Race Predictor (Running Time Calculator)

The calculator uses Pete Riegel's formula: T2 = T1 × (D2 / D1)^1.06 for males and ^1.07 for females. T1 is your known race time, D1 is your known distance, and D2 is the target distance. This exponent accounts for the increasing difficulty of maintaining pace over longer distances.

The Riegel formula is widely used and reasonably accurate for road racing distances between 1.5K and the marathon. Predictions become less reliable for very short sprints or ultra-distances. Factors like course terrain, weather, training fitness, and pacing strategy can all affect your actual result.

The fatigue exponent differs slightly between males (1.06) and females (1.07), reflecting observed differences in how performance scales with distance in large race datasets. The difference is small but adds a degree of accuracy to longer-distance predictions.

Yes, and it's a common use case. Enter your recent 5K time and select Marathon as your target distance. Keep in mind that marathon performance depends heavily on long-run training and endurance base, so the prediction is an estimate of your equivalent fitness — achieving it requires proper marathon-specific preparation.

An equivalent race performance is the projected finish time at a different distance that reflects the same level of fitness as your reference race. For example, a 25-minute 5K runner has an equivalent fitness level to roughly a 52-minute 10K or a 1:55 half marathon runner.

Use a recent race time that reflects your current fitness level. A personal best from two years ago may not accurately represent what you can do today. The more recent and well-paced the reference race, the more reliable the prediction.

The calculator supports 1 Mile, 3K, 5K, 5 Miles, 10K, 15K, Half Marathon, and Marathon as both input (known) and target distances. You can predict in any direction — for example, from a half marathon back down to a 5K, or from a 10K up to a marathon.

Once you see your predicted finish time, check the pace per mile or pace per kilometer output. You can use these paces as your target race pace. If the predicted pace feels too easy or too hard based on recent training, adjust your expectations accordingly and use it as a planning benchmark.