Cycling and swimming pace calculator

Divide power in watts by body weight in kilograms. A 75 kg rider producing 250 W has 250 ÷ 75 = 3.33 W/kg, or 1.51 W/lb at 165 lb. Riders usually quote W/kg at functional threshold power (FTP), which Allen and Coggan describe as the highest power sustainable for about an hour and estimate as 95 % of the average from a 20-minute test.

SWOLF, from swim and golf, is the number of strokes plus the number of seconds for one pool length, and as in golf a lower score is better. Garmin's example is 15 strokes in 30 seconds, a score of 45. Because it depends on pool length and on how strokes are counted (Garmin counts one per full cycle of the watch arm), scores are comparable only for the same pool and device.

Divide 3,600 by the speed in km/h to get seconds per kilometre: 30 km/h is 120 s, or 2:00 per km, and 24 km/h is 2:30 per km. For pace per mile, divide 5,793.6 by the speed in km/h, because a mile is exactly 1.609344 km: 30 km/h is 3:13 per mile.

Multiply by 0.9144, because 100 yd is exactly 91.44 m. A pace of 2:00 per 100 m (120 s) becomes 109.7 s, about 1:50 per 100 yd. Going the other way, divide by 0.9144, so 1:30 per 100 yd is about 1:38 per 100 m. The yard is defined as 0.9144 m exactly (NIST SP 811).

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “250 W at 75 kg, 30 km/h for 40 km” is checked against Python 3.8 decimal: 250/75; 250/(75/0.45359237); 3600/30 s/km; 40/30 h.

Allen H, Coggan A. Training and Racing with a Power Meter, 3rd ed. VeloPress; 2019 — power-to-weight (W/kg); Garmin Swim 2 owner's manual — swim terminology (SWOLF); NIST SP 811 — 1 yd = 0.9144 m, 1 mi = 1609.344 m exactly.