Predict Your Half marathon Time From a 5K
Ran a 5K lately? Enter your time to see what it predicts for the Half marathon, plus the pace you would need to hold.
Worked example
A 5K of 22:00 predicts a Half marathon of about 1:41:12, a pace of 7:43 per mi. Change the time above to run the same math on your own result.
How a 5K predicts your Half marathon
Predicting a half marathon from a 5K is a bigger ask. You are extrapolating across more than four times the distance, and somewhere in that gap raw speed stops being the limiting factor and endurance takes over. The formula gives you a number your legs can run, but only if your training has built the engine to back it up.
Treat a 5K based half prediction as the fast end of what is possible, not a sure thing. It answers the question, if my endurance matched my speed, what could I run? For a runner with solid weekly mileage that is a fair target. For someone who races a sharp 5K off two short runs a week, the real half will come in slower.
Where speed ends and endurance begins
A 5K takes most people fifteen to twenty-five minutes. A half marathon takes an hour and a half to two and a half hours. Those are different events physiologically. The 5K leans on your VO2 max and your tolerance for discomfort. The half leans on your aerobic threshold and your ability to keep burning fuel efficiently for ninety minutes or more. Riegel cannot see which of those you have trained, so it assumes both are in proportion.
The practical fix is to look at your long runs. If you can comfortably run ten to twelve miles in training, a 5K based prediction is believable. If your longest run is six miles, knock a few percent off the predicted time and build your 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.
Related predictions
5K to Half marathon questions
- Can a 5K really predict my half marathon?
- It can give you a ceiling. The prediction is what you could run if your endurance were as developed as your speed. With consistent long runs it is a fair target, but off low mileage your actual half will be slower.
- Why is my predicted half so much faster than I expect?
- Because a 5K rewards speed and the half rewards endurance, and the formula assumes the two are balanced. If your training is speed heavy and volume light, the real half drifts slower than the number.
- What is a better race to predict my half from?
- A recent 10K is far more reliable, and a 10 mile or 15K even more so. The closer your input race is to the half, the less the prediction has to assume about your endurance.