You’ve heard that drafting in the swim saves 21%. It doesn’t. That number is how much drag falls right behind a swimmer’s toes, and in a real race your energy saving is closer to 11%.
That’s still big. It’s also easy to waste, because the way most athletes chase a draft costs more than it saves.
Where the 21% really comes from
In a 2003 study, 11 swimmers did 4-minute swims at 95% of their best 1500 m pace. Researchers measured drag at four distances behind a leader (Chatard & Wilson, 2003). Drag fell 21% at 0 cm behind the toes and 20% at 50 cm. Swimming off to the side cut it only 6-7%.
Translation: drag is the water pushing back on you. Less drag means less work to hold the same speed. It doesn’t mean 21% less energy burned.
Think of a cyclist tucked behind a truck. The wind drops, but the truck’s exhaust still rattles you. A swimmer’s kick works the same way. It makes a calm pocket and a turbulent mess at once.
Heart rate, lactate and perceived effort all dropped in every drafting position in that study. The exact percentages for those weren’t available, so we won’t guess.
What drafting saves in oxygen
A later study measured oxygen use directly. It split the leader into two types: one who kicked and one who stayed passive (Janssen et al., 2009).
Behind a passive leader, oxygen cost fell 25%. Behind a kicking leader, it fell 11%. Beside the leader, it barely moved. Drag even rose 8-9% in that spot.
In short: the 25% is a lab best case. Real swimmers kick. Plan on 11%.
The best spot was straight behind, with your hands about half a meter from the leader’s toes.
Feet beat hips.
Drafting position by position
Here is every figure from the two lab studies in one place. Drag is the water resistance on you. Oxygen cost is the energy bill.
| Position | Drag change | Oxygen cost change | Source |
|---|---|---|---|
| Directly behind, 0 cm from toes | -21% | Lower (size not confirmed) | Chatard & Wilson 2003 |
| Directly behind, 50 cm from toes | -20% | Lower (size not confirmed) | Chatard & Wilson 2003 |
| Behind a passive leader | -20% | -25% | Janssen 2009 |
| Behind a kicking leader | -9% | -11% | Janssen 2009 |
| Beside a passive leader | +9% | Barely changed | Janssen 2009 |
| Beside a kicking leader | +8% | Barely changed | Janssen 2009 |
| Off to the side, 50-100 cm back | -6% to -7% | Not reported | Chatard & Wilson 2003 |
| Swimming alone | Baseline | Baseline | None |
Read the table top to bottom and one thing jumps out. Every good number sits behind a swimmer. Every weak one sits beside.
Why side drafting disappoints
Coaches often claim hip drafting saves 7-12%. We found no solid source for that. The lab data say the gain beside a leader is small.
There’s one nuance. A 2022 study of 15 swimmers over 200 m found less muscle fatigue when drafting at the hip. Muscle signals of fatigue drifted 6.6% drafting at the hip, versus 11.1% swimming free (Puce et al., 2022). Behind the feet did better still at 5.1%. Smaller drift means fresher arms.
So the hip is a decent fallback when you can’t get on feet. It just isn’t the prize.
The 2025 real-race evidence
Lab swims use one leader. Races have chaos. A 2025 preprint by a single author used COVID staggered starts as a natural experiment (Reichel, 2025). It found drafting paid off in speed mainly from the third swimmer in a line onward. Earlier positions mostly reduced fatigue. When races used staggered starts in 2020, the benefit was no longer measurable.
Treat this as early evidence, not settled science. But it fits a simple idea: a pack beats a pair. You want a group, not one pair of feet.
For where to line up so you find that group, see our guide to swim start seeding.
What the swim does to your bike
Here’s where the money is. Triathletes who drafted a 750 m swim rode with 4.8% better cycling efficiency than after a solo swim (Delextrat et al., 2003). Only 8 athletes took part, so read it as a signal.
Now the cost side. Nine triathletes swam 400 m, then rode a 20-minute time trial (Bentley et al., 2007).
The all-out solo swim left riders about 11% weaker on the bike. Easing to 90% effort, or drafting at the same speed, protected their power.
Put plainly: the swim is a loan against your bike. Burn it early and you pay interest for 56 miles.
Don’t sprint to catch a pack. Be in it.
How to find feet without an all-out start
The first 300 m decides most of this. The pack forms fast, and everyone thinks they must sprint to join it. You don’t.
Start where your pace belongs. If you’re a 1:40 per 100 m swimmer, don’t line up with the 1:20 group. You’ll get swum over, then spit out the back alone.
Then ease in. Swim the first 100 m a touch harder than race pace, no more. Look for bubbles and a steady kick just ahead. Swimmers at your speed will drift toward you. Feet come to you more often than you chase them.
If a gap opens, don’t bridge it with a sprint. A gap of one body length is cheap to close once the pack slows at the first buoy. A gap of ten lengths is gone. Ease into the next group instead.
Each wave start is different. In a rolling start, you enter with your pace cluster and the draft builds as you go. In a mass start, expect contact and a messy first minute. Either way, the goal is the same. Arrive at the first turn calm, on feet, with your heart rate under your ceiling.
How to practice drafting in training
Pool drafting teaches less than you’d think. Lane ropes calm the water, so the real effect shows up mostly in open water and races. Still, the pool can teach you timing and position.
Use these three drills:
- Feet-chase sets. Swim 8 x 100 m in a circle with a partner. Stay half a meter behind, then swap leaders every 100.
- Gap closers. Let a gap open to two body lengths, then close it at an even effort. This builds your calm gap-closing habit.
- Sighting off feet. Lift your eyes every sixth stroke to find the lane line or a buoy, without losing the feet.
Open-water group sessions are better still. Swim with a group of mixed speeds in a lake or the sea, and rotate through the lead. You’ll feel the real kick turbulence that the lab calls “peak flow.”
Add some tempo work at your threshold swim pace so you can hold feet when the pack surges. Our guide on masters swim squad vs solo laps covers why group sets build this pace sense. Easy aerobic fitness matters on the bike side too, which is why Zone 2 base work pairs well with swim practice.
A case study: Dana’s 70.3 swim
Take a swimmer I’ll call Dana. She’s 41, trains for her first 70.3, and holds about 1:38 per 100 m in the pool. At her first race, she saw a pack pull away and sprinted the first 300 m. Her heart rate hit 178 before the first buoy. She exited the water 40 seconds ahead of her old time, then rode 12% below her planned watts for the first 20 minutes.
At her second race, she did the opposite. She lined up with her pace group and held 1:38-1:40 for the first 300 m. She locked onto feet half a meter back, and her heart rate settled near 160. She lost the 40 seconds but hit her target watts from the first pedal stroke. (Dana’s numbers are an illustration, not study data.)
How to use open water swim drafting
- Know your pace. Use your swim pace zones to set a ceiling for the first 200-300 m. Stay under it.
- Pick a group, not a hero. A fast swimmer you can’t hold is not a draft.
- Stay half a meter back. Close enough for the 20% drag cut, far enough to sight.
- Don’t chase. If a gap opens, ease into the next group.
- Skip the pool myth. Lane ropes calm the water, so don’t expect pool drafting to feel like a race.
This works for any open-water event, not only triathlon. Marathon swimmers and swim-run athletes get the same physics.
AthleteOS builds your training plan around your swim pace zones and gives race-day pacing guidance, so you know your ceiling before the gun goes. You can start a free trial here.
Pace the first 300 m. Save the rest for the bike.