I'm designing an iPhone app that features a rather large set of onscreen rounded rect buttons. Given finger-based touchscreen UI, what do you think would be the smallest sensible button size? I need to fit as many of them as I can in the viewport without compromising the usability too much. Maybe there's an Apple-recommended minimum size? Right now it's 33x33, and it looked OK on a simulator, but I was playing with the app on a real phone last night, and it was awkward - the buttons felt too small.
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?
ReplyDeleteRecent scientific research has found that:
ReplyDelete[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.)
Apple is not consistent in this, and there are no hard rules.
ReplyDeleteThey 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
I could have sworn they said the minimum was 37 pixels, but I'm finding 44 today. Either way, 33 seems small.
ReplyDelete(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.)