Vertical measurement from case back to crystal top, affecting watch comfort, water resistance, and accommodation of movement complications.
Dress watches typically measure 7-10mm for elegant under-cuff wear. Sports watches range 11-15mm for durability and complications. Dive watches can exceed 15mm. Comfort is subjective, but watches over 14mm may feel bulky for smaller wrists or formal wear.
Automatic movements contain a rotor, mainspring barrel, gear train, and balance wheel requiring vertical space, typically 4-7mm thick. Quartz movements use compact electronic circuits and small battery, measuring just 2-4mm. This allows quartz watches to be significantly slimmer overall.
Ultra-thin movements command premium prices due to engineering complexity—Piaget's Altiplano and Bulgari's Octo Finissimo are examples. However, thickness alone doesn't determine value. Proper proportions matter more: a balanced design integrating thickness, diameter, and lug-to-lug measurements.