Free swim tool
Critical Swim Speed Calculator
Your CSS is the pace you could hold for a hard 1500m, your swimming threshold. It is the single most useful number for pacing races and setting training that actually works. Two swims and you'll know yours.
How to do the test
- Warm up properly. 400 to 600m easy with a few build efforts, so the test is a true max.
- Swim 400 all-out and record the time.
- Recover fully. 4-5 minutes of total rest.
- Swim 200 all-out and record the time.
- Enter both into the calculator below. Retest every 6 to 8 weeks to track your progress.
Do both efforts in the same session, ideally in a pool with a clock. Pace them evenly rather than sprinting the first length.
Enter your test
Two maximal swims, one honest number.
Maximal effort, mm:ss.
Maximal effort, mm:ss.
Threshold pace, mm:ss.
Your training paces
Your CSS is a baseline and a progress marker, not a pace to chase every session. These ranges show where the work sits relative to it, per 100m. They are calculated as a percentage of your threshold speed, so they line up with the swim zones in TrainingPeaks. Feel is how the zone should actually feel, with the matching RPE out of 10, for the lengths where nobody is watching a clock.
| Zone | Pace per 100m | Feel | What it's for |
|---|
Predicted race swim ranges
Estimated ranges for your CSS level, accounting for how pace naturally fades as the distance grows, not a flat pace multiplied out. Open water, wetsuit and conditions shift the real number.
Try this on our triathlon event strategy tools →What your two swims suggest
Treat this as a marker, not a target
One number on one day. Its value is what it tells you next time: retest the same way every 6 to 8 weeks, or after a block of focused work, and the change is the thing worth reading. A CSS that has not moved in four months is information too.
The formula comes from Wakayoshi et al. (1992), who validated critical swimming velocity from two maximal efforts against measured blood lactate thresholds.
Don't go it alone.
Your CSS is one number in a much bigger picture. Real progress comes from a coaching team in your corner, mapping and running the whole journey with you: training, strength and conditioning, nutrition and race-day strategy, built around your life and adjusted as you go. Let us get you to the start line ready, and across the finish line in the best possible shape.
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