The Value of 20 × 100m

Why this simple set reveals more about your swimming than almost any other.

Over recent weeks, our Wednesday Swim Squad have built up to a set of 20 x 100m. 4 sets of 5 x 100m, then 2 sets of 10 x 100m, then 1 set of 15 x 100m. This week it was 20 x 100m.

On paper, this set looks ordinary. Almost a bit dull. In reality, it’s one of the most revealing, educational, and valuable sets a coach can prescribe – and one of the most important sets a swimmer can learn to embrace.

Not because it’s the most demanding set you will ever do, but because it tells us things. 

When a coach programmes 20 × 100m – certainly when I programme it – it’s not about chasing speed or to build volume in a session. It’s a diagnostic tool disguised as a simple set. A benchmark-type of set if you like.

First, it tells us about pacing.

How a swimmer approaches the opening few 100s says a lot. Do they surge early, chasing pace, burning energy they’ll need later? Or do they show restraint, understanding that the set only really begins halfway through?

Then there’s repeatability.

We’re not looking for the fastest 100 of the night. We’re looking for the ability to reproduce the same swim again and again. Similar times. Similar stroke counts. Similar rhythm. Consistency is the what we’re looking here.

As fatigue creeps in, the set exposes technique under pressure.

This is where the truth appears. We see what breaks first – the catch, the breathing pattern, body position, or stroke rate. None of that is failure. It’s information. Clear, actionable feedback about what needs more work in training.

And then there is effort management.

Two swimmers may produce the same splits, but one does so with composure and efficiency, the other by muscling every length. The times look the same, but the demands are not. The how determines what you can repeat – and what you can’t.

And finally, there’s the mental side.

Does the swimmer stay engaged? Count properly? Make adjustments as the set unfolds? Or do they mentally drift once it becomes uncomfortable? Swimming, particularly on these longer sets, demands presence, not just fitness.

To a coach, 20 × 100m is rarely boring. It tells a story.

From the outside, a good set often looks… unremarkable.

The early 100s are almost too controlled. Stroke is long, breathing calm, technique smooth. There’s no rush to impress anyone.

The middle reps are where the set earns its value – and where many swimmers drop off, lose focus or pay the price for the early enthusiasm. In a well-swum set, reps 8 to 14 look almost identical to reps 3 to 7. Effort rises internally, but externally very little changes.

Late in the set, fatigue shows – but it doesn’t take over. The swimmer adapts rather than fights. A focus on being deliberate with technique, not ‘fatigue-sloppy’. Keeping the pressure on each pull, not slipping because that feels easier. 

The final reps aren’t a sprint and they’re not survival mode either. There’s still control. Still intent. Still more to give.

The times tell a calm story too. Minor drift is normal. Big spikes or collapses aren’t. Often the best sets finish within a second or two of the middle reps.

From the swimmer’s perspective, a good 20 × 100 is …

You feel held back early, worked but stayed composed in the middle and mentally engaged late. And you get out knowing you could have done more – not relieved that it’s finally over.

This is one of those sessions that gives back.

It teaches pacing you can trust. You learn, very quickly, what “too hard” feels like – and what sustainable really means. That instinct transfers directly to open water and racing.

It shows you your honest stroke. Not the fresh one. The one you’ll be using in the final third of a long swim. That awareness is powerful, and it’s the starting point for real improvement.

It builds confidence through control, not bravado. Finishing 20 × 100 well creates a quiet belief in your endurance.

It develops aerobic depth. This is where endurance swimmers are made. It isn’t glamorous, but it underpins everything from steady open-water swims to Ironman execution.

And it trains your mind. Staying present, counting, managing effort, and letting go of imperfect 100s is race rehearsal. The pool just gives you faster feedback.

If the set goes smoothly – great. If it falls apart – even better. Now you know exactly what to work on.

One of our swimmers told us about their experience of their well-executed set:

“It was tough, but great. No watch, just the clock and my tempo trainer set to 25s per length, bringing me in on or just under 1:40/100m. I was really pleased with that. Going off 2:05, which worked out nicely with the trainer being set to 25s. It just kept beeping and kept me honest. Fatigue started to build as we hit 10. The last 10 started to gradually slip, but I finished strong. Out of the 20, I think the slowest was about 1:43/100m, but at least 15 of the 20 were <1:40/100m.”

Progression isn’t simply about going faster. That will happen over time without forcing it. 

Good progression can look like:

• slightly less rest, same control
• subtle negative splitting
• extending the set rather than forcing intensity

Each type of progression asks a question of the swimmer:

Can you do the same thing a little more efficiently? A little more patiently? A little more tired?

It might sound like a boring set. It might sound like a big, relentless slog. But when you’ve worked your way up to it – like we have – 20 × 100m becomes something very different. It turns into a set you can settle into, manage, and even take satisfaction from. One where you finish knowing you swam well, not just that you survived. That’s when it stops being intimidating and starts being genuinely rewarding.