The formulas
Pace, time and distance are tied together by one relationship: pace = time ÷ distance. Rearranged, time = pace × distance and distance = time ÷ pace. Choose which of the three to find and the calculator solves it from the other two. Speed is the inverse of pace: km/h = 3,600 ÷ pace in seconds per kilometre.
A mile is exactly 1.609344 km, so pace per mile is pace per km × 1.609344, and mph is km/h ÷ 1.609344. A half marathon is 21.0975 km and a marathon is 42.195 km. A solved time is rounded to the nearest whole second, the way race times are quoted.
A worked example
You run 10 km in 50:00. That is 3,000 seconds over 10 km, so 300 seconds per km — a pace of 5:00 per km. Per mile it is 300 × 1.609344 = 482.8 seconds, which rounds to 8:03. Speed is 3,600 ÷ 300 = 12.00 km/h, or 7.46 mph.
Even and negative splits
An even split holds one pace for the whole distance. A negative split runs the second half faster than the first. You choose how much faster, in seconds per km or mile. The first half is set at the average pace plus half that difference and the second half at the average minus half, so the finish time does not move.
For 10 km in 50:00 with the second half 5 seconds per km faster, the first 5 km run at 5:02.5 per km and the last 5 km at 4:57.5. The halves take 25:12.5 and 24:47.5, which add to 50:00. If the halfway point falls inside a split, that split is part one pace and part the other.
Splits are shown to the whole second. To make them add up, each split is the difference between two rounded running totals, so a split can differ from its neighbour by a second without the finish time ever moving. The final split is shorter than a full km or mile when the distance is not a whole number.
Predicting a race time
The predictor uses the published formula popularised by the engineer and runner Pete Riegel: T2 = T1 × (D2 ÷ D1)1.06, where T1 is a known time over distance D1 and T2 is the predicted time over D2. A 25:00 5K, for instance, gives 1,500 × 21.06 ≈ 3,127 seconds for 10K, which is 52:07.
It is an estimate, not a promise. It cannot see your training, the course, the weather or the day, and it assumes you are prepared for the longer distance. Long distances usually run slower than the formula says for runners who have not trained for them — a marathon predicted from a 5K is the clearest case — so read longer predictions as a best case.
Related tools
To time laps while you run, use the stopwatch and enter a distance per lap to see pace for every lap. For structured workouts, try the interval timer. To add or subtract the times you get, use the time calculator.
Questions
How do I calculate running pace?
Divide your time by your distance. A 10 km run in 50:00 is 3,000 seconds ÷ 10 = 300 seconds per km, or 5:00 per km. Enter the distance and time here and choose Pace; you also get pace per mile, speed in km/h and mph, and predicted times for other distances.
What pace do I need to run a 5K in 25 minutes?
5:00 per kilometre. 25:00 is 1,500 seconds, and 1,500 ÷ 5 = 300 seconds. That is 8:03 per mile (300 × 1.609344 = 482.8 seconds) and a speed of 12 km/h.
How do I convert min/km to min/mile?
Multiply by 1.609344. A pace of 5:00 per km is 300 seconds × 1.609344 = 482.8 seconds, or 8:03 per mile. To go the other way, divide: 8:00 per mile is 480 ÷ 1.609344 = 298.3 seconds, which is 4:58 per km.
What pace is a 4-hour marathon?
Four hours is 14,400 seconds over 42.195 km, so 341.3 seconds per km: about 5:41 per km, or 9:09 per mile, at roughly 10.55 km/h. Set the distance to Marathon, enter 4:00:00 as the time and solve for pace.
What is a negative split?
A negative split means the second half of a run or race is faster than the first. Some runners use it as a pacing plan to avoid starting too fast. Choose Negative split in the splits panel and enter how many seconds per km or mile faster the second half should be; the finish time stays on target.
How accurate is the Riegel race predictor?
It is an estimate. The formula, T2 = T1 × (D2 ÷ D1)^1.06, fits how times tend to scale with distance, but it cannot see your training, the course or the weather. Predictions for much longer distances are usually optimistic for runners who have not trained for them.