How this works
Pace is the time it takes you to cover one unit of distance — usually expressed as minutes per kilometre (min/km) or minutes per mile (min/mi). It's the standard way runners describe effort, because unlike speed it scales naturally with how long it takes to finish a given race: a 5:00/km pace will get you across a 10K in 50 minutes, no matter your gear or terrain. The calculator solves for whichever of {pace, time, distance} you leave blank, given the other two. Switching the unit toggle converts between min/km and min/mi cleanly (1 mile ≈ 1.609 km), and the speed line shows the same effort in km/h and mph for cyclists or treadmill displays. The race-distance preset row uses your current pace to predict how long you'd take to finish a 5K, 10K, half marathon (21.0975 km), and marathon (42.195 km).
The formula
pace is in minutes (or seconds) per unit of distance; time is the total elapsed time; distance is in the same unit as pace. Mixing kilometres with min/mi (or vice-versa) gives nonsense — toggle the unit selector instead. The marathon distance is fixed by IAAF rules at 42.195 km (26.2188 mi) and the half at exactly half that.
Example calculation
- Distance: 10 km. Time: 0:50:00. Find pace.
- pace = 50 min ÷ 10 km = 5:00 /km.
- Convert to min/mi: 5:00 × 1.609 = 8:03 /mi.
- Speed: 60 / 5 = 12 km/h ≈ 7.46 mph.
- Holding 5:00/km, predicted marathon time = 5:00 × 42.195 = 3:30:58.
Frequently asked questions
Can I use this for cycling or walking?
Yes — the math is identical for any steady-effort sport. Cyclists usually think in km/h or mph rather than min/km, but the speed line at the bottom of the result shows both, so you can read whichever you prefer. Walkers use the same min/km convention as runners; a brisk walking pace sits around 12:00–14:00/km. Open-water swimmers and rowers occasionally use min/100m or split/500m, which this calculator doesn't directly model — but you can fake it by entering distance in km and treating it as 100m units.
How accurate is the marathon prediction from my pace?
It assumes you can hold today's pace for the full distance, which is generous. In practice, almost everyone slows down at longer distances — especially past about 2 hours, when fuelling and muscle glycogen become limiters. A common rule is the Riegel formula: T₂ = T₁ × (D₂/D₁)^1.06. So a 50-minute 10K predicts ~3:48 marathon by Riegel, not 3:30. Use the calculator's linear extrapolation as a "if everything goes perfectly" ceiling and shave at least 5–10% off for races over 90 minutes.
Why does converting min/km to min/mi look weird?
Because miles are longer than kilometres, the per-mile pace is always slower-looking (a bigger number) than the same effort per km. The conversion factor is 1.609344 km per mile, so 5:00/km becomes 5 × 1.609 = 8.046 min ≈ 8:03/mi. Don't flip the multiplication — multiplying min/km by 0.621 (the inverse) would give you min/km converted to a half-baked metric. Always pick the larger number going from km to mi.
Should I train at a single pace or vary it?
Vary it — almost every coach's plan does. The classic split is "80/20": ~80% of weekly volume at easy pace (well below threshold, conversational), ~20% at hard pace (intervals, tempo runs, race-pace work). Running every session at the same moderate pace plateaus quickly because it doesn't challenge either the aerobic base or the top end. Use the calculator to set the pace for each workout, and pair it with the Heart Rate Zones calc on this site to make sure your "easy" days are actually easy.