How to Find Your Swim Threshold Pace

The whole test is one subtraction

Swim a maximal 400. Rest about five minutes. Swim a maximal 200. Then:

CSS pace per 100 = (400 time − 200 time) ÷ 2

A 6:20 and a 3:00 gives (380 − 180) ÷ 2 = 100 seconds per 100 m.

You can do that on the poolside with wet hands and no app.

Why subtracting works

The first 200 m of any maximal swim is helped by something you can't keep spending — the fast start, and stored energy that runs out.

Subtract a maximal 200 from a maximal 400 and what's left is roughly how long the extra 200 m took once that advantage was already used up. That remaining pace is much closer to something sustainable, which is exactly what a threshold figure is meant to describe.

It's a neat piece of test design: two swims most people can manage, a watch, and no lactate meter.

Your CSS is slower than your 400 pace

This is the most useful thing on the page.

	Pace per 100
Your 400 averaged	95 s
Your CSS	100 s

Five seconds per 100 slower. That's the correct answer, not a disappointing one.

A 400 is swum partly on energy you cannot sustain. CSS estimates what remains when you cannot. People routinely take their 400 pace as their threshold, build sets around it, and find the second half falling apart — which reads as a fitness problem and is really a pacing one.

It holds across abilities, too. A swimmer going 5:00 and 2:20 gets a CSS of 80 against a 400 pace of 75 — the same five-second gap.

The gap is the measurement

CSS depends entirely on the difference between the two times, not on either one. Which means the protocol matters as much as the arithmetic:

• A hard 400 followed by a half-hearted 200 → falsely fast CSS.
• A conservatively paced 400 → falsely slow CSS.

There's a built-in check. Swim that 200 just ten seconds slow — 3:10 instead of 3:00 — and the gap between CSS and your 400 pace collapses to zero.

If your CSS comes out level with your 400 pace, one of the trials wasn't maximal. Redo it rather than training off it.

Test in the pool you train in

A 25 m pool gives twice as many push-offs as a 50 m pool over the same distance, and a push-off is faster than swimming.

So the same swimmer posts a quicker time short course. Test in a 25 m pool, then train in a 50 m one, and the sets will feel harder than your number suggests — easily misread as losing fitness when you've only changed pools. A 25 yard pool is shorter again.

The training paces around it

Common practice is roughly CSS + 2 s for longer aerobic work and CSS − 2 s for shorter, faster sets.

Those offsets are coaching convention rather than physiology, and they vary between coaches. Treat them as a starting point: if you're holding the pace comfortably to the end of a long set it's too easy, and if you're falling off it by the third repeat it's too hard.

The CSS figure is the anchor. The offsets around it are adjustable.

Retest, don't recalculate

Every four to six weeks is a common rhythm. CSS moves as fitness changes, and a figure from three months ago describes a swimmer who no longer exists.

Retesting also gives you something a single number can't — a direction. This month against last month tells you whether the training is working.

Do it under the same conditions: same pool, similar time of day, similarly rested. Otherwise you're measuring the conditions rather than yourself.

⚠️ On the test itself

Two maximal swims in one session is a significant effort, and no calculator can see you.

If you have a heart or lung condition, are returning from illness or injury, are pregnant, or simply haven't trained in a while — get the go-ahead from a clinician before testing rather than afterwards.

Never do a maximal time trial alone in open water. Stop for chest pain, dizziness or unusual breathlessness. A pace number is not worth finding out that way.