Two timed swims are enough: a 400 metres and a 200 metres, both flat out. The gap between the two times gives your critical swim speed, and from there your training paces per 100 metres.
It's the reference method in swimming, and it has a rare advantage: it needs no sensor, no heart rate monitor and no laboratory. A stopwatch at the poolside covers it.
CSS pace per 100 m—
Your training paces
| Zone | / 100 m |
|---|
Projected times at CSS pace
| Distance | Projected time |
|---|
What critical swim speed is
The reasoning is simpler than it sounds. Over 200 metres, part of your speed comes from reserves that empty quickly. Over 400 metres, that part counts proportionally less. So the time difference between the two distances measures what you can produce sustainably, without the contribution of the short reserves.
Concretely: the extra 200 metres of the 400 get swum at your sustainable pace. Dividing 200 by the time gap gives that speed in metres per second, and a hundred divided by that speed gives the pace per 100 metres.
What critical speed approximates is the pace you can hold for about thirty minutes. For many swimmers it lands near 1500 metre race pace. It plays the role in swimming that threshold plays in running: the anchor point from which every other intensity gets prescribed.
The two-test protocol
Both swims happen in the same session, and the order matters.
Warm up for twenty minutes, with a few short accelerations to open up. Swim the 400 metres flat out, evenly paced: going out too hard over that distance will distort both numbers. Recover for ten very easy minutes. Then swim the 200 metres flat out.
Three details decide whether the result is valid. Start in the water, pushing off the wall, not diving: a dive start buys about a second and a half that has nothing to do with your swimming speed. Both swims have to be honest efforts, because a 200 held back narrows the gap and inflates the CSS artificially. And the same pool serves for both, since a 25 and a 50 don't ask for the same number of turns.
If the 400 comes out faster per 100 than the 200, the calculator refuses to answer. That isn't a bug: it means one of the two swims wasn't flat out.
The paces that follow
The offsets are expressed in seconds per 100 metres around the CSS, and they're common practice.
Endurance gets swum at CSS plus 6 to 10 seconds. It's the pace of long sets, the one that builds aerobic capacity, and it's where most of the volume belongs.
Tempo gets swum at CSS plus 2 to 4 seconds. Firm enough to count, sustainable enough to string sets of 200 or 400 together.
Threshold is the CSS itself. Sets that work it are typically 100s or 200s with short recovery, ten to twenty seconds, because a long recovery turns the session into something else.
Speed gets swum at CSS minus 2 to 5 seconds, over 50 to 100 metres. It's the most technically demanding zone: as soon as speed rises, most people's stroke falls apart.
Why swimming gets steered by pace, not heart rate
Three reasons, and they stack.
Immersion and the horizontal position lower heart rate by ten to thirteen beats for the same effort, compared with the same intensity on land. Heart rate zones calculated for running are therefore wrong in the water, and applying them leads to swimming too hard while believing you're swimming easy.
Water is cooler than air, which limits cardiac drift and makes the signal less readable.
And the measurement itself is difficult: a wrist sensor underwater loses contact with every arm stroke, and a strap under a swimsuit isn't comfortable. Pace per 100 metres, meanwhile, reads off a wall clock and never lies.
Technique comes before the engine
This is the most important difference between swimming and running, and it changes how these paces should be used.
In running, air resistance is negligible at amateur pace: improving means improving the engine. In water, resistance dominates everything, and it rises very steeply with speed. Cutting your drag by a few percent, by fixing a head position or legs that sink, buys more seconds per 100 metres than months of fitness.
The practical consequence is uncomfortable for anyone who likes numbers: in an adult who learned to swim late, the main lever is almost never the intensity of the sets. It's technical work, drills, and lengths swum slowly while thinking about one thing at a time. The CSS then serves to measure progress, not to cause it.
Reference points per length
| Profile | Per 100 m, front crawl | Per 100 yd |
|---|---|---|
| Can swim, no built technique | 2:15 to 2:45 | 2:03 to 2:31 |
| Regular recreational practice | 1:50 to 2:10 | 1:41 to 1:59 |
| Good technique, consistent training | 1:30 to 1:45 | 1:22 to 1:36 |
| Club level | 1:15 to 1:25 | 1:09 to 1:18 |
These bands describe an adult swimming front crawl at a sustainable pace, not a sprint. The gap between the first two rows closes mostly through technique, not through volume.
FAQ
Is CSS the swimming equivalent of running threshold?
It's the functional cousin: the anchor point from which other intensities get prescribed. Physiologically it isn't the same measurement, and CSS has the advantage of needing no equipment beyond two stopwatch readings.
Do both tests really have to be the same day?
Yes, in the same session with ten minutes of recovery. Doing them two days apart introduces a difference in freshness that lands entirely inside the time gap, and therefore inside the result.
Why push off instead of diving?
Because a dive start buys roughly a second and a half, which comes from the glide and not from your swimming speed. On a gap between two swims, that second and a half distorts the CSS noticeably.
Can I test over 200 and 100 metres instead?
It's possible and less reliable: the shorter both distances are, the more the fast reserves weigh in, and the more the result overstates your sustainable pace. The 400 and 200 pairing is the best compromise for an adult swimmer.
How often should I retest?
Every six to eight weeks. In a swimmer working on technique, CSS can move quickly, sometimes five seconds per 100 metres in two months, which makes quarterly retesting too rare.
Does CSS work in open water?
As an intensity reference yes, as a time prediction no. Open water adds current, swell, navigation and swimming in a pack, all of which cost or return far more than a few seconds per 100 metres.
Can I use a heart rate monitor for swimming?
You can, and the signal is less useful than in running: heart rate sits ten to thirteen beats lower for the same effort, wrist measurement drops out often, and pace per 100 metres is both simpler and more reliable.
How many sessions a week to improve at swimming?
Three is the threshold where technique starts to stabilise, because the stroke is quickly forgotten. Two maintain it. One does almost nothing for technique, even if it remains worth having for other reasons.
What if my pool is 25 yards?
The Meters or Yards switch handles it, and it earns its place: most North American pools are 25 yards. The tests then get swum over 400 and 200 yards, and paces display per 100 yards. The formula is indifferent to the unit, since it divides 200 lengths by the time gap. A CSS in yards doesn't compare with a CSS in metres, a yard being 0.91 of one.
Paces don't make a season
CSS tells you how fast to swim. It doesn't tell you how many sessions, or how to place swimming alongside the rest of your training, and that's where weeks come apart. The Vitality Archetype test measures your profile across 5 dimensions in 27 questions and 3 minutes, then generates a week calibrated for your recovery and your available time, adjusted with the feedback from each session.