as a fitness benchmark when you have no recent race.
- Extrapolation
- Estimating beyond your known data, for example predicting a marathon from a 5K. The further you extrapolate, the less reliable the result.
Frequently asked questions
How does the race time predictor work?
It applies the Riegel formula, T2 = T1 x (D2 / D1)^1.06. It takes your known time over one distance, scales it by the ratio of the new distance to the old one raised to the power 1.06, and returns the predicted finish time and pace. The exponent accounts for the fact that you slow slightly as distance grows.
How accurate is the Riegel formula?
It is usually accurate to within a few percent for trained runners predicting nearby distances, such as a 10K from a 5K. Accuracy falls as the gap widens. Predicting a marathon from a 5K often reads several minutes too fast because the marathon is limited by endurance and fueling that a short race does not test.
What is the 1.06 exponent?
It is the fatigue factor. If you could hold one pace at any distance the exponent would be 1.0. Because runners slow a little as distance increases, Riegel fitted the value 1.06 to real race data so the prediction rises realistically with distance.
Which known race should I use for a marathon prediction?
Use the longest recent race you have, ideally a half marathon or a long tempo effort. The closer your input distance is to the marathon, and the more long-run training behind it, the more trustworthy the prediction. A 5K input will tend to overestimate marathon fitness.
Does the predictor account for hills or weather?
No. The formula assumes comparable, flat and fair conditions. Hills, heat, humidity, wind or altitude can all slow you down, so add a margin to the predicted time when race-day conditions are tougher than your benchmark.
Can I use it for short sprints like a 400 metre?
It is not designed for sprints. The model is fitted to endurance distances from about 1500 metres up to the marathon. Sprints rely on anaerobic energy systems that scale differently, so a sprint input gives unrealistic distance predictions.
Sources
- Time Predicting (Riegel formula and athletic performance) , Pete Riegel, Runner's World / American Scientist (1977)
- Marathon training and pacing guidance , American Council on Exercise