Critical Swim Speed Calculator (CSS)
Use our Critical Swim Speed Calculator to determine your ideal swim training pace. Enter your 200m and 400m time trials to unlock your CSS value, improve aerobic capacity, and plan smarter workouts.
How to Use the Critical Swim Speed (CSS) Calculator
The CSS Calculator determines the pace you can theoretically maintain indefinitely, ideal for setting training zones and improving endurance. Here’s how to use it:
Calculating CSS
Enter times and distances for two time trials (e.g., 200m and 400m). The calculator computes your CSS pace and training zones.
Example: 200m in 2:30, 400m in 5:15 → CSS ≈ 1:22.5/100m.
Unit Selection
Choose between meters and yards. The calculator adjusts calculations and provides pace metrics in your chosen unit.
CSS Calculation Formulas
Understand the math behind Critical Swim Speed:
CSS Formula
CSS (sec/100) = (Distance₂ – Distance₁) ÷ (Time₂ – Time₁) × 100
Calculates the pace per 100 meters/yards you can sustain aerobically.
Speed Calculation
Speed (m/s) = 100 ÷ CSS (sec/100m)
Speed (km/h) = Speed (m/s) × 3.6
Speed (mph) = Speed (m/s) × 2.237
Training Zones
Zone Pace = CSS × Percentage (e.g., 105% for Easy, 90% for Threshold)
Adjusts CSS pace for different training intensities.
Unit Conversions
1 meter = 1.094 yards
1 yard = 0.914 meters
Applied when switching between metric and imperial units.
Frequently Asked Questions
What is Critical Swim Speed (CSS)?
CSS is the pace you can theoretically maintain indefinitely without exhaustion, representing your aerobic threshold.
How do I improve my CSS?
Focus on aerobic endurance with interval sets at or near CSS pace, combined with technique work to reduce drag.
What distances should I use for CSS?
Typically, 200m and 400m time trials are used, but other pairs like 100m/200m or 400m/800m can work if times are accurate.
How often should I test CSS?
Test every 6-8 weeks during consistent training to track aerobic improvements.
Does CSS differ in pool vs. open water?
Pool CSS is typically faster due to controlled conditions and push-offs. Open water CSS may be slower due to currents and navigation.