CSS
Critical Swim Speed
CSS (Critical Swim Speed) is your threshold swimming pace—the fastest speed you can maintain without accumulating fatigue. Learn how to test and train with CSS.
Quick Answer
CSS — (Critical Swim Speed) is the swimming equivalent of lactate threshold—the fastest pace you can maintain aerobically without accumulating fatigue. It's calculated from time trials and used to prescribe swimming training zones.
What is Critical Swim Speed?
CSS represents your sustainable swimming threshold—the pace you could theoretically maintain for a long duration (30+ minutes) before fatigue forces you to slow down.
Think of CSS as swimming's equivalent to:
- FTP in cycling
- Threshold pace in running
- Lactate threshold across sports
CSS is expressed as time per 100m (or 100 yards).
Use our CSS Calculator to find your critical swim speed.
How to Calculate CSS
The Standard CSS Test
CSS is calculated from two time trials:
Test Protocol:
- Warm up thoroughly (400-600m easy)
- Test 1: Swim 400m all-out, record time
- Rest 10-15 minutes (easy swimming)
- Test 2: Swim 200m all-out, record time
CSS Formula:
CSS = (400m time - 200m time) / 2
Example Calculation
| Test | Time |
|---|---|
| 400m | 6:40 (400 seconds) |
| 200m | 3:00 (180 seconds) |
CSS = (400 - 180) / 2 = 110 seconds per 100m = 1:50/100m
Why the 400m/200m Test?
The test uses two distances because:
| Distance | What It Measures |
|---|---|
| 400m | Strong aerobic component |
| 200m | More anaerobic contribution |
The difference between them isolates the aerobic capacity—your CSS.
Alternative Test Distances
Some coaches use different combinations:
| Option | Tests | Notes |
|---|---|---|
| Standard | 400m + 200m | Most common |
| Alternative | 800m + 400m | Better for distance swimmers |
| Short | 200m + 100m | For sprint-focused |
CSS Training Zones
Once you know your CSS, calculate your swimming zones:
| Zone | % of CSS | Purpose |
|---|---|---|
| Recovery | < 75% | Easy swimming |
| Endurance | 75-85% | Aerobic base |
| Tempo | 85-95% | Sustainable hard effort |
| Threshold | 95-105% | CSS improvement |
| VO2 Max | 105-115% | Top-end speed |
| Sprint | > 115% | Anaerobic power |
Example Zones (CSS = 1:50/100m)
| Zone | Pace Range |
|---|---|
| Recovery | > 2:27/100m |
| Endurance | 2:10-2:27/100m |
| Tempo | 1:56-2:10/100m |
| Threshold | 1:45-1:56/100m |
| VO2 Max | 1:36-1:45/100m |
CSS Workouts
Threshold Sets (at CSS)
| Workout | Structure |
|---|---|
| Cruise intervals | 8 × 100m @ CSS, 10-15 sec rest |
| Threshold 200s | 6 × 200m @ CSS, 20 sec rest |
| Broken 400s | 4 × (100m @ CSS-5, 15 sec rest) |
CSS Development
| Workout | Purpose |
|---|---|
| 10 × 100m @ CSS | Build threshold endurance |
| 6 × 200m @ CSS-5 | Slightly faster than CSS |
| 3 × 400m @ CSS | Sustained threshold work |
Above CSS (VO2 Max)
| Workout | Purpose |
|---|---|
| 8 × 50m @ CSS-10 | Short, fast efforts |
| 5 × 100m @ CSS-8 | Build top-end speed |
CSS for Triathlon
Pacing with CSS
| Race Distance | % of CSS |
|---|---|
| Sprint (750m) | 100-105% |
| Olympic (1.5km) | 95-100% |
| Half Ironman (1.9km) | 90-95% |
| Ironman (3.8km) | 85-90% |
Longer swims require pacing below CSS to avoid early fatigue.
Open Water Adjustments
| Factor | Effect |
|---|---|
| Wetsuit | ~5% faster |
| Sighting | ~5% slower |
| Drafting | Up to 10% easier |
| Chop/current | Variable |
Pool CSS times typically need adjustment for open water racing.
Improving CSS
1. Technique First
CSS improvements come from:
- Better streamlining
- Efficient catch and pull
- Bilateral breathing
- Relaxed stroke
2. Volume
More swimming = better CSS:
| Weekly Volume | CSS Potential |
|---|---|
| < 5km | Limited improvement |
| 5-10km | Steady improvement |
| 10-15km | Strong improvement |
| 15km+ | Near optimal |
3. Threshold Training
Regular CSS-pace swimming:
- 2-3 threshold sessions/week
- Progressively increase volume at CSS
- Mix with VO2 Max work
CSS vs T-Pace
| Metric | Definition | Difference |
|---|---|---|
| CSS | 400m-200m calculation | Pure aerobic threshold |
| T-Pace | 1000m time trial / 10 | Practical threshold pace |
Both approximate the same threshold intensity. T-Pace is simpler but less precise.
Retesting CSS
When to Retest
| Situation | Frequency |
|---|---|
| Regular training | Every 6-8 weeks |
| After training block | At block end |
| Plateau suspected | After 4+ weeks |
| Before key race | 2-3 weeks out |
Signs CSS Has Changed
| Indicator | Interpretation |
|---|---|
| CSS sets feel easy | CSS has improved |
| CSS sets feel impossible | Fatigue or overreaching |
| Race performance change | Need new baseline |
Common Questions
Is CSS the same as race pace?
For short swims (400-800m), you can swim faster than CSS. For longer swims (1500m+), you should pace at or below CSS to avoid blowing up.
Why do my CSS times vary?
Pool conditions, fatigue, and testing consistency affect results. Always:
- Use the same pool
- Test when fresh
- Warm up consistently
- Give maximal effort
How much can CSS improve?
Beginner swimmers may improve 10-20% in a season. Experienced swimmers typically improve 2-5% per year with focused training.
Is CSS accurate for all swimmers?
CSS works best for swimmers with similar sprint and endurance abilities. Sprinters may find CSS underestimates their endurance, while distance specialists may find the opposite.