How this calculation works
The Marathon Race Time Predictor forecasts your full 26.2-mile marathon finish time, target kilometer pace, and mile splits based on recent benchmark race performances using Peter Riegel's fatigue-scaling formula.
Mathematical formula and logic
T2 = T1 × (D2 / D1)^1.06, where T1 is base race time, D1 is base distance, and D2 is marathon distance (42.195 km).
Worked example
A 1:45:00 Half Marathon predicts a Marathon finish time of 105 mins × (42.195 / 21.0975)^1.06 = 219.0 minutes = 3 hours, 38 minutes, and 59 seconds (8:21 per mile pace).
Calculation assumptions
- Peter Riegel's endurance exponent (1.06) assumes adequate long-run mileage and proper marathon fueling/tapering.
- Comparable weather, course profile, and elevation conditions.
Frequently asked questions
How accurate is Riegel's race time predictor formula?
Riegel's formula is remarkably accurate for runners who have logged adequate weekly volume (40-60+ miles/week) and 20-mile long runs. Without sufficient long-run endurance, runners often slow down in the final 10K ('the wall').
What 5K time predicts a sub-4-hour marathon?
A 5K time of approximately 25:00 (or a 10K of 52:00 / Half Marathon of 1:55:00) mathematically correlates to a sub-4:00:00 marathon prediction.
What Half Marathon time predicts a Boston Marathon qualifying time (BQ)?
To run a sub-3:00:00 marathon (common male BQ standard), you generally need a Half Marathon time of 1:26:00 or faster.
Why do runners hit 'the wall' at mile 20 in a marathon?
The human body stores approximately 2,000 calories of glycogen in muscles and liver. Without continuous carbohydrate refueling (gels, sports drinks) every 30-45 minutes, glycogen depletes around mile 20.
How much should I taper before marathon race day?
A standard marathon taper reduces weekly running mileage by 20-25% two weeks out and 50-60% during race week while maintaining brief race-pace strides to keep legs sharp.