Race Time Predictor
Predict race finish time from a known result using the Riegel formula t2 = t1 x (d2/d1)^1.06.
Covers 5K, 10K, half marathon, and marathon.
Race time prediction uses your known performance at one distance to estimate your potential finishing time at another distance. This is invaluable for setting realistic goals, pacing strategy, and planning training blocks.
The most widely used formula (Riegel’s formula, 1977): Predicted Time = Known Time × (New Distance / Known Distance)^1.06
The exponent 1.06 accounts for the physiological reality that longer races are proportionally harder — you slow down more than a linear scale would suggest.
Worked example — 5K to Half Marathon: Known: 5K in 25:00 (5.0 km) Predict: Half Marathon (21.097 km) Predicted Time = 25:00 × (21.097 / 5.0)^1.06 = 25:00 × (4.219)^1.06 = 25:00 × 4.513 = 112.8 minutes = 1:52:49
Worked example — 10K to Marathon: Known: 10K in 48:30 (10 km) Predict: Marathon (42.195 km) = 48.5 × (42.195/10)^1.06 = 48.5 × (4.2195)^1.06 = 48.5 × 4.512 = 218.8 min = 3:38:50
Common race distances and benchmarks:
- 5K: avg recreational runner 28–35 min; competitive 18–25 min
- 10K: avg 58–70 min; competitive 37–50 min
- Half Marathon: avg 2:15–2:45; competitive 1:25–1:50
- Marathon: avg 4:30–5:30; competitive 2:55–3:30
Riegel formula accuracy limits:
- Works best for distances within 2–3× of each other
- Assumes similar terrain, weather, and training specificity
- Underestimates times for first-timers who haven’t trained specifically for the longer distance
Pace calculation: Pace (min/mile) = Total Time (minutes) / Distance (miles) Pace (min/km) = Total Time (minutes) / Distance (km)