Race Time Predictor
Doubling the distance costs more than double the time.
Doubling the distance costs more than double the time. Shows the working, not just the answer.
Predicted time
3:38:55
5:11 /km · 4.2% slower than a linear scale-up
This is Riegel's formula: the new time is the old one times the distance ratio raised to 1.06. The exponent above 1 is the entire content — doubling the distance costs more than double the time, and the penalty compounds the further you extrapolate. The assumption buried in every race predictor, and the reason marathon predictions disappoint, is that you have actually trained for the longer distance. A 5 km time predicts what a marathon-trained runner with that speed could do, not what you will do off 30 km a week. Raise the exponent toward 1.10 if your endurance is your weakness and lower it toward 1.03 if it is your strength. Runners who have raced both distances usually know which side of 1.06 they sit on.
How the Race Time Predictor works
Predict a race time at a new distance using Riegel's formula, with the naive linear answer alongside so you can see the penalty. The exponent of 1.06 is why doubling a distance costs more than doubling the time, and the gap widens the further you extrapolate.
Also known as: half marathon to marathon time · what marathon time can i run · 5k to 10k time predictor · race equivalent times chart
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
How do I predict my marathon time from a half?
Multiply by roughly 2.09, not 2. Riegel's formula raises the distance ratio to the power 1.06, so a 1:30 half predicts about 3:08 rather than 3:00 — and that assumes you have trained for the marathon.
What is Riegel's formula?
T2 = T1 × (D2/D1)^1.06, fitted to race results across distances and sports. The exponent above 1 captures the fact that pace falls off as distance rises, which no linear conversion can.
Why do race predictors overestimate marathons?
Because they assume marathon-specific training. A 5 km time predicts what a marathon-trained runner with that speed could do, not what you will do off 30 km a week — the physiology is there and the endurance is not.
Should I change the exponent?
If you know yourself. Raise it toward 1.10 if endurance is your weakness and lower it toward 1.03 if it is your strength. Runners who have raced both a 5 km and a marathon usually know which side of 1.06 they fall on.
Does the formula work for other sports?
Riegel fitted it across running, swimming and cycling, and the exponent is close in each. Swimming tends to sit slightly lower because drafting and technique play a larger role than in running.
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