Your watch shows heart rate, sleep stages, calories and even a high blood pressure alert. But those numbers aren’t equally trustworthy: some are solid, some are estimates, and some are a hint rather than a measurement. Knowing which is which helps you decide whether your current watch is enough, whether you need a different one, or whether you’re simply asking too much of it.
The short answer
- Reliable: resting heart rate, steps and outdoor pace. Studies put heart rate error around ±3%, and the Apple Watch shows a ~2% median error.
- Useful as a trend, not as an exact figure: sleep stages, SpO2, skin temperature, “health age” or “readiness” scores.
- Weak: calories burned. Studies report errors from -21% to +15%, and some find more than 30%.
- An alert, not a diagnosis: ECG, irregular rhythm, sleep apnea and blood pressure.
What you can believe
Heart rate. The star metric and the best validated, especially at rest or at a steady pace. In hard intervals (sprints, weights) a wrist optical sensor drifts more; use a chest strap there.
Steps and pace. Pretty good with outdoor GPS. Less so when you push a stroller or keep your hands still.
What is an estimate
Sleep stages. A 2024 comparison against polysomnography (the clinical standard) found Apple Watch Series 8 sensitivity ranging from ~50% to ~86% depending on the stage. Translation: it tells you how much and when you slept fairly well, but telling deep from light sleep is shakier. Use it to see whether you slept less this week than last, not to obsess over REM minutes.
Calories. Watch the weekly trend, not each workout’s number. If you count calories to lose weight, don’t “eat back” what the watch says you burned.
Composite scores. Readiness, “health age” or recovery scores blend sleep, heart rate and variability using each maker’s own formula. They’re good for comparing you with yourself, not with another brand.
What is an alert (not a diagnosis)
Hypertension (Apple). The hypertension notification analyzes how your blood vessels respond to heartbeats roughly every 30 days and is FDA-cleared for Series 9 and later and Ultra 2 and later. But the clearance data show ~41% sensitivity and ~92% specificity: if it alerts you, check with a cuff; if it doesn’t, that doesn’t mean you’re fine. It’s screening, not a blood pressure monitor.
Blood pressure (Samsung). Galaxy Watch (Watch4 and later) can measure blood pressure, but it must be calibrated against an upper-arm cuff every 28 days, and Samsung says it isn’t for diagnosis. The US rollout began March 31, 2026.
ECG and sleep apnea. They detect patterns (atrial fibrillation, signs of moderate-to-severe apnea) and have clearances on several models. If one triggers, the next step is a doctor, not the watch.
How to choose based on what you want to track
| If you want… | Look for |
|---|---|
| Sleep and activity trends | Any decent watch or band; no need to overspend |
| Irregular heart rhythm alerts | An ECG cleared in your country |
| To monitor blood pressure | An upper-arm cuff; the watch as a complement |
| Hard training | GPS watch + chest strap for intervals |
And before anything else, compatibility: Apple Watch only works with iPhone, and Pixel Watch only with Android. Switching phones can strand your watch, so think twice if you plan to change platforms. Full buying criteria are in how to choose a smartwatch and, if you have an iPhone, in the best Apple Watch by budget. If the phone is your doubt, see the Top 10 iPhone, the Top 10 Android phones or the comparison.
When is it worth changing watches?
Switch if your watch lacks a sensor you truly need (ECG, say), or if the battery no longer lasts the night and you lose sleep data. Don’t switch for a “more accurate” heart rate: the gap between generations is small for everyday use.
🛒 See upper-arm blood pressure monitorsFAQ
Can a smartwatch replace a blood pressure monitor? No. Even the one that measures pressure (Galaxy Watch) needs calibration with an upper-arm cuff and isn’t for diagnosis.
Is it risky to rely on the hypertension alert? The risk is the opposite: assuming no alert means you’re fine. With ~41% sensitivity, the watch misses more than half of cases.
Which metric is most reliable? Resting heart rate and step trends. They hold up best in studies.
Is a cheap smart band enough? For heart rate, sleep and step trends, yes. For ECG or cleared alerts, usually not.
This article is informational and isn’t medical advice.
Sources
- Samsung: blood pressure monitoring
- Engadget: FDA clearance for hypertension alerts
- Euronews: how legitimate is Apple’s hypertension detection?
- Apple: Watch Series 12
- Samsung Health Monitor
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