About the Critical Swim Speed Calculator
This critical swim speed (CSS) calculator finds your threshold swim pace from two time trials: a maximal 400 and a maximal 200, swum in the same session with a full recovery between them. It returns your CSS pace per 100 metres or yards, your CSS speed and a table of training paces built around it.
CSS is the swimming equivalent of a cyclist's FTP or a runner's threshold pace — roughly the fastest pace you can hold continuously for around 30 minutes. Triathletes, open-water swimmers and masters swimmers use it to set send-off times for threshold sets and to track fitness by repeating the test every 6–8 weeks.
The method uses the distance-time slope between the two trials, so it removes the effect of the push-off and start. Swim both trials in the same pool length and stroke (usually freestyle); the training pace offsets in the table are common coaching guides rather than fixed rules.
How to use the critical swim speed calculator
- 1Warm up well, then swim an all-out 400 and record the time.
- 2Recover fully (easy swimming for 5–10 minutes).
- 3Swim an all-out 200 and record the time.
- 4Choose metres or yards and enter both times.
- 5Use your CSS pace and the training table to set send-offs for your sets.
Formula and method
Critical swim speed is the slope of the distance-time line through your 200 and 400 time trials. Because both trials share the same start, push-off and turn costs, subtracting the 200 time from the 400 time leaves the time needed to swim 200 extra metres (or yards) at a sustainable pace. Dividing that difference by two gives the pace per 100.
CSS sits close to lactate threshold and is commonly treated as the pace you can sustain for roughly 30 minutes. Speed in m/s is converted from yards at 0.9144 m per yard. The zone offsets in the table (for example CSS + 5 s for endurance) are typical coaching ranges, not part of the CSS formula.
- T₄₀₀
- 400 time trial, seconds
- T₂₀₀
- 200 time trial, seconds
- CSS
- Critical swim speed (threshold pace)
Worked examples
400 m in 6:00, 200 m in 2:50
The 400 took 190 seconds longer than the 200, so the extra 200 m were covered at 95 seconds per 100 m: a CSS pace of 1:35/100 m or 1.053 m/s. Held for 1,500 m that would be 23:45.
Fast age-group triathlete: 5:00 and 2:20
The difference is 300 − 140 = 160 seconds, so CSS pace is 80 seconds (1:20) per 100 m and CSS speed is 200 ÷ 160 = 1.25 m/s.
Yard pool: 7:30 and 3:30
450 − 210 = 240 seconds for 200 yd gives 2:00 per 100 yd. In metres per second that is 182.88 m ÷ 240 s = 0.762 m/s.
Frequently asked questions
What is critical swim speed?+
Critical swim speed is a swimmer's threshold pace: the fastest pace you can sustain for about 30 minutes without rapidly accumulating fatigue. It is estimated from a 400 and a 200 time trial and is used to set threshold and endurance training paces.
How do I do a CSS test?+
After a thorough warm-up, swim a 400 as fast as you can evenly sustain, recover with 5–10 minutes of easy swimming, then swim an all-out 200. Some coaches put the 200 first; either order works as long as both efforts are maximal.
How often should I retest CSS?+
Every 6–8 weeks is typical during a training block, or after a big change in fitness. Retesting keeps your training paces accurate; if CSS sets start to feel easy, it is probably time.
Can I use a 25 yard pool?+
Yes. Choose yards and swim 400 yd and 200 yd. Your CSS pace will be per 100 yards. Yard pools have more turns per distance, so yard paces look roughly 10% faster than metre paces for the same swimmer.
Is CSS the same as my race pace?+
CSS is close to race pace for events lasting 20–40 minutes, such as a 1,500 m or Olympic-distance triathlon swim. Shorter races are swum faster than CSS and long-distance swims (half and full Ironman) a few seconds per 100 slower.