Critical Swim Speed Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
100% FreeRuns in your browserBasic
Enter details
200 m time
:
400 m time
:
Result
Fill in the fields to see your result.
Estimate only — real results depend on terrain, weather, fatigue, conditions and individual fitness. Not medical or coaching advice.
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.
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 CSS used for?
It's a practical threshold pace for setting swim training intervals.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.