Swim Speed and Critical Swim Speed (CSS) Calculator
Swim Speed and Critical Swim Speed (CSS) Calculator
Use our Swim Speed and CSS Calculator to find your critical swim speed, set personalized training zones, and estimate your race times. Enter two swim times below and get actionable insights instantly.
Understanding Your Swim Data
Enter your times for two swim distances (e.g., 200m and 400m). The calculator will estimate your Critical Swim Speed (CSS)—the pace you can sustain for long periods without excessive fatigue.
CSS helps tailor your swim training to your real fitness. Track progress, plan race strategies, and stay motivated with clear goals.
Zone 1 (Easy): CSS + 20-30%
Zone 2 (Steady): CSS + 15-20%
Zone 3 (Threshold): CSS + 10-15%
Zone 4 (Hard): CSS + 5-10%
Zone 5 (Very Hard): CSS to CSS + 5%
Calculate Your Swim Speed Training Zones
Swim Speed Training Zones
| Zone | Intensity | Swim Speed (mm:ss/100m or 100y) |
|---|---|---|
| 1 | Easy | - |
| 2 | Steady | - |
| 3 | Moderately Hard | - |
| 4 | Hard | - |
| 5 | Very Hard | - |
Ironman Triathlon Expected Swim Times
| Event | Finish Time |
|---|---|
| Half Ironman 70.3 (HIM) 1.2 mile swim | - |
| Ironman (IM) 2.4 mile swim | - |
Need help?
Swim CSS Calculator: FAQs
The Swim Speed and Critical Swim Speed (CSS) Calculator is a tool for swimmers and triathletes. It helps you measure your current swim fitness by calculating your CSS, which is the fastest pace you can hold for a long time without getting too tired. You enter your times for two different swim distances, and the calculator does the rest.
To calculate CSS, swim two time trials—usually 200 and 400 meters or yards. Enter your times into the calculator. It uses this formula:
CSS = ((Time for Longer Distance – Time for Shorter Distance) / (Longer Distance – Shorter Distance)) × 100
This gives you your sustainable pace per 100 meters or yards.
Training zones are target pace ranges based on your CSS.
CSS + 20–30% (recovery, basic endurance)
CSS + 15–20% (aerobic)
CSS + 10–15% (lactate threshold)
CSS + 5–10% (VO2 max)
CSS to CSS + 5% (speed work)
These zones help you train each aspect of your swimming: endurance, speed, and race readiness.
CSS is backed by research in exercise physiology. It’s closely related to your anaerobic threshold—the speed you can maintain before lactic acid builds up and causes fatigue. Coaches worldwide use CSS because it’s reliable and practical.