Swim pace calculator: your CSS and training zones
Enter a 400 and a 200 time trial and get your Critical Swim Speed, plus five training zone paces per 100 meters. No signup, nothing uploaded: the math runs right here in your browser.
Enter times as minutes:seconds, for example 6:00 and 2:50. Both swims at maximal effort, from a push, fully recovered between them.
Training zone paces per 100m
Methodology: CSS = (400m - 200m) / (T400 - T200), which works out to a pace per 100m of (T400 - T200) / 2. The concept comes from sports science research by Wakayoshi and colleagues in 1992, as documented by Topend Sports. The zone offsets follow the convention used by Effortless Swimming's CSS calculator: easy swimming 8 to 10 seconds per 100 slower than CSS, steady aerobic work 4 to 7 slower, threshold right at CSS, and speed work 2 to 5 faster in short reps. Zone 5 is maximal and is not paced off a threshold number.
What is Critical Swim Speed?
Critical Swim Speed is an estimate of your threshold pace: the fastest pace you could in theory hold for a long, steady swim of roughly 1500 meters. In my racing years nobody around me called it CSS, but every threshold set I ever swam was built on the same idea: find the edge you can barely sustain, then train right at it. The two-swim test gets you that number without a lactate meter or a lab.
The math works because the 400 and the 200 stress different energy systems. Subtracting one swim from the other cancels out most of the anaerobic contribution, and what is left is a clean estimate of your sustainable aerobic pace.
How do I do the CSS test?
Swim a maximal 400, recover fully with easy swimming and rest, then swim a maximal 200, both from a push in the same session. Two things make or break the test. First, pace the 400 honestly: if you fly the first 100 and crawl home, the numbers skew. Second, go into it fresh. A test at the end of a hard week tells you about your fatigue, not your fitness. Retest every 4 to 6 weeks and your zones keep up with you.
What if I do not have test times?
A recent hard 1500 gets you close: the pace you hold in a 1500 time trial sits right around your threshold, as the coaches at Foot Traffic Coaching point out. Enter your 1500 pace per 100 as a stand-in for the Zone 3 line and work outward from there. A hard 400 and a hard 200 pulled from recent training also work as a rough version of the test. Rough numbers now beat perfect numbers never, and you can always retest properly next week.
How accurate are these zones for triathletes?
The zones are estimates from two swims, not a lab report, but they map to racing surprisingly well. Foot Traffic Coaching pegs an IRONMAN swim around CSS plus 5 to 8 seconds per 100, a 70.3 swim around CSS plus 3 to 5, and an Olympic-distance swim right at CSS. If you are training for the swim leg of a triathlon, that gives you a concrete target pace for open-water race day, not just for pool sets.
How should I use these zones in a week of training?
Most of your volume belongs in Zone 2, one session a week around Zone 3, and a smaller dose of Zone 4 and 5 work with real rest. I break down what each zone trains and how to build that week in swim training zones explained.
One honest warning from someone who swam a lot of these sets: pace only tells you how fast you went, not what your stroke did to get there. Technique is usually the first thing to break in Zone 4 and 5 work, and you rarely feel it happening. That blind spot is exactly what I built SwimLytic for: film a lap and the app scores 8 technique metrics frame by frame, entirely on your phone.