Pace or speed?
Pace is the time taken to cover one kilometre or mile (min/km, min/mi): runners use it. Speed is the distance covered in one hour (km/h, mph): cyclists use it. A 5:00 min/km pace is 12 km/h. In swimming, time is given per 100 m.
How is the prediction worked out?
With Pete Riegel's formula: T2 = T1 × (D2 / D1)1.06. It assumes training suited to the target distance; it is often optimistic for the marathon if your endurance training is limited (use an exponent of 1.08 to 1.10, adjustable).
What is maximal aerobic speed (MAS)?
It is the speed at which you use the most oxygen, which you can hold for about 4 to 7 minutes. It is measured with a field test. Training paces are given as a percentage of it: easy runs around 65%, threshold around 85%, short intervals around 100%.
Start fast or finish fast?
Going out too fast costs a lot later on. A “negative split” (slightly faster second half) is how most records are run: the tool works it out for you.
How to use it in 4 steps
- Pick what you want to find. Enter two of distance, time and pace (or speed): the third is computed at once. Common distances (5K, 10K, half, marathon) are one click away.
- Check your splits. The table gives your time at every kilometre. Choose an even pace or a negative split to start a little more cautiously.
- Print the pace band. The cut-out strip fits around your wrist or on your bib, with the key times in large print.
- Plan your sessions. Enter your MAS (maximal aerobic speed) to get easy, threshold and interval paces.
Worked examples
- Target: 50 minutes for 10K: a pace of 5:00 min/km, i.e. 12 km/h (8:03 min/mile). Halfway split at 25:00 with an even pace.
- Prediction: from that 50:00 10K, Riegel's formula (exponent 1.06) predicts about 1:50:20 for the half marathon and 3:50:00 for the marathon. For runners with little endurance background, use 1.08: the marathon becomes about 3:57.
- Paces for a MAS of 15 km/h: easy run at 65% = 9.75 km/h (6:09 min/km), threshold at 85% = 12.75 km/h (4:42 min/km), short intervals at 100% = 4:00 min/km.
Common mistakes
- Mixing up min/km and km/h: 6:00 min/km is not 6 km/h but 10 km/h. The tool always shows both.
- Ignoring the real distance: a marathon measured by a watch is often 42.4–42.6 km because of the racing line. Aim a few seconds per km faster than the theoretical pace.
- Trusting the prediction without specific training: the formula assumes preparation for the target distance (long runs for a marathon).
- Doing every run at race pace: most of your weekly distance should be run easy, clearly slower.
Going further
For trail races with lots of climbing, pace per kilometre means little: use the hiking time calculator (effort-kilometres) instead. For interval sessions, the interval timer runs your repetitions with beeps.
Guide written and checked by Erwan Hautot · updated 5 November 2026.