Swim Pace & CSS Calculator
Pace per 100, plus critical swim speed from two trials.
Work out Swim Pace & CSS. Pace per 100, plus critical swim speed from two trials. Free, with no account and nothing to install.
Swim pace
1:52 /100 m
Critical swim speed 1:38 /100 m
Critical swim speed is the gradient of the line through two time trials — the extra 200 metres divided by the extra time. It approximates the fastest pace you can hold without accumulating lactate, which makes it the pool equivalent of a running threshold test and the anchor for most structured swim training. It needs two distances. A single time cannot give it, and this returns nothing rather than a guess when only one is entered. Swim both trials rested and genuinely hard, on separate days or with a long recovery between. A 400 swum conservatively produces a CSS that is too fast to train at, which is worse than not having the number.
How the Swim Pace & CSS Calculator works
Pace per 100 metres from any swim, and critical swim speed when you supply a 400 m and a 200 m time trial. CSS approximates the fastest pace you can hold without accumulating lactate, which makes it the anchor for structured swim training.
Also known as: pace per 100m swimming · critical swim speed test · css swim pace calculator · how fast am i swimming
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
What is critical swim speed?
The gradient of the line through two time trials — the extra 200 metres divided by the extra time. It approximates your lactate threshold pace and is the swimming equivalent of a running threshold test.
How do I test my CSS?
Swim a 400 m and a 200 m, both rested and genuinely hard, on separate days or with a long recovery between. CSS is 200 divided by the difference in seconds, in metres per second.
Why does the calculator need two distances?
Because a single time cannot separate speed from endurance. The whole method rests on comparing two efforts of different lengths, so it returns nothing rather than a guess when only one is entered.
How do I train using CSS?
Threshold sets are swum at CSS pace, endurance work a few seconds per 100 slower, and speed work faster. Because CSS is a pace rather than a heart rate, it works in a pool where heart rate monitoring is awkward.
Why is my pool pace faster than my open-water pace?
Push-offs. Every turn gives several metres of free speed, which adds up to a substantial difference over a long swim — usually 5 to 10 seconds per 100 metres, before sighting and chop are counted.
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