Race Time Predictor
Predict your finish time for any race distance using the scientifically-validated Riegel formula.
Enter a recent race result
hh:mm:ss
Predicted Race Times
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Try Paicer FreeHow It Works
Enter a recent race distance and time. The calculator uses the Riegel formula to predict equivalent times for other distances, accounting for the natural pace slowdown at longer distances.
Tips for Better Results
- •Predictions are most accurate when the input race is similar in distance to your target
- •Marathon predictions from 5K times tend to be optimistic without marathon-specific training
- •Use predictions as targets, not guarantees. Race day conditions matter.
What Is the Riegel Formula?
The Riegel formula was developed by exercise physiologist Peter Riegel in 1977 to model how running (and swimming and cycling) performance declines as distance increases. It is written as T2 = T1 x (D2/D1)^1.06, where T1 is your known time at distance D1, and T2 is your predicted time at distance D2. The exponent 1.06 is an empirically derived "fatigue factor": if performance scaled perfectly with distance the exponent would be 1.0, but real-world endurance degrades slightly faster than that, which is why the formula uses 1.06 instead.
Worked Example
- Say you ran a 10K in 45:00 and want to predict your marathon time.
- D2/D1 = 42.195 / 10 = 4.2195
- 4.2195 raised to the power of 1.06 = 4.599
- T2 = 45:00 x 4.599 = 206.97 minutes, which is about 3:27:00
Race Time Equivalents (Riegel Formula)
Predicted times at other distances based on a given 5K time. Longer-distance predictions get less reliable without distance-specific training.
| 5K Time | 10K | Half Marathon | Marathon |
|---|---|---|---|
| 18:00 | 37:32 | 1:22:48 | 2:52:38 |
| 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 22:00 | 45:52 | 1:41:12 | 3:31:00 |
| 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 30:00 | 1:02:33 | 2:18:00 | 4:47:44 |
Frequently Asked Questions
Race Predictor
Race predictions using the Riegel formula are typically accurate within 2-5% for well-trained runners. Factors like terrain, weather, and training specificity can affect actual results.
The Riegel formula (T2 = T1 × (D2/D1)^1.06) predicts race times by accounting for the fact that pace naturally slows as distance increases. It was developed by researcher Peter Riegel.
Yes, use a race from the past 4-8 weeks for most accurate predictions. Your fitness changes over time, so older results may not reflect current ability.
Yes, this is one of the most reliable Riegel conversions since the distances are close. Multiply your half marathon time by roughly 2.11-2.13 (an approximation of the formula) to estimate marathon pace, but expect the real result to be slower unless you have done marathon-specific long runs.
The Riegel formula assumes your endurance scales predictably with distance, but a marathon depends heavily on fat-burning efficiency and long-run specific training that a 5K does not test. The bigger the gap between your reference race and your target distance, the less reliable the prediction.
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