Swim Critical Swim Speed (CSS) Calculator
Critical Swim Speed (CSS) is an estimate of the fastest pace you can sustain in the water without fatiguing, often used as a threshold training pace. It is derived from two maximal time trials over different distances, classically 400 metres and 200 metres. The difference in distance divided by the difference in time gives your CSS in metres per second, which converts to a pace per 100 metres. Enter your two trial times below to find your CSS pace.
CSS formula
CSS speed = (D1 - D2) / (T1 - T2)
CSS pace per 100 m = 100 / CSS speed
where D = distance in metres, T = time in seconds
CSS uses the slope between two time trials to estimate sustainable speed. The longer trial is typically 400 m and the shorter 200 m, both swum maximally with full rest between.
Worked example
A swimmer covers 400 m in 360 seconds and 200 m in 170 seconds. CSS speed = (400 - 200) / (360 - 170) = 200 / 190 = 1.0526 m/s. CSS pace = 100 / 1.0526 = 95.00 seconds per 100 m, or 1:35 per 100 m.
Frequently asked questions
What is Critical Swim Speed?
Critical Swim Speed (CSS) is the theoretical fastest pace a swimmer can maintain continuously without exhaustion. It approximates the swimming equivalent of lactate threshold and is widely used to set threshold training paces.
Which distances should I use?
The classic CSS test uses a 400 m and a 200 m time trial, each swum at maximal effort with full recovery between. You can enter other distances if you used a different protocol, but 400 m and 200 m are the standard.
How do I train with CSS?
CSS pace is commonly used as a threshold reference. Threshold sets are swum at or near CSS pace, while easier swims are slower and short sprints are faster. Recalculate periodically as fitness changes.
Sources
- Swim England: national governing body resources.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.