Predict Your Marathon Time From a 5K

How a 5K predicts your Marathon

This is the prediction to take with the biggest pinch of salt. A marathon is more than eight times the distance of a 5K, and the gap between them is where most running dreams quietly meet reality. Riegel himself noted his formula works best between about 1500 meters and the marathon, but the further you stretch it, the more it leans on assumptions about endurance a 5K simply cannot prove.

The number you get is best read as a theoretical ceiling. It says, if you had the aerobic base, the fueling, and the long runs to support your speed, this is the marathon you could run. Most people who only race 5Ks do not have that base yet, so their real marathon comes in well slower. That is not a flaw in your fitness, it is the difference between being fast and being durable.

Why the 5K to marathon jump overpredicts

A 5K is over before glycogen depletion, dehydration, or muscular fatigue become the story. A marathon is almost entirely about those things. The wall that hits many runners around 20 miles has no equivalent in a 5K, so the formula has no way to account for it. It scales your speed and hands you a time as if the back half of the marathon were just more of the same.

If a 5K is all you have, use the prediction to set a stretch goal, then reality check it against your long runs and weekly mileage. A runner averaging fifty miles a week with regular twenty milers can chase a 5K based marathon time. A runner doing twenty miles a week should add ten to twenty minutes and focus on building the distance first.

Want every distance at once? The race pace predictor shows your time for the whole range and prints a pace band, and the VDOT calculator turns this race into your training paces.