I have an Your current Apple WatchApple Watch Series 12Apple Watch Ultra 4Apple Watch Series 11Apple Watch SE 3Apple Watch Ultra 3Apple Watch Series 10Apple Watch Series 9Apple Watch Ultra 2Apple Watch Series 8Apple Watch SE (2nd generation)Apple Watch UltraApple Watch Series 7Apple Watch SE (1st generation)Apple Watch Series 6Apple Watch Series 5Apple Watch Series 4Apple Watch Series 3Apple Watch Series 1Apple Watch Series 2Apple Watch (1st generation)Apple Watch Series 5▾
and I'm looking at the The Apple Watch you are consideringApple Watch Series 12Apple Watch Ultra 4Apple Watch Series 11Apple Watch SE 3Apple Watch Ultra 3Apple Watch Series 10Apple Watch Series 9Apple Watch Ultra 2Apple Watch Series 8Apple Watch SE (2nd generation)Apple Watch UltraApple Watch Series 7Apple Watch SE (1st generation)Apple Watch Series 6Apple Watch Series 5Apple Watch Series 4Apple Watch Series 3Apple Watch Series 1Apple Watch Series 2Apple Watch (1st generation)Apple Watch SE (1st generation)▾

watchOS is the software that runs Apple Watch: the watch faces, the built-in apps and the settings. A new version can bring redesigned apps, new watch faces and new workout features.

To install a new watchOS, the paired iPhone needs a matching iOS: watchOS 27 needs iPhone 11 or later, or iPhone SE (2nd generation or later), with iOS 27. If the watch cannot run the newest version, Software Update offers the newest one it can use. After watchOS 11 came watchOS 26, so watchOS 11 to watchOS 27 is two updates, not sixteen.

Matters most if: You are buying a used Apple Watch, or your iPhone is too old for the latest iOS.

The most recent update Apple has released for the newest watchOS version this Apple Watch can run, and the day it came out, from Apple's list of security releases.

Apple's list says which updates fixed security problems. A maintenance update fixed other things and lists no security fixes. Apple says not all known security issues are fixed in older watchOS versions, even ones that still get updates.

Matters most if: You are buying a used Apple Watch, or you use Apple Pay on it.

The chip is the watch's brain. Apple names its watch chips S1, S2 and so on; the newest is the S11.

The chip decides which features a watch can get. Double tap and on-device Siri arrived with the S9 chip in Series 9 and Ultra 2: double tap uses the chip's Neural Engine, which Apple says processes machine learning tasks. Apple said the S6 chip in Series 6 let apps launch 20 percent faster.

The space on the watch for apps, music, podcasts, audiobooks and photos. GB means gigabytes. Apple Watch models have from 8 GB to 64 GB.

Storage matters most if you keep music, podcasts or audiobooks on the watch to play when your iPhone is not with you. On Series 3, the GPS + Cellular version has 16 GB and the GPS version 8 GB.

Matters most if: You want to listen to music or podcasts on runs or walks without your iPhone.

Tap the index finger and thumb of the hand wearing the watch together twice. The watch treats it like pressing the main button on the screen, so you can control it with one hand.

Answer or end a call, stop a timer, play or pause music, snooze an alarm, or scroll through the Smart Stack of widgets. It does not work in Low Power Mode, during Sleep Focus, when your wrist is down or when the watch is locked.

Matters most if: Your other hand is often busy.

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The watch can handle some Siri requests on the watch itself.

Apple says requests that need no information from the internet, such as starting a workout or setting a timer, work without Wi-Fi or cellular, and the answers come quicker. Apple added it with Series 9 and Ultra 2.

Matters most if: You use Siri for timers and workouts, or you often go out without your iPhone.

The type of screen. Apple lists an OLED screen for Series 1 and later. LTPO (low-temperature polysilicon and oxide) is a display technology Apple lists from Series 4 on, and one of the parts Apple named for the always-on screen of Series 5. LTPO2 and LTPO3 are the later versions in Apple's tech specs.

LTPO3 with wide-angle OLED is on Series 10 and later and Ultra 3 and later. Apple says Series 10's screen is up to 40 percent brighter than Series 9's when viewed from an angle, and that in always-on mode it updates once a second instead of once a minute, so some watch faces show a ticking seconds hand without you raising your wrist.

Matters most if: You want to read the watch at a glance without lifting your wrist.

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Nits measure screen brightness. This is Apple's figure for the brightest the screen can get. Apple Watch screens range from 450 nits to 3000 nits.

A higher number makes the screen easier to read outdoors in strong sun. Apple says Series 9 doubled the top brightness to 2000 nits, from 1000 on Series 8. Ultra 2 reached 3000 nits.

Matters most if: You spend a lot of time outdoors, for example running or hiking.

The optical heart sensor on the back of the watch flashes green lights against your wrist. Blood absorbs green light and more blood flows with each heartbeat, so the watch can count your heart rate in beats per minute (BPM).

The Heart Rate app shows your heart rate at rest, while walking and during and after a workout, and you can turn on alerts for a heart rate that stays above or below a level you choose. We show Apple's name for the sensor; Apple's tech specs do not say what changed between generations. Series 12 and Ultra 4 have what Apple calls the Health Sensing System, which measures heart rate as often as every 5 seconds during the day.

Matters most if: You train by heart rate, or want an alert when your heart rate stays above or below a level you set.

Blood oxygen is the percentage of oxygen your red blood cells carry from your lungs to the rest of your body. The Blood Oxygen app measures it with a red light against your wrist; a reading takes 15 seconds while you keep your arm still.

With background measurements on, the watch also measures now and then, usually when you are not moving. Apple says the readings are not intended for medical use and are only for general fitness and wellness, and the app is for people 18 and older. No Apple Watch SE model has it.

Matters most if: You want to follow your blood oxygen overnight or at high elevation.

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Two temperature sensors, one on the back near your skin and one under the display, check your wrist temperature every five seconds while you sleep.

After about 5 nights, the Health app shows each night's change from your usual temperature. It is not a thermometer: you cannot check your temperature on demand. If you use Cycle Tracking, it is also used for period predictions. Apple says it is not intended for medical purposes.

Matters most if: You use Cycle Tracking, or want to see night-to-night changes in your body temperature.

Apple describes sleep apnea as a potentially serious condition where breathing stops repeatedly during sleep. The watch uses its motion sensor to pick up small wrist movements linked to interruptions in normal breathing.

Wear the watch to bed. If nights with more breathing disturbances become frequent over 30 days, you get a notification of possible sleep apnea, with a PDF to discuss with your healthcare provider. Apple says it is not intended for people already diagnosed with sleep apnea or under 18.

Hypertension is chronic high blood pressure. The watch uses its optical heart sensor to analyze how your blood vessels respond to each heartbeat, in the background, over 30-day periods.

If it finds consistent signs of hypertension, you get a notification to discuss with your healthcare provider. Apple says it cannot identify every instance of hypertension, and it is not intended for people under 22, people already diagnosed with hypertension, or during pregnancy.

The case size in millimeters (mm), measured from the top to the bottom of the case; 10 mm is about 0.4 inch. Most models come in two sizes; Ultra models come in one, 49 mm.

The larger size has a larger screen and weighs a little more. Bands come in sizes too: before you buy one, check that it fits your case size.

Matters most if: You want the largest screen for reading, or the lightest watch to wear all day and night.

Apple's water rating. Series 2 and later are rated 50 m (meters) under the ISO 22810:2010 standard; Ultra models are rated 100 m and also meet EN13319, a standard for dive accessories. The first Apple Watch and Series 1 are rated IPX7: splash and water resistant, but Apple does not recommend submerging them.

The rating is not how deep you can go. With a 50 m rating, Apple says you can swim in a pool or the ocean, but not dive, water-ski or go into high-speed water. Apple rates Ultra models for recreational scuba diving to 40 meters (130 feet). Water resistance can wear down over time and cannot be rechecked or resealed.

Matters most if: You swim, snorkel, surf or dive.

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IP6X is a dust-resistance rating under the IEC 60529 standard. Apple Watch Series 7 and later and every Ultra model are rated IP6X.

Apple says the rating makes the watch more durable in places like the beach or the desert. It covers dust only; water resistance is rated separately.

Matters most if: You spend time at the beach, in the desert or on dusty trails.

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The deepest Apple rates the watch for scuba diving. Apple Watch Ultra models are rated for recreational scuba diving to 40 meters (130 feet).

Apple says the Depth app is not a dive computer, that Ultra models should only be used by trained divers, and to use a secondary depth gauge and timer. Series 10 and later show depth only to 6 meters (20 feet), for snorkeling, and Apple says they should not be used for scuba diving.

Matters most if: You scuba dive or free-dive.