Critical Swim Speed Calculator

Calculates critical swim speed threshold pace from 200m and 400m time trials.

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200 m time

400 m time

Result

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Estimate only — real results depend on terrain, weather, fatigue, conditions and individual fitness. Not medical or coaching advice.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

Swim 200 m and 400 m time trials.
Enter both times to get your CSS pace.

How it works

CSS pace per 100 m = (T₄₀₀ − T₂₀₀) ÷ 2 — your sustainable threshold pace.

Worked example

200 m 3:00, 400 m 6:20. CSS = (380 − 180) ÷ 2 = 1:40 /100 m.

Common mistakes

Swimming the two test efforts on different days or fatigue levels — do them in one session for a valid CSS.
Going out too hard on the 400 m — pace it evenly so the result reflects true threshold.

Tips

Times are estimates only — not medical or coaching advice. Always train safely and follow event rules.
Distances are editable: choose “Custom…” to enter your own course lengths.

Frequently asked questions

What is critical swim speed?

CSS is roughly your swimming threshold — the fastest pace you can hold without fatiguing rapidly — found from the difference between a 400 m and a 200 m time trial.

How do I use CSS in training?

Set threshold sets at your CSS pace and easy swims a few seconds per 100 m slower; retest every 4–6 weeks as you improve.

What is CSS used for?

It's a practical threshold pace for setting swim training intervals.

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Cite this tool

Zerdly. (2026). Critical Swim Speed Calculator. Retrieved from https://www.zerdly.com/tools/critical-swim-speed-calculator/

Tip: Enter any known values to calculate the remaining results.

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