Swim pace calculator

Swim a 400 and a 200, put the times in, and get the pace per 100 m that coaches build sets around.

Your details

Your critical swim speed

2:08

per 100 m

400 m pace2:00 per 100 m
200 m pace1:53 per 100 m
Threshold sets, e.g. 10 × 100 m2:08 each, 15 seconds rest
Easy swimming2:18 or slower

CSS is roughly the pace you could hold for about 30 minutes. Retest every 6 to 8 weeks; dropping a couple of seconds is good progress.

The working

The CSS test

Critical swim speed is the 200 m you add going from 200 to 400 m: (8:00 − 3:45) ÷ 2 = 2:08 per 100 m. That is roughly the pace you could hold for half an hour.

Doing the test

Simple and repeatable

Warm up 200 to 400 m easy
Swim 400 m hard but even
Rest 5 to 10 minutes, easy swimming
Swim 200 m hard
Use the same pool each time

Using CSS

Example sets

Set	Pace
10 × 100 m, 15 s rest	at CSS
4 × 200 m, 20 s rest	CSS plus 2 to 3 seconds
Easy swimming	CSS plus 10 seconds or slower

Read with care

What this cannot tell you

Open waterWaves and sighting slow you down.
TechniqueStroke work often helps more than fitness work.
HealthFeeling faint in the water: stop at the side and get out.

Questions

Common questions

What is CSS in swimming?

Critical swim speed: a threshold pace from a 400 m and 200 m test.

What is a good swim pace per 100 m?

Many adult lap swimmers are between 1:45 and 2:30.

How often should I retest CSS?

Every 6 to 8 weeks.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Before you push on

New to exercise, or got a heart, lung or joint condition? Have a word with your GP or practice nurse first. Stop and get help if you feel chest pain, faintness or unusual breathlessness, and call 999 if it is severe.