critical swim speed calculator
Critical Swim Speed (CSS) Calculator
Choose your pool, then enter your 400/200 time trials in metres or yards. Get Critical Swim Speed and five training zones in the selected pool unit.
How to use it
- 1
Select the pool you used. Warm up, then swim 400 m or 400 yd as an even, maximal effort and record the time.
- 2
Rest fully (5–10 minutes of easy swimming), then swim 200 in the same pool unit all-out and record that time.
- 3
Enter both times as m:ss — the 400 time goes in the first field, the 200 in the second.
- 4
Read your Critical Swim Speed per 100 m or per 100 yd, matching your selected pool.
- 5
Use the five-zone table to pace recovery, aerobic, threshold, race-pace, and sprint sets in the displayed pool unit.
CSS is your threshold swim pace
Critical Swim Speed is the swim's answer to FTP on the bike and threshold pace on the run: the fastest pace you hold in a steady aerobic state before fatigue compounds, roughly the pace you'd sustain in a long, hard continuous swim. Anchoring your sessions to CSS instead of a guessed effort turns 'swim hard' into a number you pace to the second per 100 m — and gives every other discipline a threshold to match.
The 400/200 test does the math for you
CSS is the slope of the distance-versus-time line through two maximal efforts. Swim a 400 m and a 200 m time trial fresh, and the speed that separates them is the speed you hold once the anaerobic kick of the short swim drops out: speed = (400 − 200) ÷ (T400 − T200). This calculator converts that speed straight into a pace per 100 m, so you skip the spreadsheet and the unit conversions.
Five zones, built from your CSS
Your CSS pace seeds five training zones using the Swim Smooth offset convention — recovery and aerobic swimming sit a few seconds slower than CSS, threshold sets land right on it, and race-pace and sprint work run faster. These are the exact zones PaceBeats writes your swim sets against, so the pace targets you read here are the pace targets your plan prescribes.
Common questions
How do I do a CSS swim test?
Warm up thoroughly, then swim a 400 m time trial as an even, maximal effort from a push start and record the time. Recover with 5–10 minutes of easy swimming, then swim a 200 m time trial all-out and record that. Enter both into the calculator. Do the test rested in a pool you know, pace the 400 evenly rather than sprinting the first 100, and re-test every four to six weeks to track fitness.
What's the difference between CSS and threshold pace?
They describe the same idea — your sustainable threshold — from two angles. CSS is derived from the 400/200 distance-time slope, which lands close to your true critical speed, while 'threshold pace' is sometimes set from a single 1,000 m or 30-minute time trial. Both target the aerobic-anaerobic boundary; the 400/200 protocol just removes most of the anaerobic contribution that a single all-out effort leaves in, so CSS is the cleaner anchor for pacing endurance swim sets.
Related tools
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Next step
Turn these numbers into an adaptive plan.
PaceBeats combines the relevant calculations with your training history, constraints, and deterministic checks to build and adapt your week.