Race Time Predictor
Enter a recent race result and predict your finish time at any other distance — from 1500m to marathon. Based on Riegel's formula, the gold standard for race time equivalency.
How Riegel's formula works
The race time predictor is built on a formula first published by American engineer and runner Peter Riegel in Runner's World in 1977. It remains the most widely used performance equivalency formula in running.
k = fatigue exponent (1.04 for well-trained runners, 1.06 for regular, 1.08 for beginners)
The exponent k is where the runner profile matters. It models how quickly a runner slows down as distance increases. A well-trained runner with strong aerobic base maintains pace better over longer distances (lower k), while a beginner slows more sharply (higher k).
Example: a runner who finishes a 10K in 50:00 as a regular runner (k = 1.06) would predict a marathon in approximately 3:49. The same runner classified as well-trained (k = 1.04) predicts 3:43 — reflecting better endurance efficiency.
Choosing your runner profile
| Profile | Exponent (k) | Who it fits | Training context |
|---|---|---|---|
| Beginner | 1.08 | Under 2 years running, limited race experience | 1–3 runs/week, no structured training |
| Regular | 1.06 | Consistent runner with some race history | 3–4 runs/week, occasional long runs |
| Well trained | 1.04 | Competitive runner, high weekly mileage | 5+ runs/week, structured training blocks |
Example predictions by race distance
These tables show predicted finish times using common reference performances — useful for benchmarking before you run the calculator with your own numbers.
From a 10K finish time (regular runner, k = 1.06)
| 10K time | 5K equiv. | Half marathon | Marathon | Pace/mi (marathon) |
|---|---|---|---|---|
| 35:00 | 16:40 | 1:17:30 | 2:42:30 | 6:13/mi |
| 40:00 | 19:06 | 1:28:35 | 3:05:45 | 7:06/mi |
| 45:00 | 21:30 | 1:39:40 | 3:28:30 | 7:59/mi |
| 50:00 | 23:54 | 1:50:25 | 3:49:55 | 8:47/mi |
| 55:00 | 26:18 | 2:01:10 | 4:13:20 | 9:41/mi |
| 60:00 | 28:42 | 2:12:00 | 4:36:00 | 10:33/mi |
From a 5K finish time (regular runner, k = 1.06)
| 5K time | 10K equiv. | Half marathon | Marathon | Pace/km (marathon) |
|---|---|---|---|---|
| 18:00 | 37:42 | 1:24:00 | 2:55:30 | 4:09/km |
| 20:00 | 41:55 | 1:33:20 | 3:15:00 | 4:37/km |
| 22:00 | 46:05 | 1:42:40 | 3:34:30 | 5:05/km |
| 25:00 | 52:24 | 1:56:40 | 4:03:30 | 5:47/km |
| 28:00 | 58:41 | 2:10:40 | 4:32:15 | 6:27/km |
| 30:00 | 1:02:54 | 2:20:00 | 4:51:30 | 6:55/km |
How accurate is the prediction?
Riegel's formula is reliable within specific conditions — and significantly less accurate outside them. Understanding when to trust it (and when not to) helps you use the results appropriately.
Similar distances
Predicting a marathon from a half marathon, or a 10K from a 5K. The closer the distances, the more reliable the result.
3.5 min – 4 hour efforts
The formula was derived from efforts in this range. Times outside this window — very short sprints or ultra distances — become less reliable.
Trained for target distance
If you've done the long runs for a marathon, predictions from 10K time are solid. Without that base, your actual marathon will be slower than predicted.
Large distance jumps
Predicting marathon from a 5K, or half marathon from a 1500m. The formula extrapolates but aerobic specific fitness matters more at these gaps.
Heat, hills, or wind
The formula assumes flat, neutral conditions. A PR on a hilly course or in heat will underestimate your fitness for a flat, cool target race.
Ultra distances (50K+)
Ultra performance depends heavily on nutrition, sleep strategy, terrain, and mental resilience — factors the formula cannot model. Use only as a rough reference.
How to use race time predictions in training
The prediction isn't just about race day — it's one of the most useful tools for setting training paces. Here's how to put it to work.
Set a realistic goal pace. If you've run a 50-minute 10K and the predictor gives you 3:49 for the marathon, that's your starting point for goal race pace — approximately 8:45 per mile or 5:26 per kilometer. Don't ignore the number; if it feels too fast in training at that pace, it probably is.
Calibrate your training zones. Your predicted race times map directly to effort-based training zones. Easy runs should be 60–75 seconds per mile slower than predicted marathon pace. Tempo runs sit around half-marathon pace. VO₂ max intervals target 5K pace.
Track fitness improvements. Run a time trial or tune-up race every 6–8 weeks and update your prediction. Seeing your predicted marathon drop from 4:10 to 3:58 over a training block is one of the most motivating things in running.
Assess readiness. If your long runs feel manageable at predicted marathon effort and your shorter race times are hitting targets, you're ready. If your 10K time hasn't improved in a block, neither has your marathon fitness — regardless of mileage.
Who was Peter Riegel?
The formula behind this calculator has a surprisingly human origin story.
Peter "Pete" Riegel (1935–2018) was an American mechanical engineer and passionate distance runner based in Columbus, Ohio. He was best known in the running community not as an elite athlete but as a meticulous volunteer — he spent decades measuring road race courses to USATF standards and served as a technical official at major events.
In 1977 he published a short article in Runner's World proposing a simple mathematical model for predicting performance across distances. The formula was elegant: a power law relating time and distance with a single fatigue exponent. It was practical, required no laboratory testing, and needed only a recent race time as input.
Nearly 50 years later, Riegel's formula remains the foundation of most race time prediction tools — a remarkable legacy for what started as a single magazine article. More sophisticated models exist, but none have displaced it as the universal starting point for runners trying to answer the question: "What can I run?"
Frequently asked questions
Enter your 10K finish time in the calculator above, select "10K" as the known distance, choose your runner profile, and click predict. The calculator will show your estimated marathon time alongside predictions for every other standard distance.
As a rough benchmark: a 10K time in minutes, multiplied by approximately 4.6 (regular runner), gives a reasonable marathon estimate. A 50-minute 10K × 4.6 ≈ 3:50 marathon.
Yes — enter your 5K time, select the 5K as your known distance, and the predictor will output a half marathon time. The prediction assumes you're adequately trained for the half marathon distance. If your longest long run is 8 miles, the formula will give you a time you're not yet fit enough to run.
A useful rule of thumb: your half marathon time is approximately 2.2 times your 5K time for a regular runner. A 25-minute 5K suggests roughly a 55-minute 10K and a 1:55 half marathon.
The profile adjusts the Riegel exponent (k), which controls how much you're predicted to slow down as distance increases. A beginner (k = 1.08) slows more sharply over longer distances. A well-trained runner (k = 1.04) maintains pace more efficiently.
In practice this means: for the same 10K time, a beginner profile predicts a slower marathon than the well-trained profile — because the formula assumes the beginner will fade more in the late miles. The difference between profiles is roughly 6–10 minutes over a marathon.
The most common reason is that the reference performance isn't representative of current fitness — a PR from a year ago, or a time set on a fast course in perfect conditions, will produce an overly optimistic prediction.
Other factors the formula doesn't account for: insufficient training for the target distance (especially long run mileage for marathons), race-day heat, hilly courses, and pacing errors. If predictions consistently feel ambitious, try the Beginner profile or use a more recent race time.
It depends on what you're predicting. For distances within a similar range (5K to marathon), Riegel is highly accurate and straightforward. More complex methods like VO₂ max-based predictions or age-grading can be more precise, but require more data inputs.
For marathon-specific prediction from a half marathon, the Pete Magill or Jack Daniels VDOT methods can be slightly more accurate because they're calibrated specifically on marathon data. But for a free, single-input tool with no lab testing required, Riegel's formula has stood the test of nearly 50 years of use.
Take your predicted marathon time and divide by 26.2 (miles) or 42.2 (kilometers) to get target pace per mile or kilometer. For a 3:49 predicted marathon: 3:49 = 229 minutes ÷ 26.2 = 8:45 per mile, or ÷ 42.2 = 5:26 per km.
Use this as your goal pace for the first 18–20 miles, then decide based on feel whether to push in the final miles. The most common marathon mistake is starting 15–20 seconds per mile faster than goal pace — by mile 22 that gap becomes very expensive.