# Race Time Predictor (Riegel Formula)

URL: https://speedycalculator.com/calculators/sports/race-time-predictor/
Last updated: 2026-10-25

> The race time predictor estimates your finish time for a new distance from a recent race result using Peter Riegel’s formula, T₂ = T₁ × (D₂ ÷ D₁)^1.06.

## Inputs

- Recent race distance (km, mi)
- Recent race time (min, h)
- Target race
- Fatigue exponent

## Results

- Predicted time
- Predicted pace

## Formula

```
T₂ = T₁ × (D₂ ÷ D₁)^1.06
```

## Explanation

Riegel’s formula reflects that runners slow slightly as distance increases. It works best for races between about 3.5 minutes and 4 hours.

### How to use the Race Time Predictor

- **Recent race distance**
- **Recent race time**
- **Target race**
- **Fatigue exponent**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

The known time is scaled by the distance ratio raised to the fatigue exponent 1.06.

### Inputs explained

FieldWhat to enterRecent race distanceUnits: km, miRecent race timeUnits: min, hTarget raceChoose one: 5K, 10K, Half marathon (21.1 km), Marathon (42.2 km)Fatigue exponentA number

### What each result means

- **Predicted time** = [Recent race time]×([Target race]÷[Recent race distance])^[Fatigue exponent]×60
- **Predicted pace** = [Recent race time]×([Target race]÷[Recent race distance])^[Fatigue exponent]÷[Target race]

## Worked example

Inputs — Recent race distance: 10 km; Recent race time: 50 min; Target race: Half marathon (21.1 km); Fatigue exponent: 1.06.

Results — **Predicted time = 1:50:19**; Predicted pace = 5:14.

## FAQ

**Is the prediction reliable for a first marathon?**
It is often optimistic unless your endurance training is solid.

**Why 1.06?**
Riegel fitted it to world-class performances; some runners fit 1.07–1.08 better.

**What formula does the Race Time Predictor use?**
T₂ = T₁ × (D₂ ÷ D₁)^1.06.


## Sources

- Riegel PS. Athletic records and human endurance. American Scientist, 1981