Critical Swim Speed (CSS) Calculator

What this tool does

Critical swim speed is the pace swimmers can in theory hold without piling up fatigue, found from two time trials: CSS = (400 − 200) ÷ (T₄₀₀ − T₂₀₀). Dividing the extra 200 m by the extra time cancels the effect of the push-off and the fast start, and what is left over is D′, a fixed distance you can swim above CSS. The idea was proposed in the early 1990s (Wakayoshi and colleagues) and coaches use it to set threshold sets — often described as roughly the pace you could hold for about 30 minutes. It needs a good rest between the two trials and an all-out effort in both; treat it as a training guide and re-test every month or two.

Formula

CSS = (400 − 200) ÷ (T₄₀₀ − T₂₀₀) m/s · pace per 100 m = (T₄₀₀ − T₂₀₀) ÷ 2 · D′ = 200 − CSS × T₂₀₀

Variables

Symbol	Meaning	Unit
`m4`	Your 400 m time — minutes	min
`s4`	Your 400 m time — seconds	s
`m2`	Your 200 m time — minutes	min
`s2`	Your 200 m time — seconds	s
`PS`	CSS pace per 100 m (seconds)	s
`PM`	CSS pace per 100 m — whole minutes	min
`PR`	CSS pace per 100 m — remaining seconds	s
`CS`	Critical swim speed	m/s
`DP`	Anaerobic distance capacity D′	m
`T15`	Time for 1500 m at CSS pace	min

Worked example

Your 400 m time — minutes6 min
Your 400 m time — seconds0 s
Your 200 m time — minutes2 min
Your 200 m time — seconds50 s

CSS pace per 100 m (seconds)95.0 s
CSS pace per 100 m — whole minutes1 min
CSS pace per 100 m — remaining seconds35.0 s
Critical swim speed1.053 m/s
Anaerobic distance capacity D′21.1 m
Time for 1500 m at CSS pace23.8 min

Limitations

The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
This tool is informational only. It does not diagnose, treat or replace a qualified health professional. Speak to a doctor before making decisions about your health.