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I read this blog post about the ideal button size of touch screen control sizes. It was based on the study form the MIT touch lab that gives some measures of the size and sensitivity of the finger. He quotes the ideal size of 45-57 pixels instead of the 34-44 in the Windows and iPhone UI Design guides.

As the author points out, the contact size is much larger than the suggested size for Windows and Apple devices. I am wondering why there is such a difference, and whether it makes more sense to increase the size of controls on touch screen applications, or if this is not really a usability issue at all?

Also, it seems that many mobile websites and apps seem to completely ignore these principles. I am sure the screen size poses a limitation to how big the interface controls can be, so is this simply a trade-off between being able to fit in as much information versus how easy it is to interact with the control? It feels as if the stylus is not something that is in much use.

A similar question has been raised on the minimum/smallest size that you can get away with, but I would consider this to be a different question to what the optimum size should be, because the smallest size accommodates the minimum usability requirement, whereas the optimum size caters for the best usability requirement.

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I help elderly people with their tablets, smartphones, and e-readers on a regular basis and I can certainly attest that button size is a usability issue. –  jimirings May 1 '13 at 23:11
    
You might be interested in this previous question. There are many factors that affect minimum button size (including hardware). It's also worth pointing out that the minimum size isn't the same as the "optimum" size. –  Kit Grose May 2 '13 at 1:12
    
probably a duplicate of ux.stackexchange.com/questions/1226/… the accepted answer states: "tl;dr? It all boils down to the pixel density". –  Mike Trusov May 2 '13 at 2:38
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Theses days, a measurement in pixels isn't all that useful given the wide range of pixel densities on devices. –  DA01 May 2 '13 at 2:49
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3 Answers

The guidelines given for the various platforms are all based on the idea that the minimum size should be 9-12mm. The variation between the guidelines is mostly due to differing pixel densities on the devices, and hence different number of pixels needed to reach the 9-12mm figure.

Note, that this isn't an ideal dimension to make the button easier to use, but what you should never go below. In practice, however, due to space constraints it often ends up being the standard size that is used.

What the 'ideal size' is really depends on the application. I worked on one mobile application where three buttons in a bottom aligned actionbar had to be used very often. By increasing the size of the buttons by about 20% we found a measurable improvement in the UX. This was important enough for this application that we were willing to give up some space for the bigger buttons. In other applications this may not be a good tradeoff.

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MIL-STD-1472G uses mm instead of px. See page 32 for a recommended 13mm x 13mm. Also, there are two dimensions to consider: button size and distance between buttons. You do not want to reduce the size of the buttons and reduce the distance between the buttons. In support of John's statement '...the 'ideal size' is really depends on the application...' notice the increased size requirement (+5mm) for situations in which the user is expected to be wearing gloves. –  user1757436 May 2 '13 at 18:17
    
The variation between the guidelines is mostly due to differing pixel densities I'm pretty sure most of the guidelines are measuring in "device independent pixels", which may or may not corresponds to physical pixels –  Lie Ryan May 3 '13 at 4:25
    
@LieRyan Yes, they are given in 'device independent pixels' but not platform independent pixels. 10px on iOS is not the same size as 10px on Android. –  JohnGB May 3 '13 at 5:49
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Yes, size can be an issue. Touch targets need to be bigger than typical desktop targets because the finger precision is worse than the pointer+mouse precision. With this worst precision the odds to do missed taps is bigger (like not touching a button or touching the incorrect one).

Usually, touch studies use as principal study variable the error rate. The suggestions are not for perfect error-prone touch targets, but rather to minimize the number of errors to a practical level while balancing other important variables, like screen information density.

Target size influences error rate (from Microsoft)

In traditional touch interfaces, like kioscs, targets are usually a lot larger than in mobile phones, because the size of the screen lets the designer do this. In mobile phones some trade-offs need to be made, and this is the reason because some target sizes differ between style guides and past studies.

Any measurement in pixels is not useful to design for touch interfaces. You must consider the human finger size in relation to the screen pixel density. You must know the pixel density of the devices you are designing for and convert the metric units to pixels.

The optimal touch target size, without considering any other variable, would be the theoretical infinite size, but it's usually suggested in style guides to use a general target sizes of 9mm (0,5% missed taps) and 7mm (1% missed taps) as a minimum.

Luke Wroblewski has a good compilation of style guide recommendations, and I would suggest you to have a look at Microsoft recommendations (based in user testing sessions). Missed taps graph is from their studies.

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Useful links, thanks. –  JohnGB May 2 '13 at 12:17
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I've battled with people quoting this finding on several projects now. The discrepancy, imho, exists because of the researcher's ability to exist in a controlled environment.

When you have complex pages to deal with, you can't make every tapable action the size of a large man's thumb print. If you did, you'd end up with a cluttered mess of a UI. People are more adept with their touch devices than we sometimes give them credit for.

For critical actions, I've found anything from 36 to 48px to work reliably. For secondary and tertiary actions, like small icons or text links, I just make sure there is no less than 24px (48 in @2x terms) of total depth.

Bottomline: With anything other than your "critical path" actions, you have a little wiggle room. People aren't completely haphazard with their devices these days, they're aware of the cost of lazy tapping. When they want to interact with one of your less important actions, they'll be a little more careful with their fingers ;)

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