This calculator uses the Riegel formula: T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 (rather than 1.0) accounts for the natural fatigue factor — running twice as far takes more than twice as long due to the exponential relationship between distance and performance decline.
The formula works well for distances between 1.5 km and 50 km when you're racing at full effort. It assumes equivalent fitness and race conditions. Predictions may be less accurate when crossing very different terrain types (e.g., track 5K to hilly marathon).
Frequently Asked Questions
For most trained runners predicting within one or two distances (e.g., 5K to 10K, or 10K to half marathon), the Riegel formula is accurate within 3–5%. Accuracy decreases for very large distance jumps (e.g., 5K to marathon), for new runners who haven't raced at maximum effort, and in extreme temperatures or hilly conditions.
The Riegel formula reflects physiological reality — most runners' marathon pace is significantly slower than their shorter race pace would suggest, due to glycogen depletion and the need to maintain form over 26.2 miles. Many runners also underestimate the training required specifically for the marathon. The prediction is realistic, not pessimistic.
The three most effective training methods: 1) Increase weekly mileage (more aerobic base = faster race pace), 2) Add one tempo run per week at your 10K race pace, 3) Add one interval session (e.g., 6×800m at 5K pace with 90s rest). Give any new training approach 8–10 weeks to show results.