Race Time Predictor (Riegel Formula)
Predict 10K, half, and marathon times from a recent race result.
What it does
Runners love translating one result to another distance, and the Riegel formula is the standard translator. Published by researcher Peter Riegel in 1977 from world record analysis, it states that time grows a touch faster than distance: multiply by the distance ratio raised to 1.06.
The tool above applies that to your recent race. Enter the distance and time, pick a target, and read the predicted time with its even pace plus a marathon projection at the same fitness.
The 1.06 exponent captures slowing with distance. Twice the distance costs about 2.08 times the duration, which is why 25 minutes for 5K predicts near 52 minutes for 10K rather than 50 flat.
Specific endurance limits the magic. A 5K specialist without long runs rarely hits the marathon number, while a marathoner often beats short predictions. The formula assumes training to match the target.
How to use
- Enter a recent all out race, not a tempo run or a stale best.
- Enter its exact distance and finishing time.
- Pick the target distance in km or miles.
- Train to the predicted pace, then adjust for course and weather.
Formula
- T1, D1
- Known race time and distance, the anchor of the prediction.
- D2
- Target distance in the same unit as D1.
- 1.06
- Riegel fatigue exponent from record data. Larger values suit beginners.
Some calculators use 1.07 or 1.08 for recreational runners, which softens long predictions slightly. At 1.06 the marathon from a 5K runs a touch optimistic for most amateurs, so treat it as a ceiling with full training.
Worked example
Example: 5K in 25:00 predicts 10K
Known 5 km in 25 minutes flat, target 10 km.
- Ratio: 10 / 5 = 2.
- Power: 2^1.06 = 2.0849.
- Time: 25 x 2.0849 = 52.12 min, shown as 52:07.
- Pace: 52.12 / 10 = 5.21 min per km, shown as 5:13.
- Marathon same fitness: 25 x (42.195/5)^1.06 = 239.78 min, shown as 3:59:47.
Reference table
| Distance | Predicted | Pace per km |
|---|---|---|
| 5K | 25:00 | 5:00 |
| 10K | 52:07 | 5:13 |
| Half marathon | 1:55:00 | 5:27 |
| Marathon | 3:59:47 | 5:41 |
Limits
- Assumes training fits the target. Short specialists miss long numbers.
- Flat fast course assumed. Hills, heat, and trails slow reality.
- One exponent for all. Beginners fade faster than 1.06 suggests.
- Stale bests mislead. Use races from the last two months.
Common mistakes
- Predicting from training runs instead of raced efforts.
- Ignoring the endurance gap between 5K speed and marathon legs.
- Chasing the marathon number without the long runs behind it.
- Forgetting course profiles. Hilly races run minutes slower.
- Rounding the exponent to 1.0, which just doubles and lies.
Try it yourself
Quick checks. Answers sit tucked under each question.
10K in 52:07 predicts what half marathon?
Ratio 21.0975/10 = 2.1098. 52.12 x 2.1098^1.06 = 52.12 x 2.2067 = 115.0 min, about 1:55:00.
Why is doubling not exact?
Fatigue compounds. The 1.06 exponent adds about 4% per doubling: 25 x 2.0849 instead of 25 x 2.
Questions
How accurate is Riegel?
Within a few percent for trained runners at near distances. Jumps from 5K to marathon vary most.
What exponent should beginners use?
Try 1.07 or 1.08 for softer long predictions until endurance catches up.
Can I predict shorter from longer?
Yes, the formula runs both ways, though speed work still decides short results.
Do age grade tables do the same?
Related but different. Age grading compares to age records; Riegel translates your own distances.
Why do marathons miss most?
Fueling, pacing, and durability dominate past 30K. The formula knows fitness, not stomachs.
Which race predicts best?
The closest distance to the target, raced recently on a similar course.