Free tool
CSS calculator. Critical swim speed.
Swim a 400 m and a 200 m all out, type the two times and get your CSS pace per 100 m. Free, no sign-up.
What it is
Critical swim speed, in plain words.
Critical swim speed, CSS for short, is an estimate of the fastest pace you can hold over a long swim without slowing down. Many coaches use it as your threshold pace.
You get it from two timed swims, a 400 m and a 200 m, both all out. The difference between the two times, divided by 2, is your pace per 100 m.
(6:00 − 2:50) ÷ 2 = 1:35
The test
How to do the CSS test.
All you need is a pool and a way to time yourself. Start both swims the same way, for example with a push off the wall, so the two times compare.
- Warm up. Swim easy for a while and add a few short, faster lengths, so you are ready to go hard.
- Swim 400 m all out. Freestyle, at the fastest pace you can hold to the end. Note your time.
- Rest fully. Swim easy or stay still until your breathing is back to normal.
- Swim 200 m all out. Freestyle again, as fast as you can hold to the end. Note your time.
- Use the times. Type them in the calculator above.
In training
How swimmers use the CSS pace.
The CSS pace is a reference, not a rule. How you use it depends on your level and your goals.
- Target times. Your CSS pace per 100 m, 50 m or 25 m gives you a time to aim for on each repeat.
- Threshold sets. Repeats close to your CSS pace with short rest, such as 100 m repeats, are a common way to train endurance.
- Easier and harder. Easy swimming and technique work go slower than your CSS pace; short sprints go faster.
- Progress. Repeat the test every few weeks in the same pool. A faster CSS means faster target times too.
Your CSS, in your plan.
In the SwimSensei app you can add your CSS pace, or your 400 m and 200 m times, to your plan settings. The sessions ahead are then built around that pace.