The Classic Armband fits the iPhone 13 Pro with no case or a thin case and is the best armband choice if you want the least amount of bulk on your arm while running or working out.
The Classic M6 Modular Armband for iPhone 13 Pro fits cases up to 150 x 75 x 10 mm.
If your case is thick (i.e. Otterbox, Lifeproof) and you prefer to keep the case on during your workouts, we recommend the Velocity V7 Armband.
Strap Size Arm Size (upper arm circumference)
Small 7 - 9.5 inches (20 - 25 cm) - For THIN arms or wrists only
Medium 10 - 14 inches (25 - 36 cm) - Fits most arms/most common
Large 13 - 18 inches (33 - 46 cm) - Fits large arms or over clothing
Extender + 4 inches (10 cm)