Sage + Stone · sensors graded on accuracy

Your wrist is lying to you.

Watches and smart rings are convenient — and loose. They miss beats the moment you move, because the wrist is a bad place to read a heartbeat. The accurate place is your forearm. Sage + Stone grades every sensor on how close it gets to a medical ECG, points you to the best one, and unifies all of it in one trusted layer.

See the accuracy grades Reserve the Sage + Stone armband
The verdicts

Heart-rate sensors, graded on accuracy

One question: how close is it to a medical ECG? Placement decides almost everything — and we'll send you to the best one, even when it isn't ours.

A

Chest strap e.g. Polar H10

ECG-grade — the reference nearly every study uses. The only catch is comfort: a band around your ribs. If you want the single most accurate reading, this is it.

Where to buy →
A−

Optical armband e.g. Polar Verity Sense

Forearm/bicep PPG is nearly chest-strap accurate and far comfier — independent testing puts a good armband neck-and-neck with the H10 at rest. The sweet spot for most people. It's also exactly what we're building.

Where to buy →
B

Wrist watch Apple Watch, Garmin

Convenient, and that's the point — but the wrist is bony, loose, and motion-prone. Fine for steps, sleep, and trends; it drifts during a hard workout.

Keep yours
B−

Smart ring Oura

Genuinely excellent for sleep and resting trends. Not built for real-time or exercise heart rate — wrong sensor for that job.

Sleep only
!

WHOOP the lock-in flag

Accurate enough — but the band is useless without a $199/yr subscription and a closed app. You rent the hardware and don't own your data. We grade the sensor; we flag the cage.

Rented, not owned
!

COROS HRM the claim flag

A good armband — that markets itself as "clinical-grade" with no published validation. Decent unit, unsupported claim. Exactly the kind of claim we refuse to make about our own.

Unproven claim

Grades reflect placement and accuracy evidence for the category, not a lab test of a specific unit. Heart rate and HRV here are for fitness and recovery — not a diagnosis, and not a medical device.

The Sage + Stone optical heart-rate armband worn on a forearm Founding reservation

The accurate armband — that talks to everything.

We're building the armband we'd actually recommend: forearm optical HR + HRV (real RR-intervals), ANT+ and Bluetooth so it pairs with Garmin, Zwift, and Peloton — and native Sage + Stone, so every reading lands in one trusted layer. One-time price. No subscription. You own the sensor, and you own the data.

$59 · chest straps we graded accurate run $90+ · $0 today, notify-only

Our promise, and our firewall: we will not call it "clinical-grade" or claim a number until we pay an independent lab to validate it against an ECG — and then we publish the report. Reserve now at the founder price; nothing is built or charged until it's validated and enough of us want it.

$0 todayA reservation, not a purchase. We only ask for a card if and when it ships.
Validated firstNo accuracy claim until an independent ECG test passes — published in full.
You own itOne-time price, open ANT+/BLE — the opposite of a rented, locked band.

Built to order — waitlisters get the drop and the founder price first. Ships ~6–8 weeks; full refund anytime before it ships, so we never sit on dead stock. And we won't claim it's accurate until we publish the independent ECG test behind it.

No-risk

Graded on accuracy — sold with a guarantee

30day money-back

If it doesn't hit the spec we publish, send it back for a full refund — and cancel a built-to-order unit anytime before it ships. We won't publish an accuracy number we can't defend with the data standing behind this guarantee.

How do you know it's accurate?

We publish the independent ECG-reference test behind every accuracy claim. No published number, no claim — that's the whole brand.

What if I don't like it?

30-day money-back, and a full refund anytime before a built-to-order unit ships. Made to order, so no dead stock and no regret.

Who makes it?

We spec it; it's built by a vetted OEM and graded on our public rubric before it ships. We disclose plainly that we sell our own armband.

The "Where to buy" links in the grades above are affiliate links — we may earn a small commission at no cost to you, and it never moves a grade.

We grade sensors — and we're making one. Here's the line we hold.

  • We don't grade our own band on our engine. An independent lab validates it against an ECG and we publish the raw report — it earns a seal, not a Sage + Stone letter grade.
  • Every competitor stays graded. If a Polar armband is the better buy for you, we say so and link you straight to it — like we do right above.
  • We disclose that we make it, on every page that mentions it.
  • One-time price, open ANT+/BLE. The opposite of WHOOP's rented, locked hardware — because a trust brand shouldn't cage your data either.

Sage + Stone takes no money from graded brands. "Where to buy" links may earn a small commission at no cost to you, and never affect a grade. Heart-rate and HRV features are for general wellness and fitness — they do not diagnose, treat, or monitor any medical condition.