Pace Calculator

Enter any two of distance, time and pace, and the calculator returns the third, in minutes per kilometre and minutes per mile as well as km/h and mph. It also lists split times at each kilometre or mile for the target pace, so that a plan for a 10 km in 50 minutes becomes "pass 5 km at 25:00", and race predictions for 5 km, 10 km, half marathon and marathon based on your entered performance.

These are even-pace times. In practice most runners slow over longer distances, which is what the prediction model accounts for.

The predictions use Riegel's formula, the most widely used model: time for the new distance = your time × (new distance ÷ old distance)^1.06. The exponent captures the fact that pace slows as distance increases; a 25-minute 5 km predicts about 52 minutes for 10 km rather than 50, and about 4 hours for a marathon. The formula is reliable between distances of similar type (5 km to half marathon) and optimistic for the marathon unless your training includes long runs; treat the marathon figure as a ceiling. Predictions from a recent race are better than from a training run, and a well-paced race is a better predictor than one where you faded.

Write the split times on your hand or set them as lap alerts on a watch, and check at each marker. Going out 10–15 seconds per kilometre faster than target in the first kilometre is the most common way to lose a race in the last three. For interval sessions, the interval timer runs work and recovery periods, the lap timer records each rep, and the speed calculator converts between pace and speed for cycling or rowing.