# Free online race time predictor

> Enter a finish time from any race and this calculator predicts how fast you could run every other standard distance — from 1 mile to marathon — using the proven Riegel formula.

Predict finish times for 1 mile, 5K, 10K, half marathon, and marathon from any known race performance using the Riegel formula.

**Interactive version:** https://onlinecalculator.me/health/race-time-predictor/
**Category:** Health

## How to use

1. Select the race distance you have a time for.
2. Enter your hours, minutes, and seconds.
3. Click **Predict Race Times** to see predictions for all five standard distances.

## The Riegel formula

Source: Riegel PS. Athletic Records and Human Endurance. *American Scientist*, 69(3):285-290, 1981.

```
T2 = T1 × (D2 / D1)^1.06
```

- `T1` — your known finish time (seconds)
- `D1` — your known race distance (meters)
- `D2` — the target race distance (meters)
- `T2` — predicted finish time (seconds)

The exponent **1.06** reflects the non-linear relationship between distance and pace: runners slow slightly per mile as races grow longer.

## Race distances used

| Race | Distance |
|---|---|
| 1 Mile | 1,609.34 m |
| 5K | 5,000 m |
| 10K | 10,000 m |
| Half Marathon | 21,097.5 m |
| Marathon | 42,195 m |

## Worked example

Known: 5K in 25:00 (1,500 seconds).

Predict 10K: T2 = 1500 × (10000/5000)^1.06 = 1500 × 2^1.06 ≈ 1500 × 2.083 = **3,125 s ≈ 52:05**

Predict Marathon: T2 = 1500 × (42195/5000)^1.06 ≈ 1500 × 8.439^1.06 ≈ 1500 × 9.288 ≈ **13,932 s ≈ 3:52:12**

## Notes

- Results are best treated as training targets, not race-day guarantees.
- The Riegel formula tends to over-predict marathon times for recreational runners who need more recovery pacing.

## Frequently asked questions

### What is the Riegel formula?

The Riegel formula predicts race times across distances: T2 = T1 × (D2/D1)^1.06. T1 is your known time for distance D1. The exponent 1.06 accounts for the physiological cost of sustaining pace over longer distances — it's slightly harder than a straight linear scale.

### How accurate are these predictions?

The Riegel formula is a well-validated approximation, but individual results vary based on training, pacing strategy, terrain, and fitness. Most runners find it most accurate within a 2x distance range (e.g., predicting a half marathon from a 10K is more reliable than predicting a marathon from a 5K).

### Why does marathon time scale non-linearly from 5K time?

Running efficiency drops with distance due to glycogen depletion, muscle fatigue, and heat. The exponent 1.06 in the Riegel formula captures this effect — a typical 25-minute 5K runner finishes a marathon in around 4:00-4:30, not the 3:45 a simple linear scale would predict.

### Can I use a recent training run to predict race time?

Yes, but race times are typically faster than equivalent training efforts. Use race results for the most accurate predictions. A hard tempo run time will give optimistic predictions.

### What distances does this calculator support?

1 Mile, 5K, 10K, Half Marathon (21.097 km), and Marathon (42.195 km). Distances use standard road-race lengths in meters.

## Sources

- [Athletic Records and Human Endurance](https://www.jstor.org/stable/27851417) — American Scientist
- [An empirical study of race times in recreational endurance runners](https://pmc.ncbi.nlm.nih.gov/articles/PMC5000509/) — BMC Sports Science, Medicine and Rehabilitation

## Related calculators

- [Running pace calculator](https://onlinecalculator.me/health/running-pace/) — Calculate pace per mile or km from any race distance and finish time.
- [VO2 max calculator](https://onlinecalculator.me/health/vo2-max/) — Estimate aerobic fitness from a walk test, run test, or resting heart rate.
- [Calories burned calculator](https://onlinecalculator.me/health/calories-burned/) — Estimate calories burned running, walking, or cycling by weight and distance.
- [Heart rate zones calculator](https://onlinecalculator.me/health/heart-rate-zones/) — Find your five training zones from max heart rate or estimated max.

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