Swimming Intensity Zones: Complete Guide to Training by Pace
Quick summary: The 6 zones are A1 (recovery), A2 (aerobic base), A3 (endurance), TH (threshold/CSS), VO₂ (power) and LAC (speed). 60-70% of your volume should be A1-A2, 15-20% A3-TH and 10-15% VO₂-LAC.
Swimming intensity zones are effort ranges based on your CSS (Critical Swim Speed). Each zone targets a different energy system and produces specific physiological adaptations. Training in the right zone is what makes the difference between swimming kilometers without improving and actually progressing.
The 6 swimming intensity zones
| Zone | Name | % CSS | RPE (1-10) | Feel | Typical sets |
|---|---|---|---|---|---|
| A1 | Basic aerobic | 80% | 3-4 | Conversational, very easy | Warm-up, cool-down |
| A2 | Medium aerobic | 88% | 5-6 | Comfortably challenging | 10×200m, 5×400m r/15-20" |
| A3 | Intense aerobic | 95% | 6-7 | Demanding, controllable | 5×300m, 8×200m r/20-25" |
| TH | Threshold | 100% | 7-8 | Hard but sustainable | 8×100m, 5×200m r/15-20" |
| VO₂ | Max O₂ consumption | 108% | 8-9 | Very hard, heavy breathing | 8×100m, 6×150m r/25-30" |
| LAC | Lactate tolerance | 115% | 9-10 | Maximum, muscle burn | 12×50m, 8×75m r/30-60" |
Zone A1 — Basic aerobic (recovery)
Pace: CSS + 15-20 seconds per 100m. This is the active recovery zone. Used for warm-ups, cool-downs and regeneration sessions. Perceived effort is low: you can hold a conversation. Don't underestimate this zone — active recovery is fundamental for absorbing training load.
Zone A2 — Medium aerobic (base)
Pace: CSS + 8-12 seconds per 100m. This is where the aerobic base is built. Most training volume should be in this zone. It improves cardiovascular efficiency, mitochondrial density and the ability to use fat as fuel.
Zone A3 — Intense aerobic (endurance)
Pace: CSS + 3-5 seconds per 100m. Transition zone between aerobic work and threshold. Develops swimming-specific endurance and prepares the body for more intense efforts.
Zone TH — Threshold
Pace: exactly your CSS. This is the most important zone for improving performance. Training at threshold pace directly raises your CSS, which shifts all other zones to faster paces.
Zone VO₂ — Maximum oxygen consumption
Pace: CSS − 3-5 seconds per 100m. Develops maximum aerobic power. These are short, intense sets with controlled rest. Crucial for the final meters of any race.
Zone LAC — Lactate tolerance (speed)
Pace: CSS − 8-12 seconds per 100m. Maximum sustainable intensity over short distances. Improves lactate tolerance and top speed. Essential for the 100 freestyle.
How to distribute zones in a training plan
A well-structured training plan distributes volume across zones following this general rule:
| Zone group | % of weekly volume | Purpose |
|---|---|---|
| A1 + A2 | 60-70% | Aerobic base, recovery |
| A3 + TH | 15-20% | Threshold, specific endurance |
| VO₂ + LAC | 10-15% | Speed, power |
This distribution varies depending on the block phase: early weeks prioritize aerobic work while later weeks increase threshold and speed work.
The "gray zone": Without intensity zones, most swimmers fall into the gray zone — they swim too fast for aerobic and too slow for threshold. The result is fatigue without adaptation. Training by zones ensures every meter you swim has a clear physiological purpose.
Calculate your zones with Swimer
Swimer automatically calculates your 6 intensity zones from your CSS and applies them in every session of your plan. Calculate your CSS for free and get your zones instantly.
Paso a paso
- Calculate your CSS — Swim 400m and 200m at maximum effort and use the formula CSS = (D400 - D200) / (T400 - T200) to get your reference pace per 100m.
- Derive your 6 zones — From your CSS, calculate each zone: A1 (80% CSS), A2 (88%), A3 (95%), TH (100%), VO₂ (108%) and LAC (115%).
- Distribute weekly volume — Assign 60-70% of volume to A1-A2, 15-20% to A3-TH and 10-15% to VO₂-LAC for optimal polarized distribution.
- Train each zone in specific sessions — Alternate aerobic base sessions (A2), threshold (TH), speed (VO₂/LAC) and recovery (A1) throughout the week.
Preguntas frecuentes
How many intensity zones are there in swimming?
There are 6 main zones based on CSS: A1 (recovery, 80% CSS), A2 (aerobic base, 88%), A3 (endurance, 95%), TH (threshold, 100%), VO₂ (aerobic power, 108%) and LAC (speed/lactate, 115%).
How much volume should I swim in each zone?
The recommended distribution is: 60-70% of volume in zones A1-A2 (aerobic base), 15-20% in A3-TH (endurance and threshold), and 10-15% in VO₂-LAC (high intensity). This is known as polarized distribution.
How do I know what zone I'm swimming in?
Each zone has a pace per 100m calculated from your CSS. For example, if your CSS is 1:40/100m, your A2 zone would be approximately 1:54/100m. A watch or app like Swimer shows you the exact paces for each zone.
What happens if I always swim in the same zone?
Swimming at the same pace every session (usually zone A2-A3) produces stagnation. Your body adapts and stops improving. You need to vary stimuli: easy recovery sessions, threshold work and high-intensity sets.