The arithmetic behind pace
Pace is time per unit distance, so the calculator converts your hours, minutes and seconds into total seconds and divides by the distance in kilometres (miles are first multiplied by 1.609344). That gives seconds per km, which is formatted as M:SS. Pace per mile is seconds per km × 1.609344. Speed is the reverse ratio: kilometres divided by hours for km/h, and km/h ÷ 1.609344 for mph.
Worked example
10 km in 50:00 is 3,000 seconds ÷ 10 = 300 seconds per km, so 5:00 per km. Per mile that is 300 × 1.609344 = 482.8 seconds, shown as 8:03 per mile. Speed is 10 ÷ 0.8333 h = 12.00 km/h (7.46 mph). Holding 5:00/km, the finish-time panel gives:
| Race | Distance used | Predicted time |
|---|---|---|
| 5K | 5 km | 25:00 |
| 10K | 10 km | 50:00 |
| Half marathon | 21.0975 km | 1:45:29 |
| Marathon | 42.195 km | 3:30:59 |
Why predictions are optimistic
The race times are simple multiplication: pace × distance. They assume you could hold the same pace at every distance, which almost nobody can. The jump from a 10K effort to a marathon typically costs a lot of pace through glycogen depletion and muscle fatigue, so a 3:31 marathon forecast from a 50-minute 10K should be read as an upper bound. The page does not apply an endurance model such as Riegel's formula, so it will not adjust for the fall-off.
Common mistakes
- Entering 90 in the minutes field. Minutes and seconds are capped at 59. For 1 hour 30 minutes, put 1 in hours and 30 in minutes.
- Mixing the unit. If your watch recorded 6.2 miles, set the unit to miles. A 10 in the distance box with kilometres selected is a very different run.
- Using GPS distance for track times. A 5K road race may be measured slightly long by a watch because of corners, so your calculated pace can look faster or slower than the official result.
- Reading pace and speed as interchangeable. A pace of 5:00/km is 12 km/h. Treadmill displays often show speed, so use km/h or mph as a cross-check against the speed boxes here.
Pace targets pair naturally with fuelling: the TDEE calculator estimates daily calories, and training dates can be counted with the date difference calculator.