Race Time Predictor
Predict your finish time at any distance based on a known race time using Riegel's formula.
PhD Sports Science, Registered Nutritionist (RNutr)
Sports scientist and registered nutritionist specialising in metabolic health, athletic performance and dietary analysis.
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About the Race Time Predictor
Predicting your race time at an unfamiliar distance is one of the most practically useful tools in an endurance athlete's arsenal. Whether you're deciding whether to enter a marathon based on your recent half marathon time, or trying to set a realistic goal pace for an upcoming 10K, Riegel's formula provides a principled mathematical estimate grounded in decades of real race data.
The Riegel formula, published by Peter Riegel in a 1977 paper in American Scientist, is T2 = T1 × (D2/D1)^1.06. The exponent 1.06 — slightly above 1 — reflects the well-established physiological reality that pace slows as distance increases. Running twice as far takes more than twice as long because aerobic energy demand increases, fatigue accumulates, and glycogen depletion becomes more significant. The 1.06 value was derived by Riegel from analysis of world record data across distances from 100 metres to 50 miles.
The formula is most accurate for distances within a factor of 2–3 of each other. Predicting a marathon from a 5K will overestimate performance, because the 5K can be run largely anaerobically, drawing on energy systems that are disproportionately depleted at marathon distance. A 10K to half marathon prediction is typically within 2–3 minutes. Using your most recent race time (not a training run time) gives the most accurate result, as race conditions produce genuine maximal effort.
How it works
Riegel Race Predictor: T2 = T1 × (D2 ÷ D1)^1.06 Where: T1 = Known race time (in minutes) D1 = Known race distance (in km) D2 = Target race distance (in km) T2 = Predicted race time (in minutes)
Where
T1Your known race time in minutes — must be from a genuine race effortD1The distance of your known race in kilometresD2The distance you want to predict1.06The Riegel fatigue exponent — reflects the slowing effect of increasing distanceWorked example
5K time: 25:00 (25 minutes) → Predict half marathon time
T2 = 25 × (21.1 ÷ 5)^1.06
= 25 × (4.22)^1.06
= 25 × 4.556
= 113.9 minutes
= 1:53:54
Pace: 113.9 ÷ 21.1 = 5:24 per km
Note: This assumes equivalent training and similar race conditions.
Tips to improve your result
- 1.
Use a recent race time, not a training run. Races produce maximal effort; training runs don't, so they will underestimate your race potential.
- 2.
The formula tends to underestimate marathon times for most recreational runners. Many runners find the actual marathon demands 5–10% more time than predicted from a half marathon.
- 3.
If you are heat-sensitive, add 4–8% to your predicted time for races above 20°C. Heat increases cardiovascular strain significantly beyond what the formula accounts for.
- 4.
Altitude adds roughly 1–2% per 300m above sea level above your acclimatised altitude. A race at 2,000m will be approximately 5–8% slower than at sea level.
- 5.
Long-course races (road/trail) with significant elevation gain require a separate correction — add approximately 1 minute per 100m of net elevation gain per 10km of racing.