Marathon Time Predictor
Predict your full marathon finishing time using a recent 5 km, 10 km, or half marathon result. Useful for setting a realistic race goal before your first or next 42.2 km event.
About this calculator
Marathon time prediction uses scaling multipliers derived from the relationship between race performances at different distances. The formula applied here is: predicted marathon time = recent race time × multiplier, where the multiplier is 9.3 for a 5 km time, 4.66 for a 10 km time, and 2.1 for a half marathon time. These multipliers reflect the compounding physiological demands — aerobic capacity, glycogen depletion, and pacing discipline — that grow disproportionately as distance increases. For example, a 25-minute 5 km predicts a marathon of roughly 3 hours 53 minutes (25 × 9.3 = 232.5 minutes). More accurate predictions generally come from longer reference races because the physiological demands are closer to the marathon distance. Predictions assume optimal training, pacing, and race conditions.
How to use
Say you recently ran a 10 km race in 48 minutes. Select 10 km as your Race Distance and enter 48 as your Race Time in minutes. The calculator applies the 10 km multiplier: 48 × 4.66 = 223.68 minutes. Convert to hours and minutes: 223.68 minutes = 3 hours 43 minutes and approximately 41 seconds. This gives you a realistic marathon target of around 3:43:41, assuming your training and race-day conditions are comparable to your 10 km effort.
Frequently asked questions
How accurate is a marathon time prediction based on a 5km result?
A 5 km-based prediction is the least accurate of the three options because the physiological demands differ most from a marathon. Factors like glycogen depletion, heat tolerance, and long-run endurance play almost no role in a 5 km but dominate the marathon. A half marathon result typically gives the most reliable prediction. Use 5 km predictions as a rough guide only, and adjust your target after longer training runs closer to race day.
What multiplier is used to predict marathon time from a half marathon?
This calculator uses a multiplier of 2.1 applied to your half marathon finishing time in minutes. So a 1:55:00 half marathon (115 minutes) predicts a marathon of 115 × 2.1 = 241.5 minutes, or roughly 4 hours 1 minute 30 seconds. The 2.1 multiplier is slightly higher than 2.0 because the second half of a marathon is almost always slower due to fatigue and glycogen depletion — a phenomenon known as the 'marathon wall.' Solid long-run training can help you get closer to a 2.0 ratio.
Why does marathon time prediction get harder as distance increases?
Beyond roughly 30 km, the body's glycogen stores become critically depleted, forcing a greater reliance on fat oxidation, which is a much slower energy pathway. This physiological shift — commonly called 'hitting the wall' — causes pace to slow in a way that shorter race performances cannot fully predict. Additionally, cumulative muscle damage, heat regulation demands, and mental fatigue all compound. That is why even well-trained runners often find their actual marathon time exceeds their predicted time if they set out too fast.