Apple Watch vs Fitbit for Health Tracking: Series 9 vs Charge 6

The first thing to understand about this comparison is that the two devices are not the same shape. The Apple Watch Series 9 is a 41mm or 45mm square computer with a bright always-on OLED, a digital crown, an app store, and a battery that will not see a second day under normal use. The Fitbit Charge 6 is a slim 36.7mm by 22.7mm aluminium-edged band with a small AMOLED, no app store, no speaker, and a week of battery if you turn the screen off at night. Calling these “competitors” only makes sense if you define the category by the sensor stack rather than the form factor.

For health tracking specifically, the sensor stack is what matters. So we are going to do this comparison anyway, on the narrow question of which one is the better device for ECG, AFib screening, SpO2, skin temperature and sleep. Workouts and notifications and watch faces are not the subject here. If you want a fitness-first comparison, the best fitness trackers page is the right place.

A note on which Apple Watch we are picking. We default to the Series 9 because it is the model with the full health-sensor stack — wrist temperature sensor, ECG, blood-oxygen, AFib history, fall and crash detection. The cheaper SE (2nd Gen) drops the temperature sensor, the ECG and the blood-oxygen sensor, which means for the purposes of this article it is barely an Apple Watch at all. We will note where the SE is the budget alternative and where it is not. The Charge 6 has no internal SKU split; it is one device at one price.

The specs, side by side

We will work through the rows that actually decide the purchase. Some of them are obvious; some are not.

ECG: same regulatory category, different user experience

Both devices have FDA-cleared single-lead ECG. That is the headline. The implementation is not the same.

On the Apple Watch, you open the ECG app, rest a finger on the digital crown, and 30 seconds later you have a sinus-rhythm or possible-AFib classification with a Lead I rhythm strip you can export as a PDF. The classification covers sinus rhythm, AFib, AFib with high heart rate, low or high heart rate inconclusive, and poor recording. The on-screen prompt and the haptic confirmation make it the fastest consumer ECG to actually take — you do not have to remember a sequence of menus.

On the Charge 6, ECG was always promised but did not ship as a working user feature on day one. The hardware was there. Fitbit needed a separate FDA submission for the band form factor (different electrode geometry from the Sense), and the on-device ECG app came online over a series of firmware updates through 2024. Once it is enabled, the experience is similar: hold the band edges with thumb and forefinger of the opposite hand, wait 30 seconds, get sinus / possible AFib / inconclusive. The PDF export goes through the Fitbit app, not directly off the device, which means the ergonomics of bringing a reading to a cardiologist are slightly clunkier than Apple’s “share as PDF” sheet.

The AFib history feature, which Apple introduced as a separately cleared use case in 2022, deserves its own paragraph. It is for users who have already been diagnosed with AFib by a cardiologist. The watch then estimates the fraction of time spent in AFib over a week, charts the trend, and lets the user share it back to the clinician. This is a different FDA pathway from the initial-detection workflow, and as of mid-2026 only the Apple Watch has it. Fitbit’s irregular-rhythm notification (cleared in 2022 for Sense and Charge 5, inherited by the Charge 6) covers the screening side but does not estimate AFib burden over time. If you have already been diagnosed and your cardiologist wants the longitudinal data, this is a real reason to pick Apple.

SpO2: similar hardware, asterisks on both sides

Both watches have a red and infrared LED pair on the back and both produce overnight oxygen-saturation trends. Both come with the usual caveats — accuracy degrades on darker skin pigmentation (Sjoding et al., NEJM, 2020), motion artifacts ruin spot readings, and the FDA position is that wrist SpO2 is wellness data, not a clinical pulse-ox replacement. The SpO2 wearables explainer goes into this in more depth.

The Apple asterisk is regulatory rather than physical. From January 2024 through August 2024, Apple shipped the Series 9 (and the Ultra 2) in the United States with the blood-oxygen feature disabled in software, after a US International Trade Commission ruling in the Masimo patent dispute. Watches sold in the US during that window have the sensor hardware on board but cannot show the feature. Watches sold before that window, and watches sold after Apple’s revised workaround was approved, do. If you are buying a Series 9 used or open-box from that period, check the unit. International (non-US) units were unaffected throughout.

The Charge 6 has no equivalent regulatory hole. Estimated SpO2 has been a Fitbit feature since the Charge 4 in 2020 and the trend is shown overnight on the wrist without a subscription. Spot SpO2 readings are not as immediately available as on Apple, where you can tap an icon and get a 15-second measurement.

Skin temperature: a feature both have, but with different framings

Apple introduced wrist temperature with the Series 8 and Ultra in 2022, and the Series 9 carries the same two-sensor module — one against the skin, one toward the watch face — designed to estimate the temperature differential overnight. The reading is reported as a deviation from a personal baseline, not as an absolute number, and only when the watch detects you are asleep with Sleep Focus on. It powers two things: a wrist temperature card in the Health app, and the cycle-tracking retrospective ovulation estimate.

The Charge 6 has continuous overnight skin temperature on the same deviation-from-baseline model. Same caveat — it is not a clinical thermometer, the sensor is on the wrist not the core, and a single elevated reading is meaningless without a baseline of at least a few nights. Fitbit shows the deviation in the daily readout for free, but the higher-resolution multi-night temperature trend is part of Sleep Profile, which is gated behind Premium. Apple shows the equivalent for free.

We have a longer piece on what skin temperature data is actually good for — short version: useful for noticing fever onset, illness, and (with caveats) ovulation phase, less useful for the absolute “what is my temperature right now” question.

Sleep tracking: the battery problem decides this

Both devices track sleep stages. Both rely on PPG and accelerometer signals. The accuracy gap between them, in our own testing and in the published validation work, is real but small.

The deciding factor is not per-night accuracy. It is that the Apple Watch has an 18-hour battery, which in practice means most users charge it overnight or for an hour in the morning. The Charge 6 runs four-to-six days. That difference compounds: if you charge the Apple Watch overnight you do not get a sleep recording, and if you charge it in the morning you have to remember to put it back on before you go out. The longitudinal trend — the thing sleep data is actually useful for — gets gappier on Apple’s battery cadence than on Fitbit’s. People who care about sleep data buy a Charge 6 (or a ring) for exactly this reason, and we say so in the sleep tracker roundup.

Apple’s own answer is to charge during a known awake window — the 60 minutes around your morning shower, say — and the Series 9 fast-charges most of the way back in about 45 minutes. That works if you have a routine. It does not work if your routine breaks.

Fall and crash detection, Emergency SOS

This is one of the rows where the two devices are not playing the same game. Apple has both fall detection (since Series 4) and crash detection (since Series 8), plus Emergency SOS. The cellular models can place that call without an iPhone nearby, which matters for runners and older users living alone. The Charge 6 has none of these. There is no fall detection on the band. There is no emergency call.

For some buyers — older parents, anyone with a known seizure or syncope risk, someone who cycles solo — this is the entire conversation. The Charge 6 is a health band; the Apple Watch is a health band that can also call an ambulance. We would not recommend the Charge over the Apple Watch for an 80-year-old, full stop.

The platform lock

Apple Watch requires an iPhone. There is no setup path on Android, there is no workaround, and Apple has not signalled any intention to change this. If your phone is a Pixel or a Galaxy, the Series 9 is not on your shopping list. The cellular models additionally tie to the original buyer’s Apple ID, which complicates secondhand purchases — a used cellular Apple Watch is sometimes locked.

Fitbit works on iOS and Android. The asterisk, since August 2023, is that you must use a Google account to set up the device. Existing Fitbit accounts have been migrated on Google’s schedule; new buyers cannot avoid this. For some readers that is fine; for others it is the same kind of platform-lock objection that Apple-skeptics raise about Cupertino, just pointed at Mountain View. Our health-data privacy piece covers what each company actually says it does with your data.

Data export

If you ever want to take your data with you, both companies do support full export. Neither is fast.

Apple Health export lives at Health app → profile → Export All Health Data. The output is a single zip file containing an XML document of every recorded data point, plus a workouts directory, plus the ECG PDFs. For an account with a few years of continuous data this is hundreds of megabytes and the export can take ten or fifteen minutes to assemble. The XML format is documented; community tools parse it readily.

Fitbit data export goes through Google Takeout. You select Fitbit, you wait for Google to produce the archive (this can take hours), you download a zip of per-day JSON files. The structure is less elegant than Apple’s single XML but it is machine-readable. Both are fine. Neither is what a clinician wants — that is the ECG PDF, which both devices produce.

The subscription situation

Apple does not gate any health feature behind a subscription. Fitness+ exists, but it is workout video content; the health metrics are all included with the watch.

Fitbit gates a lot. Daily Readiness Score, the deep sleep analytics (Sleep Profile, sleep animal), stress management score, and the longer health-trend windows all require Fitbit Premium at $9.99 a month. The free Charge 6 still gives you ECG, SpO2, sleep stages, skin temperature, resting heart rate, and the basic dashboard. What it does not give you is the synthesised “should you train hard today” readout that Premium calculates. We covered this in our Charge 6 review and the verdict has not moved: most buyers will not need Premium, but Fitbit clearly wants you on it, and they have steadily moved features behind the paywall since Google bought them.

A Garmin or Apple buyer pays once and is done. A Fitbit buyer pays $160 for the band and then makes a recurring decision about $120 a year. That is part of the cost calculation, even if you decide not to subscribe. The Fitbit vs Garmin piece walks through this if you are also considering the Garmin side.

What we would actually buy

Two pieces of advice, depending on which problem you are trying to solve.

If you are an iPhone user, you are willing to charge a device every day, and you want a real screen-on-wrist computer that also happens to do most of the health-sensor work a wrist can plausibly do — the Apple Watch Series 9 is the answer. Crash detection, fall detection, AFib history for diagnosed users, no subscription, the fastest ECG workflow, decent skin temperature, decent SpO2 (subject to the Masimo asterisk on US 2024 units). The price of admission is the iPhone and the battery routine. Buy at Apple.

If your budget for the watch is $249 rather than $399, and you are sure you do not need ECG or SpO2 or skin temperature, the Apple Watch SE 2nd Gen is fine. It still has fall and crash detection and Emergency SOS, which for many buyers is the actual reason to wear an Apple Watch. Check price on Amazon. But understand what you are giving up: most of this article is about features the SE does not have.

If you want a slim, light, 24/7 health monitor with multi-day battery, and you do not need a real watch, the Fitbit Charge 6 is the better device for the job. Better sleep tracking (because it stays on through the night), continuous skin temperature, AFib screening, on-demand ECG since the 2024 firmware, SpO2 trend. It works on Android. The Premium upsell is annoying but the free tier is honest. The form factor is the actual difference: you can wear a Charge under a dress shirt and forget about it. You cannot do that with a Series 9. Check price on Amazon.

The wrong purchase, in either direction, is buying the device that fits your phone and then resenting it for not being the other thing. If sleep matters most, do not buy the Apple Watch and then complain about the battery. If you want crash detection and AFib history, do not buy the Charge 6 and then wonder why it has neither. The form factors are different on purpose, and the right call depends on which problem is louder in your life.