Free Tool

Critical Swim Speed (CSS) Calculator

Calculate your Critical Swim Speed and swimming training zones using the 400m/200m test protocol.

About Critical Swim Speed

Critical Swim Speed (CSS) represents your swimming lactate threshold pace. It's calculated from the difference between your 400m and 200m time trial times, providing an anchor point for all swimming training zones.

Heart Rate Methods
Karvonen Tanaka Gulati (Women) MAF 180 Joe Friel 80/20
Power & Pace
Coggan Power VDOT CSS (Swim)
By Sport
Running Cycling Swimming Triathlon

Auto-Calculate from Your Data

Upload Strava, Garmin, or Intervals.icu exports to estimate your zones

Strava vs Garmin vs TrainingPeaks: Zone Differences

Different platforms calculate training zones differently. Here's what each expects:

Strava
Zone System 5-zone (% Max HR)
Algorithm % Max HR
Customizable No

Uses simple % of max HR. Zones are fixed percentages. Cannot customize.

Garmin
Zone System 5-zone (customizable)
Algorithm % Max HR or % HRR
Customizable Yes

Supports both %Max HR and %HRR (Heart Rate Reserve). Fully customizable zones.

TrainingPeaks
Zone System 5 or 7 zone
Algorithm % LTHR (Joe Friel)
Customizable Yes

Uses LTHR-based zones. Supports pace, HR, and power. Highly customizable.

Polar
Zone System 5-zone
Algorithm % Max HR or % HRR
Customizable Yes

Uses either method. Includes sport profiles with different zone settings.

Related Methods