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Riegel's original assumption
Based on Riegel's (1977) formula with 1.06 as the default exponent. Actual results vary with training, course, weather, and elevation — marathon time especially depends on whether you've trained past 30km.
You can't hold the same pace forever
Running a 5K in 20 minutes doesn't mean you can finish a marathon at that pace — pace always slows as distance grows. The Riegel formula fits that slowdown to real race results: predicted time = base time × (target distance ÷ base distance)^1.06.
1.06 assumes you trained for that distance
Someone who only runs 5Ks will get an overly generous marathon prediction from this formula, because marathon performance depends on training volume more than any other race distance — if you've never run past 30km, the prediction won't hold. That's why this calculator offers three exponents instead of one; which assumption you pick matters as much as the number itself.
It works in the other direction too
The same formula predicts a 5K time from a marathon result — but there speed is the missing variable, so a long-distance runner's 5K prediction tends to come out faster than reality. Either direction only holds if training is similar across distances.
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Frequently asked questions
QWhy do I get three different marathon predictions?
The Riegel exponent (1.06) is calibrated for someone trained at that specific distance. Runners who mostly train short distances see their endurance drop off faster than the formula assumes, so this calculator offers a higher exponent for them — pick the one closest to your actual training.
QCan I predict a shorter race from a longer one?
Yes, the same formula works both ways. A marathon time predicts a 5K time using the same exponent, though speed rather than endurance becomes the limiting factor in that direction.