Critical Swim Speed calculator
Turn a 400 and 200 time trial into your estimated threshold pace, useful training ranges, and exact pace projections.
400 + 200 test
Enter your swims
Use two maximal, rested swims from the same session and course. The standard protocol is 400 followed by 200.
Long test
Maximal effortShort test
Fully recoveredResults and zones update instantly.
400m + 200m · SCM
Critical Swim Speed test
Estimated threshold pace
1:35.0- Long average
- 1:30.0 /100m
- Short average
- 1:25.0 /100m
- Test spread
- +5.0 sec/100m
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.
- 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.
How to run the 400/200 CSS test
Consistency makes the comparison useful: use the same pool, course, timing method, and similar conditions each time.
- 01
Warm up completely
Prepare as you would for a time trial, including easy swimming, technique work, and short pace builds.
- 02
Swim a maximal 400
Record the full time. Pace it hard and evenly enough that the result represents your best sustainable effort.
- 03
Recover, then race 200
Swim easy until recovered, then complete a maximal 200 in the same pool course and record the exact time.
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) = 1.05m/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.
Should I swim both tests on the same day?
For the most comparable result, swim both tests in the same course and session while rested, with enough easy recovery between them to produce a maximal 200.
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.
How often should I retest CSS?
Retesting every four to eight weeks is common when training is consistent. Also retest after a meaningful break or when threshold sets feel persistently too easy or too hard.
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.