Glossary
Riegel formula
The 1981 equation behind most race predictors: T2 = T1 x (D2/D1)^1.06. Raising the exponent to 1.07 adds nearly 4 minutes to a predicted marathon.
What it is
One line of arithmetic. Your new time equals your old time multiplied by the ratio of distances, raised to a power.
Peter Riegel fitted the exponent 1.06 to race records across distances and published it in American Scientist in 1981. It has been in nearly every race calculator since.
| From an 18:00 5K | Exponent 1.06 | Exponent 1.07 |
|---|---|---|
| 10K | 37:29 | 37:45 |
| half marathon | 1:22:57 | 1:23:56 |
| marathon | 2:52:38 | 2:56:22 |
What it is not
It is not calibrated to you. The exponent came from record performances, which belong to people with deep endurance bases.
It is also not fixed. Many calculators use 1.07 or 1.08 for the marathon without saying so, which is one reason two apps give different race predictions from the same input.
What moves it
- The exponent. One decimal place is worth minutes.
- The distance gap. Error grows the further you extrapolate.
- Nothing about your training. The formula has no such input.
When to ignore it
Ignore it for marathon predictions off short races if your weekly volume is under about 60 km (37 mi).
Ignore it entirely for ultra distances, where the relationship it fits stops holding.