Minimum sensible button size on iPhone

Recent scientific research has found that:

[A] target size of 9.2 mm for discrete tasks [i.e., single-target pointing tasks] and targets of 9.6 mm for serial tasks should be sufficiently large for one-handed thumb use on touchscreen-based handhelds without degrading performance and preference.

Cited from Target Size Study for One-Handed Thumb Use on Small Touchscreen Devices (Parhi, Karlson, & Bederson 2006). Other sources agree on this "close-to-0.4-inch-rule" (e.g. Designing Gestural Interfaces (Saffer 2008, p. 42)).

Given the iPhone's pixel density of 163 PPI (6.417px/mm), you should preferably aim no lower than 59px diagonal for any target.

(Please note that this is verified for one-handed thumb use only.)

I could have sworn they said the minimum was 37 pixels, but I'm finding 44 today. Either way, 33 seems small.

(Note that images can be smaller because they're often part of a larger rectangular area.)

(You might get away with fewer pixels on other devices, if the pixel size is larger.)

Apple is not consistent in this, and there are no hard rules.

They recommend you provide Fingertip-Size Targets in the UI Guidelines, and then go on to say that the calculator buttons are 44x44.

Also in that document they say that images for toolbar and navigation icons should be 20x20, and for tab bar icons should be 30x30.

I think you have to simply test your application and make sure that it's easy to press the buttons - even when you have fairly large hands...

-t

Apple says that the avg finger tap is 44x44 (from WWDC). All table rows are recommended to be at least that height. It is common for icons to appear 32x32, but have padding to make the touchable area 44x44. Of course, people can tap a 1x1 if they are very careful, but why make people try harder than they need to in order to use your app?