Critical Swim Speed (CSS) calculator
Calculate CSS from a 400 and 200 swim test. Get threshold pace per 100 metres or yards, practical training-zone ranges, and projected times.
How to run a reliable 400/200 CSS test400 + 200 test
Enter your swims
Use two maximal, rested swims from the same session and course. The standard protocol is 400 followed by 200.
Follow the full test protocolLong test · usually 400
Maximal effortShort test · usually 200
Fully recoveredResults and zones update instantly.
CSS updated: 1:35.0 per 100m.
400m + 200m · SCM
Critical Swim Speed test
Estimated threshold pace
1:35.0- Long test
- 400m · 6:00
- Short test
- 200m · 2:50
Training zones
Practical starting ranges built around your estimated CSS.
What the result means
The separation between the two tests is in a useful range for a first CSS estimate. Zone ranges are starting points, not individualized or medical advice.
Plan your next CSS retest- CSS speed
- 1.05 m/s
- Course
- SCM · 25m
| Distance | Time at CSS pace |
|---|---|
| 25m | 0:23.8 |
| 50m | 0:47.5 |
| 100m | 1:35.0 |
| 200m | 3:10.0 |
| 400m | 6:20 |
| 800m | 12:40 |
| 1500m | 23:45 |
One useful benchmark
What CSS tells you—and what it does not.
CSS estimates aerobic threshold from two maximal efforts. It is especially useful for setting repeatable threshold sets and checking whether training pace changes over time.
It is not a diagnosis, a fixed physiological boundary, or a guaranteed race prediction. Use the zone ranges as starting points and adjust them using repeat quality, recovery, and coaching context.
Before you calculate
The result is only as good as the two swims.
Both efforts need to be maximal, in the same course, with the same start and timing method, and enough recovery between them for a genuine 200. The guide has the full warm-up, pacing, and recovery protocol.
The formula
Two efforts isolate sustainable speed.
CSS speed = (long distance − short distance) ÷ (long time − short time). The result is distance per second, which Swimio converts into pace per 100.
Example
400m in 6:00 + 200m in 2:50
(400 − 200) ÷ (360 − 170) = 1.0526m/s
CSS = 1:35 /100m
From threshold to workout
Translate CSS into splits and send-offs.
The pace calculator turns your CSS into repeat targets, changing pace profiles, cumulative splits, and practical send-offs for the pool.
Open pace calculatorRecommended workflow
- 1. Test: calculate CSS from two time trials.
- 2. Plan: choose the target distance and pacing profile.
- 3. Train: use the exact splits in a structured session.
Critical Swim Speed questions
What is Critical Swim Speed?
Critical Swim Speed, or CSS, is an estimate of the fastest pace you can sustain aerobically without steadily accumulating fatigue. Swimmers often use it as a practical threshold pace for training.
How is CSS calculated from a 400 and 200?
Subtract the 200 distance from the 400 distance, then divide by the difference between the two times in seconds. That produces speed. Swimio converts it into a pace per 100 metres or yards.
Does the CSS calculator work in yards?
Yes. Select SCY and the result stays in yards. Select SCM or LCM for metres. The calculator avoids implying a performance conversion between courses.
Can I calculate CSS with distances other than 400 and 200?
Yes, the calculator accepts any custom long and short pair. Keep the same pair every time you test, though, or the results are not comparable — 400 and 200 is the standard choice.
Is CSS a race-time predictor?
No. CSS estimates threshold pace, not a guaranteed race result. Starts, turns, drafting, distance, fatigue resistance, and race conditions all affect performance.