Apple Watch Ultra 4 and Series 12 serve different workouts

Apple’s Series 12 offers the core fitness and wellness tools most active adults may want, while the Ultra 4 adds features suited to diving and travel beyond cellular coverage. Both models check pulse every five seconds and collect HRV readings up to 24 times more frequently than earlier watches. Their new readiness rating combines recent exercise, sleep, training demand and overnight measurements into a 0-to-10 assessment of daily capacity.

The biggest differences are physical size and staying power. The 42-millimeter Series 12 felt comfortable enough for the reviewer to wear overnight, though its display was harder to glance at during exercise. Its battery supports up to 24 hours of normal use. A 15-minute charge can provide as much as 12 additional hours.

The Ultra 4 has a 49mm case, making workout figures and messages easier to see. Its separate control for stopping and restarting exercise also reduces screen navigation during a run. The tradeoff is a heavier feel at bedtime. Battery capacity reaches 50 hours, and the reviewer found short charging sessions effective. It also includes maps that work without a connection, navigation for remote areas and diving functions. A forthcoming Health app update will add a body-age comparison based on measures including sleep, VO2 max, resting pulse and HRV.