iPhone Diff: what changes between two iPhones, iPads, Macs, Apple Watches or AirPods

Updates & compatibility1 gain · 4 changes

Latest watchOS
watchOS 10.6.2›watchOS 26.6
What does this mean?
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.
Last security update
watchOS 10.6.2 on February 2, 2026›watchOS 26.6 on July 27, 2026
watchOS 10.6.2: maintenance update (no published security fixes). Apple Watch Series 4 and Apple Watch SE (2nd generation) can't run watchOS 27. Apple says not all known security issues are fixed in older versions. Apple's note on software updates
What does this mean?
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.
Apple repair support
Varies by version — see Apple's list›Repaired by Apple
What does this mean?
Apple counts from when it stopped selling the model, not from its release date. Vintage: Apple stopped selling it 5 to 7 years ago; repairs only while parts last. Obsolete: over 7 years ago; Apple no longer repairs it.
Checked against Apple's vintage and obsolete products list on 2026-10-11.
Chip
S4›S8
What does this mean?
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.
Storage
16 GB›32 GB
What does this mean?
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.
Double tap gesture
No (same on both)
What does this mean?
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.
More in the glossary →
On-device Siri
No (same on both)
What does this mean?
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.

Price1 change

Price check

Apple Watch SE (2nd generation) — launched at $249

Used on eBay
Refurbished on Back Market
Apple does not currently sell this model refurbished

What your Apple Watch Series 4 sells for

Apple Trade In estimate
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Links open in a new tab. Prices are set by each seller and change daily.

Launch price (US)
$399›$249−$150
What does this mean?
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.

DisplaySame

Display type
LTPO OLED (same on both)
What does this mean?
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.
More in the glossary →
Always-On display
No (same on both)
What does this mean?
The watch face and the time stay visible when your wrist is down, instead of the screen going dark.
You can see the time, and your numbers during a workout, without raising your wrist. The screen dims while your wrist is down and goes back to full brightness when you raise it. It is not available in Low Power Mode.
More in the glossary →
Peak brightness
1000 nits (same on both)
What does this mean?
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.

Health sensors1 loss

ECG app (electrical heart sensor)
Lose
Availability varies by country. Apple's feature availability page
What does this mean?
ECG stands for electrocardiogram: a recording of the electrical signals that make your heart beat. The ECG app records one in 30 seconds while you hold a finger on the Digital Crown.
The app checks the recording for atrial fibrillation (AFib), a form of irregular heart rhythm, and saves it in the Health app, where you can export a PDF for your doctor. Apple says the app cannot detect a heart attack, blood clots or a stroke, and is not intended for people under 22. No Apple Watch SE model has it.
Matters most if: You want to record your heart rhythm on the spot, for example when you feel a rapid or skipped heartbeat.
More in the glossary →
Heart rate sensor
Second-generation optical heart sensor (same on both)
What does this mean?
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 app
No (same on both)
What does this mean?
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.
More in the glossary →
Temperature sensing
No (same on both)
What does this mean?
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.
Sleep apnea notifications
No (same on both)
What does this mean?
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 notifications
No (same on both)
What does this mean?
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.

Safety1 gain

Crash Detection
Gain
What does this mean?
The watch can detect a severe car crash, such as a front, side or rear-end collision or a rollover.
After a crash it sounds an alarm and shows an alert. If you don't respond, it calls emergency services and messages your emergency contacts with your location. It needs a cellular connection on the watch or a nearby iPhone, or Wi-Fi calling. Apple says it cannot detect all car crashes.
Matters most if: You drive often, especially alone.
Fall Detection
Yes (same on both)
What does this mean?
The watch can sense a hard fall. It taps your wrist, sounds an alarm and shows an alert, so you can call emergency services or tap I'm OK.
If you don't move for about a minute, a 30-second countdown starts; when it ends, the watch calls emergency services and messages your emergency contacts with your location. Apple says it cannot detect all falls. It turns on automatically if you entered an age of 55 or older.
Matters most if: You are 55 or older, or you often work out alone.
Emergency SOS via satellite
No (same on both)
What does this mean?
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.

Battery & chargingSame

Battery life
18 hours (same on both)
What does this mean?
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.
Battery life in Low Power Mode
Not stated by Apple (same on both)
What does this mean?
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.
Fast charging
No (same on both)
What does this mean?
The watch can charge from 0 to 80 percent in about 30 minutes to an hour, depending on the model, with Apple's USB-C magnetic fast charging cable and a USB-C power adapter.
Series 10 and later reach 80 percent in about 30 minutes; Series 7 to Series 9 take about 45 minutes. On Series 12, 15 minutes of charging gives up to 12 hours of use.
Matters most if: You track sleep and want to top up quickly instead of charging overnight.

Design & durability2 changes

Weight by version: Apple Watch Series 4

40 mm, aluminum: 30.1 g
40 mm, stainless steel: 39.8 g
44 mm, aluminum: 36.7 g
44 mm, stainless steel: 47.9 g

Weight by version: Apple Watch SE (2nd generation)

40 mm, aluminum (GPS): 26.4 g
40 mm, aluminum (GPS + Cellular): 27.8 g
44 mm, aluminum (GPS): 32.9 g
44 mm, aluminum (GPS + Cellular): 33 g

Case materials
Aluminum, stainless steel›Aluminum
What does this mean?
What the case is made of: aluminum, stainless steel, titanium or ceramic depending on the model, or 18-karat gold on the first Apple Watch Edition.
Aluminum is the lightest. Where Apple lists the front glass, aluminum cases have Ion-X glass (Ceramic Shield 2 on Series 12), and stainless steel, titanium, ceramic and gold cases have sapphire crystal. The colors you can choose also depend on the material.
Matters most if: You want the lightest case, or a sapphire front.
Weight (case)
30.1–47.9 g›26.4–33 g
What does this mean?
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.
Case sizes
40 mm, 44 mm (same on both)
What does this mean?
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.
Water resistance
50 m (same on both)
What does this mean?
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.
More in the glossary →
IP6X dust resistance
No (same on both)
What does this mean?
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.
More in the glossary →
Dive depth
None (same on both)
What does this mean?
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.

ConnectivitySame

GPS + Cellular model
Yes (same on both)
What does this mean?
Whether the model comes in a GPS + Cellular version, with its own mobile connection for calls, messages, music and notifications without your iPhone nearby. A GPS-only model uses your iPhone, or Wi-Fi, for those.
Cellular needs a service plan, set up with the carrier your iPhone uses; check with your carrier first, since prepaid and some older accounts may not be supported. Cellular models are made for the country where they were sold, and no single model works with cellular service everywhere.
Matters most if: You want to leave your iPhone at home on runs or walks, or you are buying a used watch sold in another country.
Ultra Wideband chip
None (same on both)
What does this mean?
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.
More in the glossary →
Precision Finding for iPhone
No (same on both)
What does this mean?
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.

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