Apple Watch Ultra vs Apple Watch Ultra 4

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 price Apple charged in the US on launch day for the cheapest version, before tax.

This is not what it costs today. Older models drop in price over time, so check the used and refurbished links for current prices.

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.

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.

On Apple Watch Ultra 3 and later, the watch itself can connect to a satellite when there is no cellular or Wi-Fi coverage, to text emergency services and share your location.

You need a clear view of the sky, and a message can take a few minutes to send. Apple says satellite features are free for 2 years after you activate the watch, and are not offered on models sold in certain countries or regions.

Matters most if: You hike, ski, camp or travel where there is no cell signal.

Apple's battery rating: how many hours the watch lasts on a full charge in Apple's own test.

Apple calls 18 hours all-day battery life. Ratings for Series 11 and later and Ultra 3 and later include sleep tracking: Apple's 24 hours for Series 12 is 18 hours of daily use plus 6 hours of sleep tracking. Apple says battery life varies with use, settings and signal strength.

Matters most if: You want to wear the watch to bed for sleep tracking.

How many hours the watch lasts on a full charge with Low Power Mode on, in Apple's own test. Low Power Mode saves battery by turning some features off.

It turns off the Always-On display, background heart rate and blood oxygen readings, and gestures like double tap. Apple says other notifications may be delayed and emergency alerts may not arrive. Workouts still measure heart rate and pace.

Matters most if: You travel or camp and cannot charge every night.

How much the watch case weighs, in grams (g); 28 grams is about 1 ounce. Apple lists a weight for each case size and material, and on some models for the GPS and GPS + Cellular versions.

Comparisons show the lightest to the heaviest case Apple lists; the spec page lists each one. Bigger cases and heavier materials weigh more.

Matters most if: You plan to wear the watch all day and to bed for sleep tracking.

Ultra Wideband (UWB) is a short-range wireless technology Apple describes as spatial awareness. Apple's list puts Series 6, 7 and 8 and the first Ultra on the first-generation chip, and Series 9 and later and Ultra 2 and later on the second generation; Apple Watch SE models are not on the list.

With the second-generation chip, the watch can guide you to your iPhone with Precision Finding. With some cars, Apple Watch Series 6 or later can unlock the car as you walk up to it (passive entry). Ultra Wideband availability varies by country or region.

Matters most if: You often misplace your iPhone, or your car works with car keys in Apple Wallet.

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The watch guides you to a misplaced iPhone: it shows the direction and distance, such as 50 feet (about 15 meters), even if the iPhone is in another room.

Both the watch and the iPhone need the second-generation Ultra Wideband chip; on iPhone that means iPhone 15 models or later, or iPhone Air, but not iPhone 16e or iPhone 17e. Without it, the watch can still make your iPhone play a sound. Availability varies by country or region.

Matters most if: You often lose your iPhone around the house.