Critical Swim Speed (CSS) Calculator
Sets your threshold swim pace from two time trials.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
CSS (m/s) = (400 − 200) ÷ (T400 − T200) pace per 100 m = (T400 − T200) ÷ 2
T400 and T200 are the two times in seconds. The result is the speed you could in theory hold without tiring quickly, and it is close to threshold pace.
What to watch out for
Wakayoshi and colleagues (1992) tested the critical power idea in swimming, using six speeds in a flume. This two-distance version is a poolside shortcut. Warm up, swim 400 m all-out, rest for 5 to 10 minutes, then swim 200 m all-out. Do both in the same session, with push starts.
It is an estimate. Pace the 400 m evenly, because a fast start and a slow finish will spoil the figure.
Frequently Asked Questions
What is the CSS for a 7:00 400 m and a 3:10 200 m?
1:55 per 100 m, or 0.87 m/s.
How do I train with CSS?
Sets such as 10 × 100 m at CSS pace, with 10 to 20 seconds of rest, are a common way to work at threshold.