💡 Predictions assume you're trained for the target distance. A 22:00 5K runner who has never run over 8 miles will not run the predicted 3:35 marathon.
FAQ
How accurate are these predictions?
Within 2–4% for trained runners predicting a nearby distance from a recent all-out race. Marathon predictions from a 5K are the least reliable — endurance, fueling, and long-run history matter more than raw speed at 26.2 miles.
What formula is used?
Riegel's formula: T2 = T1 × (D2/D1)^1.06. The 1.06 exponent models how humans slow as distance increases. It's the same formula used by most online calculators and many coaches.
My marathon was much slower than predicted — why?
The usual suspects: insufficient weekly mileage (the formula assumes you're trained for the distance), fueling mistakes, hot weather, or going out too fast. Treat the prediction as a ceiling, not a promise.