Predict Half Marathon Time from a 25-Minute 5K
A 25-minute 5K predicts a half marathon time of approximately 1 hour 56 minutes.
How to use this tool
- Enter known distance, known time — minutes, known time — seconds and target distance in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your predicted minutes and the full breakdown beneath it.
See what half marathon time your 25-minute 5K suggests using the race time prediction formula.
Frequently asked questions
- What is the Riegel formula?
- T2 = T1 × (D2 / D1)^1.06, where T1 is your known time over distance D1, T2 is the predicted time over target distance D2. The exponent 1.06 reflects the fact that runners slow slightly at longer distances.
- How accurate is the Riegel predictor?
- It is most accurate within a factor of ~2–3× of the known distance. Predicting a marathon from a 10 km is reasonable; from a 400 m sprint, much less so. Terrain, weather, and fitness level all affect actual results.