CSS calculator (critical swim speed)
Work out your CSS from the 400 m and 200 m tests and get your six swim paces in min/100 m. For triathlon and swimrun.
Updated 16 September 2026
CSS — critical swim speed — is the pace you could theoretically hold for a very long time without accumulating fatigue. It is the swimming equivalent of running threshold, and the anchor every set is built on.
The test is simple and fits in one session: 400 m all out, full recovery, then 200 m all out. Those two times are all it takes.
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How it is computed
The formula is direct: CSS = (400 − 200) ÷ (400 m time − 200 m time). It gives a speed in metres per second, then converted into a pace per 100 metres. It amounts to measuring the slope of your distance-time curve — what you hold as the effort lengthens.
The six zones follow as a percentage of that pace: recovery at 130-120 % of CSS time, endurance 120-113 %, tempo 112-106 %, threshold 105-100 %, VO2max 99-92 %, sprint 92-80 %. A higher percentage means a slower pace, since we reason in time per 100 m.
Two conditions for a valid test: genuinely full recovery between the two efforts (8 to 10 minutes), and a push start rather than a dive for both. A conservative 200 m artificially inflates CSS.
The plans built from this number
Now the plan that goes with it
These numbers are exactly the ones Endu uses to build a full preparation: phases, volume, detailed sessions, exported to your watch. 14-day trial, cancel any time.
Create my planFrequently asked questions
- Does my CSS apply in open water?
- The zones still apply, but your actual pace will be slower: expect 5 to 15 seconds per 100 m more depending on water state, visibility and how often you sight. In a wetsuit you get part of that back through flotation.
- How often should I retest?
- Every 6 to 8 weeks. In swimming, progress comes largely from technique and arrives in steps: testing too often shows nothing but noise.
- Why 400 and 200 m rather than 1,000 m?
- Because two distances are needed to draw a line, and this pair brackets threshold well without eating a whole session. A single 1,000 m would give an average pace, not a critical speed.
- Useful for swimrun?
- Yes, and it matters most there: in swimrun you swim with shoes on, often with a pull buoy, towing or being towed. Knowing your pool CSS gives the baseline; then add 10 to 20 seconds per 100 m for real conditions.