Race Shape › Calculators › CSS swim calculator
CalculatorCSS swim calculator
Swim a 400 m and a 200 m time trial, type both times, and get your Critical Swim Speed — your threshold pace per 100 m — with every training zone and a rough Midmar Mile time.
| Zone | Pace | What it is for |
|---|---|---|
| Recovery | 2:09/100 m | Easy enough to think about your stroke rather than your lungs. |
| Endurance | 2:04/100 m | Steady. You could keep going for a long time. |
| CSS | 1:59/100 m | Threshold. Hard but repeatable across a whole set. |
| VO₂max | 1:56/100 m | Faster than threshold, in short reps with real rest. |
| Speed | 1:53/100 m | Short and quick. Technique first, effort second. |
How CSS is worked out
Critical Swim Speed is roughly the fastest pace you can hold without tiring quickly — the swimming equivalent of threshold. The formula is simple: subtract your 200 m time from your 400 m time and divide by two. That gives seconds per 100 m. A swimmer who does 400 m in 7:30 and 200 m in 3:32 has a CSS of 1:59 per 100 m.
Do the test fresh, after a proper warm-up: the 400 m first, then rest for five to ten minutes, then the 200 m. Swim both as evenly paced as you can. A 400 m that dies in the last 100 m gives you a CSS that is too slow.
Using the zones
Recovery and endurance swimming are about technique and volume. CSS sets are threshold work — hard but repeatable across a whole set, usually 100–400 m reps on short rest. VO₂max and speed are faster and shorter. In open water expect to be slower than your pool pace: no walls to push off, sighting costs you, and a wetsuit helps less than people hope.
The honest bit about swimming
For most amateur swimmers, improvement comes from technique rather than fitness. A plan can prescribe the work; it cannot see that your hips are sinking or your catch is dropping. If you can afford one thing, buy a few sessions with a coach on the pool deck.
Common questions
What is a good CSS?
Around 2:00 per 100 m is a solid recreational swimmer, 1:40 a strong club swimmer, and sub-1:30 competitive.
How long will the Midmar Mile take me?
Roughly your CSS pace for 1.6 km plus about 6% for open water — the calculator shows it. Even pacing beats a fast start in a mass start.
How often should I retest CSS?
Every six to eight weeks, or when your sets start feeling easy at your current paces.
Is CSS the same as threshold pace?
Close enough to train by: it is the swimming equivalent, hard but sustainable for long sets. More on CSS.
Put this calculator on your club’s website
Free for running clubs, coaches, race organisers and bloggers. Copy the code below into any page that accepts HTML — WordPress, Wix and Squarespace all do — and your members get the same live css swim calculator, with nothing to install and no sign-up.
Keep the credit line under it, please: that is the only thing we ask in return.
Related
Build a swim plan from your CSSFree to build. See your whole block before you pay.