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Critical Swim Speed

CSS calculator & swim zones.

Derive Critical Swim Speed from two time trials, get five training zones in pool intervals, and predicted 70.3 and Ironman swim splits.

distance (m)
time mm:ss
distance (m)
time mm:ss
Your CSS
1:40per 100m

Swim training zones

Z1 · Easy
2:00 – 2:10per 100m
Z2 · Steady
1:55 – 2:00per 100m
Z3 · Mod-Hard
1:50 – 1:55per 100m
Z4 · Hard
1:45 – 1:50per 100m
Z5 · Very Hard
1:40 – 1:45per 100m
70.3 swim split
35:24
Ironman swim split
1:14:02

Frequently asked

What is Critical Swim Speed (CSS)?

CSS is your lactate-threshold pace in the water — the fastest pace you can hold for 30-60 minutes. It anchors every swim training zone.

How do I test it?

After a thorough warmup, swim two all-out time trials at different distances (e.g. 200 then 400, with 10-15 min rest). Plug in distance and time for each — CSS is the velocity between them.

What distances should I use?

200/400 m, 100/200 yd, or 300/600 m. The longer trial should be roughly 2× the shorter one, and both should land between 2-8 minutes.

How accurate are the 70.3 / IM swim splits?

They assume open water with wetsuit and decent pacing. Real splits depend on chop, drafting and wetsuit fit.

How often should I retest CSS?

Every 6-8 weeks during a block, or at the start of every race-specific phase.

From the coach

Swim CSS is the test. The block is the upgrade.

1:1 coaching builds the swim block that drops your CSS — open water, pool, or wetsuit-legal.

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