A running pace calculator that works in every direction. Give it any two of pace, finish time and distance, and it solves for the third — then it goes further, laying out even splits for each kilometre or mile and predicting your finish time across every common race distance from 1 km up to 50 km. It is built for anyone planning a goal race, pacing a long run, checking treadmill numbers, or simply converting a min/km pace into min/mile and back.
How it works
Everything reduces to one relationship: distance = speed × time, or equivalently pace = time ÷ distance. When you choose pace, the tool divides your finish time by your distance. When you choose time, it multiplies your pace by the distance. When you choose distance, it divides the time by the pace. The field you are solving for is disabled so you always provide exactly two knowns, which removes the classic mistake of accidentally over-specifying the problem.
Internally the calculator converts your figures to a canonical pair — total seconds and distance in kilometres — so it can show you four consistent readings at once: pace per km, pace per mile, km/h and mph. Switching the unit selector never changes the underlying maths; it only changes how the answer is presented, so a 5:00 min/km pace always reads as 8:03 min/mile, 12.00 km/h and 7.46 mph.
The splits table walks the distance one unit at a time and reports the
cumulative elapsed time to reach each marker at a steady effort, including a final
partial split for fractional distances such as the 0.195 km tail of a marathon.
The race predictions use Peter Riegel’s well-known endurance formula,
T2 = T1 * (D2 / D1) ^ 1.06, which scales a known effort to other distances while
accounting for the small, predictable slowdown that comes with going longer.
Worked example
Suppose you run 10 km in 50:00. Pace = 3000 s ÷ 10 = 5:00 min/km, a speed
of 12.0 km/h. Switch to miles and the same effort is 8:03 min/mile at
7.46 mph. The splits table shows you should pass 5 km at 25:00 and 8 km at
40:00 to stay on track. Riegel then predicts a half marathon of roughly
3000 * (21.0975 / 10) ^ 1.06 ≈ 1:48:30 and a marathon near
3000 * (42.195 / 10) ^ 1.06 ≈ 3:46. Treat the marathon figure as optimistic
unless your long-run endurance matches the speed.
Formula note: pace = time ÷ distance; speed = distance ÷ time; predicted time
T2 = T1 × (D2 ÷ D1)^1.06. Marathon = 42.195 km, half marathon = 21.0975 km, and 1 mile = 1.609344 km exactly.
Every calculation runs locally in your browser — no numbers are ever uploaded.