How to use this pace calculator
- Distance: type the distance and pick kilometres or miles, or tap a preset for 5K, 10K, half marathon or marathon.
- Time: fill in the hours, minutes and seconds boxes.
- Read your pace, your speed and the predictor table below them. All three update as you type.
Nothing is submitted anywhere. The arithmetic runs in your browser.
How pace is calculated
Pace is time divided by distance. The calculator adds your hours, minutes and seconds into one number of seconds, divides by the distance, and prints the answer as minutes and seconds per unit (mm:ss), which is how runners talk. Speed is the same division turned round: distance divided by time, in km/h and mph.
Take the values the page loads with, 5 km in 25 minutes. That is 1,500 seconds over 5 km, so 5:00 per kilometre, 8:03 per mile, 12.0 km/h and 7.5 mph.
What the predictor table is doing
The five rows under your results come from Riegel’s formula, T2 = T1 × (D2 ÷ D1)1.06. Pete Riegel, an engineer and marathoner, published it in Runner’s World in 1977 after fitting world-class results from 800 m up to the marathon.
The exponent carries the whole idea. If human endurance were flat, the exponent would be 1.0 and doubling the distance would double the time. It is 1.06 instead, and 21.06 is 2.085, so doubling the distance multiplies your time by 2.085. Per kilometre that is roughly 4.2% slower each time the distance doubles.
So the default 25-minute 5K predicts 7:31 for a mile, 52:07 for 10K, 1:55:00 for a half and 3:59:46 for a marathon. Riegel put two limits on it. It fits efforts lasting about 3.5 to 230 minutes, and it assumes you have actually trained for the longer distance. A fast 5K does not hand you a sub-four marathon on its own.
Why your watch and the race clock disagree
Certified road courses are not measured along the line you run. Under World Athletics Technical Rule 55, the course is measured along the shortest possible route an athlete could legally take, with a calibrated bicycle, and a short course prevention factor of 1.001 is built into the calibration so the course can never come out short. That factor adds one metre per kilometre, or about 42 m over a marathon (AIMS course measurement).
You do not run that line. You swing wide on bends, cross to a drinks table, weave past slower runners in the first mile. Your watch dutifully records every extra metre and then divides your time by the bigger number, which flatters your pace. Log 42.9 km for a marathon and a four-hour finish reads as 5:36 per kilometre on the watch against 5:41 on the official 42.195 km. Add the GPS error itself, which is worse under trees and between tall buildings, and the gap widens. The practical rule: for a pace you can compare with race results and training targets, type the official distance into the box, not the number on your wrist.
Common race distances
- 5K: 5 km (3.107 miles)
- 10K: 10 km (6.214 miles)
- Half marathon: 21.0975 km (13.109 miles)
- Marathon: 42.195 km (26.219 miles)
The marathon looks like an odd number because it is an imperial one in disguise. The distance was fixed at 26 miles 385 yards, which converts to 42,194.99 m. Race calendars round it to 42.2 km and the presets above do the same, a difference of 5 m that moves your pace by well under a second per kilometre.
The predictor table underneath uses the unrounded figures, 21.0975 km and 42.195 km, so its five rows stay consistent with published race results. Parkrun, run at 5K every Saturday morning, is the distance most people end up measuring themselves against.
Using pace to plan your training
Most plans are written in pace rather than time, because pace still means something whether the run is 20 minutes or two hours. Once you know the pace you hold for 5K, the rest of the week falls out of it. Easy runs sit around 60 to 90 seconds per kilometre slower than 5K pace, slow enough to hold a conversation, and they make up the bulk of the mileage in almost every plan you will meet. Tempo work sits nearer 10K to half marathon pace. Long runs stay easy, sometimes with the last few kilometres lifted towards marathon pace.
Do the sums before you go out. Guessing at pace mid-run is how a good session turns into a slog.
Frequently asked questions
How is running pace calculated?
Pace is total time divided by distance. This tool turns your hours, minutes and seconds into total seconds, divides by the distance, and shows the result as mm:ss per kilometre and per mile.
How do I convert pace per mile to pace per kilometre?
Multiply pace per mile by 0.6214, or divide pace per kilometre by 0.6214 to go the other way. One mile is 1.60934 km, so the per-mile figure is always the larger of the two. An 8:00 mile is a 4:58 kilometre.
What pace do I need for a four-hour marathon?
Four hours is 14,400 seconds over 42.195 km, which is 5:41 per kilometre or 9:09 per mile. Tap the Marathon preset and enter 4 hours to see it.
How accurate is the race time predictor?
Treat it as a ceiling rather than a forecast. Riegel's formula is usually within a few per cent for runners whose training matches the target distance, and it drifts optimistic when it stretches a short race out to a marathon, because it knows nothing about your longest run, your fuelling or the heat on the day.
Why does my watch show a faster pace than my official race result?
Because it recorded a longer distance. Courses are certified along the shortest legal line plus 0.1%, and you ran wider than that, so your watch divides the same time by more kilometres.
What is a good running pace?
The one you can hold for the distance you have entered. Plenty of recreational runners cover a kilometre in 6 to 7 minutes, around 10 to 11 minutes per mile, and trained club runners are considerably quicker. Judge it against your own recent times rather than anyone else's.