How it works
Critical swim speed is the pace you could hold without tiring quickly — a swimmer’s threshold. Swim 400 m flat out, rest fully, then 200 m flat out. CSS per 100 m is (T₄₀₀ − T₂₀₀) ÷ 2.
Sets around CSS pace build the aerobic engine faster than anything else in the pool; easy and aerobic swims sit a few seconds slower, and short, hard repeats a few seconds quicker.
A worked example
A 400 m in 6:40 and a 200 m in 3:05 give a critical swim speed of 1:48 per 100 m — (400 − 185) ÷ 2 = 107.5 seconds — and these pool paces:
| Zone | Pace per 100 m | What it is for |
|---|---|---|
| Easy | 2:06–1:58 | Technique focus. |
| Aerobic | 1:57–1:52 | Steady sets, short rest. |
| Threshold | 1:51–1:46 | CSS pace — the main aerobic driver. |
| Vo2max | 1:45–1:40 | 50s and 100s, full effort. |
More free tools
- Race time predictor — a realistic time and pace for every distance from one race you have run.
- Running pace zones — six training paces from one recent race, anchored on threshold.
- Cycling power zones — six Coggan power zones from Functional Threshold Power.
- Swim pace zones — critical swim speed from a 400 m and a 200 m time trial, and four pool paces.
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