How to use the Swim Pace
- Enter the distance swum and total time.
- Choose metres or yards, since the two differ by about 9%.
- Read pace per 100 and the equivalent splits.
- Enter two time trials to estimate critical swim speed.
How the calculation works
Pace per 100 is total seconds ÷ (distance ÷ 100). Pool units matter: 100 yards is 91.44 metres, so a yard pace looks roughly 9% faster than the same effort in metres. Converting between short course and long course also matters, since a 25 m pool gives extra push-offs worth around one second per turn.
Critical swim speed approximates threshold pace from two time trials, typically 400 m and 200 m: CSS = (400 − 200) ÷ (t400 − t200), expressed as metres per second and converted back to a per-100 pace. It is the pace you can theoretically hold for a long continuous effort, and it anchors interval sessions — threshold sets sit at CSS, endurance work a few seconds slower, and speed work faster.
Pace/100 = total time / (distance / 100) ; CSS = (D₂ − D₁) / (t₂ − t₁)Source: Critical swim speed two-distance protocol as described in swimming coaching literature.
Worked example
A swimmer covers 400 m in 6:32 and 200 m in 3:04.
- 400 m = 392 s, 200 m = 184 s.
- CSS = (400 − 200) / (392 − 184) = 200 / 208 = 0.9615 m/s.
- Pace per 100 = 100 / 0.9615 = 104 s = 1:44.
- 400 m pace was 1:38 per 100, faster than threshold as expected.
Critical swim speed is about 1:44 per 100 m — the anchor pace for threshold sets.
Frequently asked questions
Why is my yard pace faster than my metre pace?+
A yard is 8.4% shorter than a metre, so the same speed yields a faster per-100 figure.
What is CSS used for?+
Setting threshold interval paces. Most structured swim sessions are prescribed relative to it.
Does pool length change my time?+
Yes. More turns means more push-offs, so short-course times are typically faster than long-course.
How often should I retest?+
Every six to eight weeks is enough; CSS moves slowly and frequent testing eats training time.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.