Critical Swim Speed (CSS) Calculator
Your critical swim speed and CSS pace per 100 m or 100 yards from a 400 and a 200 time trial: CSS = (400 − 200) ÷ (T400 − T200).
Result…
How it's calculated
CSS (distance per second) = (400 − 200) ÷ (T400 − T200) CSS pace per 100 = 100 ÷ CSS = (T400 − T200) ÷ 2 1 yd = 0.9144 m
Critical swim speed (CSS) is a swimmer's threshold pace, worked out from two maximal time trials: a 400 and a 200, with a rest between them. The extra 200 metres of the longer swim, divided by the extra time it took, gives a speed you could in theory hold for a long time; in practice it is close to the pace of an all-out 1500 m. Swim Smooth, a swim coaching company, sets its training zones from it, and the idea comes from the "critical speed" model in sports science. The pace per 100 is simply the difference between the two times divided by two. The test works in a yards pool too: swim 400 and 200 yards and the pace comes out per 100 yards.
Your numbers, step by step
About the Critical Swim Speed (CSS) calculator
This critical swim speed calculator turns two time trials, a 400 and a 200, into your CSS: the threshold speed in metres per second and the pace per 100 m and per 100 yards. The rule is CSS = (400 − 200) ÷ (T400 − T200), so the pace per 100 is simply the gap between the two times divided by two. Swim Smooth describes CSS as the pace you can sustain over 1500 m at maximum effort, worked out from a 400 m and a 200 m time trial, and builds its training zones on it. Pool swimmers and triathletes use it to set interval paces and to track fitness from one test to the next. The test works in a yards pool too. The steps show each time in seconds, the division and the conversion between metres and yards, so you can check every figure.
How to use it
- Choose the pool unit in “Test distances in”: metres or yards.
- After a good warm-up, swim the 400 flat out and evenly paced, and type the time into “400 time trial” as minutes:seconds, like 6:40.
- Rest or swim easily for 5–10 minutes, then swim the 200 flat out and enter it in “200 time trial”.
- Read your CSS pace per 100, the same pace in the other unit, and your speed in m/s.
- Use the CSS pace as the target for threshold sets, and retest after a block of training.
Worked example
A swimmer does the test in a 25 m pool: 400 m in 7:05 (425 s) and 200 m in 3:22 (202 s). CSS = 200 ÷ (425 − 202) = 200 ÷ 223 = 0.897 m/s. The pace is 223 ÷ 2 = 111.5 s per 100 m, shown as 1:52 /100 m, which is 1:42 per 100 yards. For comparison, the 400 itself was swum at 1:46.3 /100 m and the 200 at 1:41.0 /100 m, and holding CSS pace for 1500 m would take about 27:53.
Updated
Questions people ask
How do I calculate my CSS from a 400 and a 200?
Subtract the 200 time from the 400 time in seconds and divide 200 by the answer to get your speed. For the pace per 100, just halve the difference: a 400 in 7:05 and a 200 in 3:22 differ by 223 s, so CSS pace is 111.5 s, about 1:52 per 100.
What is the formula for critical swim speed?
CSS = (400 − 200) ÷ (T400 − T200), with times in seconds. It gives metres per second in a metre pool or yards per second in a yard pool, and 100 ÷ CSS turns it into a pace per 100.
Why does the calculator say my 400 time should be more than twice my 200 time?
Because the 400 should be swum at a slower pace per 100 than the 200. If it was faster, the 200 was probably not flat out, or the times were mixed up, and the formula would give a CSS faster than either swim. Repeat the 200 or check the times.
Can I do the CSS test in a yards pool?
Yes. Swim 400 yd and 200 yd, choose yards, and the pace comes out per 100 yards. The calculator also converts it to per 100 m using 1 yd = 0.9144 m, so at the same speed a pace per 100 yd takes 0.9144 times as long as a pace per 100 m.
What are the limits of a CSS result?
CSS is an estimate from two efforts on one day, so pacing, fatigue and turns all move it. It describes pool swimming, not open water with waves or a wetsuit. For a training plan built around a health condition or an injury, talk to a coach or a doctor. For running paces, see the pace calculator.