Apple Watch Ultra 4 & Series 12: BETTER HR, Steps & ECG! (All You Need To Know)
In a Nutshell
Apple’s Series 12 and Ultra 4 introduce a new optical/electrical heart-sensing system on the S11 chip that samples heart rate every 5 seconds and HRV up to every 5 minutes, even in the background, while a new step-counting algorithm claims best-in-class accuracy. These higher-frequency data feed a continuously updating readiness score, a redesigned Health app with day-long vitals, and a new “health age” metric. Battery life and multi-day workout modes are also extended, but the most critical accuracy claims still need independent validation.
These notes were generated by AI and may contain inaccuracies.
The biggest update in the new Apple Watches is the biggest upgrade to any Apple Watch in a long time when it comes to health tracking. The new Apple Watch Series 12 and Apple Watch Ultra 4 have a new health sensing system. Both watches contain a redesigned optical and electrical heart rate sensor powered by the new S11 chip. All these upgrades should lead to better heart rate tracking during exercise, better heart rate tracking during the day at a much higher frequency, better step counting, and better heart rate variability.
The optical heart rate sensor uses repositioned photodiodes and larger, more power-efficient green LEDs. This means brighter green lights and better sensors to detect the reflections of that light, which should improve heart rate even more. The electrical sensor has also been redesigned to improve ECG measurements. The Series 12 starts at $309 in the United States, while the Ultra 4 starts at $799.
Heart rate on the Apple Watch was always one of the strong points, one of the best, if not the best, on the wrist heart rate measurements. Apple claims they are now the best, based on a new study with the new sensor. However, what is potentially even more important is that Apple has changed the hardware and firmware so they can now collect heart rate and heart rate variability data at a much higher frequency.
Previous Apple Watches measured heart rate with different frequencies depending on context. During user-initiated measurements, the foreground algorithm used green PPG signals together with accelerometer data and measured one heart rate value roughly once every 5 seconds. Background measurements during the day used low-powered infrared light and provided roughly one heart rate value every 5 minutes.
With the new Apple Watch Series 12 and Ultra 4, background measurements are now reduced to every 5 seconds. The approximate 5-second frequency that was previously associated with foreground exercise measurements is now being extended to passive background monitoring. This is especially important when active but forgetting to start exercise.
Historically Apple prioritized accuracy over availability, meaning the watch may withhold a value when the signal is too noisy rather than give a potentially incorrect number. Apple Watches have generally been among the better smartwatches for heart rate tracking, but difficult exercises like rapid changes in intensity or wrist tension could impact heart rate sensing. Especially during weight lifting, the watch missed some peaks in heart rate because it stopped detecting due to increased tension on the wrist.
There is now a new step counting system on the Apple Watch Series 12 and Ultra 4. Apple says the new machine learning algorithm improves all-day step counting as well as distance estimates during indoor walking and running workouts. Based on Apple's comparison with other leading smartwatches using a 1,000 step test validated by video counting, the company claims this is now the most accurate step counting of any smartwatch.
The claim of most accurate step counting requires testing to see performance in real conditions, including checking for false positive steps when driving or riding a bike.
The Ultra 4 and Series 12 can measure HRV as often as every 5 minutes and up to 24 times more frequently than before. HRV is becoming much more visible in Apple's software with a dedicated section in the heart rate app. It is included in overnight vitals as well as a new daytime vitals view.
Apple is separating HRV into two parts. Recovery HRV helps identify daily signals associated with stress and recovery, while overall HRV provides broader context around general and cardiovascular health. Overnight recovery HRV will be compared with the user's personal baseline, while daytime view may identify changes before they become apparent overnight.
Apple has traditionally reported HRV primarily as an SDNN value, whereas most people use RMSSD. The additional heart rate and HRV data feed directly into Apple's new readiness feature. Readiness combines recent activity, training load, vitals, and sleep score to produce a score from 0 to 10, then assigns one of four recommendations: Recover, pace yourself, ready, or go for it.
Apple's readiness score can update throughout the day, not just in the morning. A more demanding workout or change in daytime vitals may alter the results after the initial morning calculation. Users can inspect the factors that drive the score rather than receiving only a single unexplained number.
Apple is redesigning the health app with a new insights tab that creates summaries updating throughout the day using heart, sleep, readiness, fitness, vitals, and cycle tracking information. A for you section presents personalized recommendations and educational material, such as suggesting a wind-down routine after several late nights.
A new longevity tab takes a longer-term view across heart health, sleep, mental well-being, movement, metabolic health, hearing, and nutrition. Its most prominent new metric is health age, which compares parts of measured health with chronological age. The calculation can include VO2 max, resting heart rate, sleep, and HRV. Users can enrich this analysis with laboratory values such as A1C and LDL cholesterol.
The redesigned health app includes at-home movement evaluations using the iPhone camera together with heart rate information from the Apple Watch. These assessments examine flexibility, strength, balance, movement mechanics, and cardio fitness. The camera-based analysis runs entirely on the device, and videos are not recorded, stored, or shared. There is also a dedicated test for VO2 max estimate that can be done with an Apple Watch, AirPods Pro 3, or compatible third-party heart rate sensor.
In the United States, users can purchase a laboratory panel directly through the Health app for $119 through Quest service. This includes more than 50 biomarkers and should be available at close to 2,000 Quest Diagnostics locations.
Apple Watch Series 12 retains 24 hours of normal use battery life, but outdoor workout battery life increases to 10 hours, which is 25% more than previous models. 15 minutes of charging can add up to 12 hours of use. The Apple Watch Ultra 4 receives larger improvements with up to 50 hours in normal use and up to 84 hours in low power mode.
For endurance sports, the Apple Watch Ultra 4 can track an outdoor workout for up to 18 hours in normal workout mode. Extended workout mode increases this to 25 hours while preserving heart rate and GPS. Max extended mode can track outdoor runs, walks, and hikes for up to 45 hours. These longer modes preserve heart rate and GPS, including GPS readings every second in max extended workout, but reduce or disable other features and metrics.
Both watches gain audio intelligence features scheduled to arrive later in beta in 2026. Sound recognition can identify selected sounds such as alarms, sirens, and doorbells. Life rewind shows a text representation of the previous 15 seconds after pressing the digital crown two times. Siri recap creates a high-level title and key points from a conversation. Shazam can automatically surface detected music in the smart stack. None of these features store audio recordings, and raw audio is processed inside a hardware-isolated secure enclave and then deleted, though life rewind text and Siri recap summaries are saved with end-to-end encryption.
Some audio intelligence features require an Apple intelligence-compatible iPhone and will be initially limited to English and not initially available in the European Union.
Apple has substantially improved its existing optical and electrical heart sensing platform, dramatically increasing sampling frequency and building more interpretation on top of these measurements. This means heart rate every 5 seconds throughout the day, HRV as often as every 5 minutes, daytime vitals, and a continuously updating readiness score. The most important claims still require objective testing for accuracy during difficult workouts, reliability of increased sampling during the day, and quality of new HRV outputs.
Keep The Quantified Scientist in your library
Save the videos and channels worth coming back to, and find them again in one place.
