Critical Swim Speed (CSS) Calculator

Calculate your Critical Swim Speed from 400m and 200m time trials to determine your aerobic threshold pace for effective training.

400m Time Trial

sec

200m Time Trial

sec

Tab to move between fields

CSS Pace per 100m

97.5 sec

CSS Pace per 50m48.8 sec

CSS Speed1.026 m/s

400m Avg Pace /100m90 sec

200m Avg Pace /100m82.5 sec

How to Use This Calculator

Complete a 400m time trial and a 200m time trial in training.
Enter your 400m Time and 200m Time in seconds (convert from minutes:seconds if needed).
Review your CSS Pace per 100m — this is your Critical Swim Speed threshold pace.
Use CSS Pace to set your threshold intervals and aerobic training targets.
Compare your 400m and 200m average paces to assess pacing consistency.

How the result changes with 400m Time Trial

400m Time Trial: Your all-out 400m swim time in seconds. Swim after a proper warm-up for accurate results.
200m Time Trial: Your all-out 200m swim time in seconds. Rest at least 10-15 minutes between trials.

Identify Input Parameters
400m Time Trial = 360, 200m Time Trial = 165 = 2 input(s) provided
Calculate CSS Pace per 100m
CSS Pace per 100m
97.5 = 97.5
Calculate CSS Pace per 50m
CSS Pace per 50m
48.8 = 48.8
Calculate CSS Speed
CSS Speed
1.026 = 1.026

Engine last updated August 6, 2026. Checked against 2 independently-derived tests — how we verify calculators.

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