Pace Calculator

Pace is time per unit distance — typically minutes per mile or minutes per kilometer. Speed is distance per unit time — typically miles per hour or kilometers per hour. They're inversely related: 8 min/mile = 7.5 mph; 5 min/km = 12 km/h. Runners think in pace; cyclists think in speed; the calculator converts.

Mile-vs-kilometer conversion is the source of most pace-math errors. 1 mile = 1.609344 km. A 6:00/mile pace = 3:44/km. A 5:00/km pace = 8:02/mile. Doing this in your head mid-run is hard; many running watches let you toggle units to whichever you trained in. For race-day, pre-compute the splits in the unit the course is marked in.

Marathon pacing rule of thumb: even or slightly negative split (second half faster) almost always outperforms positive split (front-loaded). Most casual marathoners go out too fast in the first 5–10K and pay for it after mile 20. Disciplined pacing — first half at goal pace, second half at goal pace or slightly faster — produces better times for the same fitness.

Half marathon and shorter: pacing tolerance is wider, but the same principle holds — the cost of going out too fast (oxygen debt, glycogen depletion, lactate buildup) is asymmetric with the benefit. Even-split or slight-negative-split races tend to feel hard but produce strong final results.

Most well-designed running programs prescribe specific paces for specific workouts. Easy/aerobic pace (~70% of HRmax, conversational): roughly 60–90 seconds per mile slower than 5K race pace. Build aerobic base; spend most weekly volume here.

Threshold/tempo pace: roughly 5K race pace + 30–45 seconds. Roughly the pace you could sustain for an hour at maximum effort. Develops lactate threshold; do 20–40 minutes of continuous tempo or 4–8 minutes of cruise intervals.

VO2 max pace: roughly 5K race pace or slightly faster. Done as 3–5 minute intervals with equal recovery. Builds maximum oxygen uptake; do less of this — perhaps 4–8% of weekly mileage at most.

Runners often want quick mental conversions for common race distances. A 4:00/km pace covers 5K in 20:00, 10K in 40:00, half-marathon in 1:24:23, marathon in 2:48:46. A 5:00/km pace: 5K in 25:00, 10K in 50:00, half in 1:45:28, marathon in 3:30:56. A 6:00/km pace: 5K in 30:00, 10K in 1:00:00, half in 2:06:34, marathon in 4:13:08.

The marathon time isn't simply 2× the half-marathon time at the same pace — the longer distance means cumulative fatigue, glycogen depletion, and form breakdown that slow most runners 5–10% per mile after mile 20. Plan slower marathon paces than half-marathon paces for the same fitness level.