The two-point test
Critical swim speed comes from the slope between two maximal efforts: (400 m time − 200 m time) ÷ 2 gives seconds per 100 m. A 6:00 400 m and a 2:50 200 m produce a CSS of 1:35 per 100 m. Any CSS swimming calculator runs that same division — the hard part is swimming both trials honestly, with adequate rest and no pacing off a friend.
Why not just use the 400 m time?
Because the 400 m includes a pacing and anaerobic contribution that you cannot sustain in a long set. CSS approximates the aerobic ceiling — the pace you could hold for roughly 1500 m — which makes it the right anchor for threshold work. It is the swimmer's equivalent of a running threshold test.Turning CSS into sessions
The training table gives five intensities: technique at CSS + 20 s, aerobic base at CSS + 8 s, threshold at CSS, VO2max at CSS − 6 s, and sprint at CSS − 12 s. Most weekly volume belongs at the first two; one or two threshold sets a week is enough to move the number.
Frequently Asked Questions
How often should I test CSS?
Every 6–8 weeks. Testing more often adds fatigue without adding information.
Do I need a 400 m pool?
No — any measured distance works, as long as the two trials are exactly 400 and 200 and both are maximal.
Why is my CSS slower than my 200 m pace?
Because 200 m is partly anaerobic. CSS deliberately sits below that, at a pace you can repeat for long sets.
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Privacy & your data
SwimRunPace calculates everything in your browser. Distances, times and body weights are never uploaded — there is no account, no server-side record and no training history stored between visits.