Free Tool
Critical Swim Speed Calculator — CSS Pace from 400 & 200
Swim a hard 400 and a hard 200 (same stroke, same pool). Enter both times to get your critical swim speed pace per 100, documented training zones, and projected times at CSS.
CSS pace /100
—
Critical swim speed
Speed
—
200 ÷ (T400 − T200)
Time difference
—
T400 − T200 (must be positive)
Enter your 400 and 200 times. Everything updates as you type.
Training zones
| Zone | Offset | Pace /100 |
|---|
Projected times at CSS
| Distance | Time |
|---|
How critical swim speed is calculated
Critical swim speed is a two-point field estimate from a maximal 400 and a maximal 200 (same stroke, same pool, full recovery between). The identities used here are:
- pacePer100Sec = (T400 − T200) ÷ 2
- speed = 200 ÷ (T400 − T200) (course units per second)
Example: T400 = 360 s (6:00), T200 = 170 s (2:50) → difference 190 s → CSS pace 95 s = 1:35 /100, speed ≈ 1.053 units/s.
The 400 must be slower than the 200. If the times are equal or inverted, the model is invalid and this tool reports an error instead of inventing a pace.
Documented training-zone offsets
Zone paces are CSS plus a fixed offset in seconds per 100. These are common coaching conventions for set design — not lab lactate or VO₂max thresholds.
| Zone | Offset | Typical use |
|---|---|---|
| Easy | CSS+6s/100 | Warm-up, recovery, aerobic base |
| Steady | CSS | Continuous endurance at threshold pace |
| Threshold | CSS | CSS-pace / threshold sets |
| VO2 | CSS−4s/100 | Hard aerobic / VO₂ intervals |
| Sprint | CSS−8s/100 | Speed / race-pace reps |
This tool does not invent stroke factors, turn penalties, or yard↔meter “course conversion” fudge. For pool pace, splits, and send-offs from a single swim, use the swim pace calculator.
How to run the test
Warm up thoroughly. Swim an all-out 400, rest fully (often 5–15 minutes depending on fitness), then an all-out 200 in the same stroke and the same pool. Enter both clock times here. Retest every few weeks after a consistent training block — one wild day is not a new threshold.
CSS is a field estimate for training pace, not a medical or lab VO₂ verdict. Open-water, current, and mixed-stroke sessions will not match a clean pool TT.