About this tool
This running pace calculator solves whichever of pace, time, and distance you do not know from the two you do. Enter a distance (or pick 5K, 10K, half marathon, or marathon) and a finish time to get your pace; enter a pace and a distance to get your predicted finish; or enter a pace and a time to see how far you would run. Results are given as pace per kilometer and per mile, speed in km/h and mph, and a table of even splits at every kilometer or mile so you can check yourself against the clock during a race. Below that, the Riegel formula projects your equivalent finish times at other race distances, from 1,500 m to the marathon, from the single performance you entered. It is for runners planning a race, setting treadmill speeds, pacing a long run, or working out what a target time actually feels like per kilometer.
How it works
Pace, time, and distance are linked by time = pace × distance, so any one can be solved from the other two: pace = time ÷ distance and distance = time ÷ pace. Everything is converted to seconds and kilometers internally; one mile is exactly 1.609344 km, a half marathon is 21.0975 km, and a marathon 42.195 km. Pace per mile equals pace per km × 1.609344, and speed in km/h is 3,600 ÷ pace in seconds per km. Splits are simply the pace multiplied by each whole kilometer or mile, with the final partial split scaled to the remaining distance. Race predictions use Peter Riegel's 1981 formula T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known time over distance D1 and the exponent 1.06 describes how pace slows as the distance grows; a 50-minute 10K therefore predicts a marathon of 3,000 s × 4.2195^1.06, about 3 h 50 min. The formula assumes you are equally well trained for both distances.
Frequently asked questions
How accurate is the Riegel race predictor?
For distances between about 1,500 m and the marathon, and for runners who have trained appropriately for the target distance, Riegel's formula is typically within 2 to 3 percent, which is a few minutes over a marathon. It tends to be optimistic when predicting a long race from a short one, because most people have not built the endurance the formula assumes; predicting from a half marathon to a marathon is more reliable than from a 5K. Other models such as Cameron's or the Daniels VDOT tables give similar figures.
What is a good running pace?
It depends entirely on the runner. Recreational runners commonly finish a 5K between 25 and 35 minutes, which is roughly 5:00 to 7:00 per km (8:00 to 11:15 per mile). A 10K under 50 minutes (5:00/km) and a marathon under 4 hours (5:41/km) are common milestone goals. Elite men run marathons at about 2:55 per km and elite women at about 3:15 per km. Easy training runs should feel conversational, typically 60 to 90 seconds per km slower than 10K race pace.
How do I convert pace per mile to pace per kilometer?
Divide the pace per mile by 1.609344. An 8:00 per mile pace is 480 seconds ÷ 1.609344 = 298 seconds, or 4:58 per km. Going the other way, multiply pace per km by 1.609344: 5:00 per km is 300 × 1.609344 = 483 seconds, 8:03 per mile. The calculator shows both at once so you do not have to convert during a race.
Should I run even splits?
For most road races even or slightly negative splits (second half marginally faster) produce the best times, because starting too fast burns glycogen and accumulates fatigue that costs more later than it gained early. The splits table gives the even-pace clock times at each kilometer or mile; many runners aim to be within a few seconds of these through the first two-thirds of the race and then push. Hilly courses and wind justify uneven splits, so adjust for the course profile.