Triathlon swim training: how to train the swim leg

The swim is the shortest leg of a triathlon and almost never the one that wins it. But it is the leg most likely to wreck the other two, and that changes what you should be training for.

Last updated 11 August 2026 · 9 min read

Swim distances and what each one demands

The four standard triathlon swims are 750m (sprint), 1500m (Olympic or standard), 1.9km or 1.2 miles (half-Ironman and 70.3), and 3.8km or 2.4 miles (full Ironman). Each one is a different physiological problem, not just a longer version of the last.

A 750m swim is close to a hard continuous effort: you can start uncomfortable and hang on. A 3.8km swim is an aerobic grind where going 5 seconds per 100m too hard in the first kilometre costs you an hour of misery later in the day. The training that prepares you for one is not the training that prepares you for the other.

Those times assume a wetsuit-legal open water swim with a reasonably straight course. Add a few minutes for a choppy sea swim, poor sighting, or a non-wetsuit race.

What you are actually training for

The swim is roughly 10 to 20% of your race, the larger share at the shorter distances. You are training to exit the water fresh, not to set a swim personal best, because the time you can gain by swimming harder is small and the time you can lose by arriving on the bike wrecked is large.

Run the numbers on a 70.3. Improving your swim from 40 minutes to 36 is a four minute gain, genuinely worth having. But swimming those same 36 minutes at an effort you cannot sustain, so that you climb onto the bike with your heart rate pinned and your shoulders cooked, routinely costs more than four minutes over the following five hours. Age-group races are full of people who swam a good split and then rode badly for reasons they never connected back to the water.

So you are not chasing raw speed. You are chasing an efficient stroke you can hold for the full distance at an aerobic effort, plus the open water skills that stop you wasting energy on things unrelated to fitness: swimming off-course, fighting a wetsuit, or spending the first 400m in a controlled panic.

How often should you swim?

Two to three swims a week is the consistency sweet spot for most age-group triathletes. Three is where the majority of people racing 70.3 or Ironman start to see genuine improvement, and consistency beats one long heroic session every time.

Swimming is unusually skill-dependent, which is why frequency matters more here than in cycling or running. Feel for the water decays fast: most swimmers notice their stroke going hollow after ten days out of the pool. Three sessions of 45 minutes beat two of 75 minutes for almost everyone, even though the second option has more total volume.

A realistic weekly pattern by race distance:

• Sprint and Olympic. Two swims a week, one technique-and-intervals session and one steady endurance session, is enough to race well. A third is a bonus, not a requirement.
• 70.3. Three swims a week during the build phase, at least one of which contains a long continuous piece of 1,000m or more.
• Ironman. Three swims, with one session that regularly goes past 3,000m of total volume so that 3.8km is never a distance you have to imagine.

If you can only reliably manage two, take the two and stop feeling bad about it. Two consistent swims for six months beats four a week for three weeks followed by a shoulder niggle and a month off.

Structuring 16 to 24 weeks

Most 70.3 build-ups run 16 to 24 weeks, split into base, build and peak. Base is mostly long steady aerobic swimming, build adds structured intervals, and peak makes the training look like the race.

Base (the first 40% of your plan, weeks 1 to 8 of a 20-week build)

The great majority of your volume here should be steady aerobic swimming with technique work folded in: 400s, 600s and 800s at a comfortable, repeatable pace on short rest, plus drills that address whatever your stroke actually does wrong. This is the least exciting phase and the one that determines your ceiling. Test your critical swim speed near the end of it, so everything afterwards has a number to work from.

Build (the next 40%, weeks 9 to 16 of a 20-week build)

Now balance the two things that make a triathlete fast in the water: longer sets of 500m to 1,000m that teach you to hold form when tired, and shorter intervals of 100m to 200m at threshold that raise the pace you can hold at all. A typical build week has one threshold session, one endurance session with a long piece, and one shorter technique or open water session.

Peak (the final 4 to 6 weeks, taper included)

Specificity. Sets at goal race pace, long continuous swims without walls, wetsuit swims, sighting every six to eight strokes, and if at all possible a rehearsal swim in open water at close to race distance. The aim is that nothing on race morning is a first.

Two workouts that do most of the work

If you only had two swim sessions to repeat all season, make one a threshold set anchored to your CSS pace and the other a long endurance session built around a continuous race-distance piece.

That set is 2,000m and works for every race distance. For Ironman, extend it to 12 or 14 repeats rather than swimming them faster.

Swap 1,900m for 750m, 1,500m or 3,800m depending on your race. For Ironman, most people do not need to swim the full 3.8km continuously more than two or three times in a build-up. Confidence is the point, not accumulation.

If you want more session templates by ability, the swim workouts by level guide has full sessions for beginner through advanced.

Pool fitness is not open water confidence

These are two separate abilities, and pool fitness does not automatically transfer. Plenty of swimmers who comfortably do 3km in a pool have a genuinely bad time in their first open water race, because nothing in a pool prepares you for no line, no walls, cold water and other people.

What actually changes: you cannot see where you are going, so you have to sight, and every sight costs a little speed and body position. There are no push-offs, which over 1.9km removes a lot of free assistance you have been quietly enjoying. The water is colder, murkier and sometimes moving. And there are bodies around you.

The fix is to practise the specific skills rather than to swim more in the pool. Sight every three to five strokes in chop or on a turning course, every six to eight in calm water on a long straight leg, lifting only the eyes rather than the whole head. Practise bilateral breathing so a chop or low sun on one side does not ruin your race. Swim continuous sets without stopping at the wall. Then go and do it for real: three or four open water swims before race day is the minimum that meaningfully changes how the swim feels. The open water swimming training guide covers the progression in detail.

Wetsuits, and why you must practise in one

A wetsuit makes you faster by lifting your hips and legs, but it also restricts shoulder rotation and changes your stroke. If race morning is the first time you have swum in yours, the suit will feel like it is fighting you.

The buoyancy effect is large: for swimmers with heavy legs and a low body position, a wetsuit is worth several seconds per 100m for free. But that same lift alters your kick and rotation, and the neoprene across the chest and shoulders makes the recovery heavier and the breathing feel tighter. Swimmers regularly describe their first wetsuit swim as constricted or breathless, and in a crowded race start that sensation is easy to mistake for panic.

Three practical things. Get the fit right (too big and it fills with water, too small and it genuinely restricts breathing). Swim in it three or four times before you race, including one swim at close to race distance. And practise taking it off, which sounds trivial and costs people a minute in transition every weekend.

Wetsuit-legal water temperature rules exist, and the thresholds and mandatory-wetsuit limits vary by federation and by race. Check your event's rules rather than assuming, and have a plan for a non-wetsuit swim in case the water is warm on the day.

Mass starts, drafting and not panicking

Start wide or towards the back if the washing machine worries you, settle into your own rhythm before you race anyone, and then use drafting, which is completely legal in the swim.

Starting wide and back costs a few seconds. Starting in the middle of a fast pack you do not belong in costs you being swum over, losing your rhythm, and five minutes recovering. That trade is not close for most age-groupers. Take the calm water.

Panic in a mass start is almost always a breathing problem rather than a swimming problem. Cold water on the face triggers a gasp reflex, adrenaline pushes your breathing rate up, and your stroke rate follows. The remedy is deliberate: swim the first two or three minutes slower than feels right, breathe every two strokes, and if it gets away from you, roll onto your back for five seconds. If the event allows an in-water warm-up, take it, because the gasp reflex fades once you are wet.

Once you are settled, look for feet. Drafting is illegal on the bike in most age-group racing but entirely legal in the swim, and it is one of the biggest free savings in the sport: sitting on someone's feet, or just off their hip, noticeably reduces the effort of holding a given pace. The skill is picking feet moving at a pace you can hold rather than one that drops you, and letting go when they slow down.

Pacing, taper and the swim-to-bike transition

Anchor your race pace to CSS, taper by cutting volume while holding intensity in the final one to two weeks, and remember that the swim ends with you standing up, running, and getting on a bike.

Critical swim speed is the closest thing swimming has to FTP, and it is the right anchor for triathlon pacing at 1.9km and beyond. Find it with a 400m and 200m time trial, then race at or slightly slower than CSS: roughly CSS pace for 1500m, CSS plus 2 to 5 seconds per 100m for a 70.3, and CSS plus 5 to 8 seconds per 100m for Ironman. The wetsuit will make that feel easier than the same pace in a pool, which is exactly the point.

Taper the swim like any other discipline: reduce volume noticeably over the last one to two weeks while keeping some race-pace intensity in each session, so you arrive sharp rather than flat. Cutting intensity entirely is the classic taper mistake, and it makes race pace feel foreign on the day.

Finally, rehearse the exit. Swimming hard to the beach, standing up fast and running to transition is a shock to the system, and it is why some people feel dizzy in the first minutes on the bike. Ease off slightly in the last 100m, kick a little more in the final minute to get blood back into your legs, then ride the first few kilometres more conservatively than your ego suggests. The swim was the shortest leg of the race. What matters is the state you are in when it ends.

Common questions