Best heart rate monitors for swimming

Why your strap goes silent the moment you push off

Bluetooth Low Energy and ANT+ both transmit in the 2.4 GHz band. Water absorbs radio energy at that frequency extremely strongly — this is the same physical property that makes a microwave oven work at a nearby frequency. A signal that carries ten meters through air struggles to cross a few centimeters of water.

So a chest strap on your sternum cannot reliably reach a watch on your wrist while you are swimming. The link drops on every stroke and recovers only when both devices happen to surface at the same moment. What you get is not a slightly worse heart-rate trace, it is a trace with most of the session missing.

This is worth being precise about because it is routinely misdiagnosed as a water-resistance failure. Your strap is not broken and it is not leaking. It is transmitting perfectly well into a medium that will not carry the signal.

The specification that actually solves it

On-board memory. A device that records internally does not need a live link at all. It logs beat-by-beat data through the whole session and syncs to your phone afterwards, when you are dry and the radio works again.

That is why the ranking on this page is not the same as our site-wide accuracy ranking. Sensor accuracy is necessary but it is not what separates these devices in the pool. Storage is.

The second consideration is water rating, and it is the easier one. Polar states WR30 for both straps here and WR50 for the Verity Sense. All three are comfortably rated for pool swimming. Note that these are pressure ratings rather than depth ratings, and none of these devices is a diving instrument.

The three, and why they rank in this order

1. Polar H10 — memory plus the reference-standard measurement

Polar states internal memory for one session of up to 30 hours, alongside WR30 water resistance and electrical ECG measurement. It is also the device researchers use as their criterion measure — including in the 2025 Sensors placement study — which is the strongest evidentiary position a heart-rate sensor can occupy.

For pool swimming it does the complete job: it measures with the best-established principle, it stores the session while the radio is useless, and it costs less than any watch. The practical annoyance is that a chest strap can migrate during push-offs and turns, so it needs to be tighter than you would wear it on land.

2. Polar Verity Sense — the armband with a swim mode

Polar publishes on-board recording, WR50 and a dedicated swim mode. The recording is the part that matters here for exactly the same reason as on the H10 — it does not need a live link — and the armband form factor tends to stay put through push-offs and turns better than a chest strap does.

Its accuracy record on land is genuinely strong — the same 2025 study measured the lowest resting error of any device tested at roughly 1.6% MAPE, and a bias under 0.1 bpm with concordance of 0.997 on the graded treadmill protocol. It ranks second here rather than first because it is optical, and nobody has published a measurement of any optical sensor underwater. Water on and around a skin-contact optical sensor changes the optical path, and we have no data on what that does. The device is excellent; the swimming claim specifically is unverified.

3. Polar H9 — the one that cannot do this

We rank the H9 highly elsewhere. It has the same electrical measurement principle and the same electrode design as the H10, three radios including the 5 kHz analog that old gym equipment listens for, WR30, and 400 stated hours on a replaceable coin cell — at a meaningfully lower price.

It has no internal memory. It cannot record a session on its own. In the pool that is not a minor limitation, it is total: the strap will happily measure your heart rate and there will be nothing to receive it and nowhere to put it.

It is on this page precisely because it is the trap. It looks like the H10 on a spec sheet, it is cheaper, and it is the wrong purchase for this one use case. Buy it for cycling with a head unit, not for swimming.

What about swim-capable watches?

Most modern sports watches are rated for swimming and will record wrist heart rate in the pool. Two things are worth knowing before you rely on that.

First, wrist optical sensing is the weakest placement in the published evidence even on dry land — the same study found proximal placements beat the wrist at every intensity tested, with wrist limits of agreement of about ±10-19 bpm during a cycling warm-up. Second, nobody has published any measurement of wrist optical heart rate during swimming at all.

So the honest position is that a watch will produce a number and nobody can tell you what it is worth. If you are training by heart rate in the water, the strap-plus-memory approach is the one with a defensible measurement behind at least part of it.

Open water, and a genuine safety note

Everything above applies more strongly in open water, where you are further from any receiver and surface less predictably. The recording approach is the only one that works.

One thing no device on this page does is give you live heart rate you can react to mid-swim. If that is what you need — for a medical reason rather than a training one — none of these is the answer, and that is a conversation to have with a doctor rather than a purchase to make.

What we could not verify

• Any underwater accuracy figure, for any device. Every measurement we cite was taken on land. No published validation of heart-rate accuracy during swimming exists that we could find, optical or electrical.
• Real-world memory behavior.Polar's 30-hour figure is a published specification, not something we have filled.
• The swim mode in use. Polar publishes it; we have not swum with one.
• Long-term water exposure. Chlorine and salt degrade fabric straps and seals, and nobody has published a durability comparison.