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You can read everywhere that bigger targets are easier to hit. But say I have a given input modality and want to know the minimum button size for the interface. How would I go about that?

Fitts' law says

T = a + b log2(1 + D/W)

So I guess I could define a maximum T knowing a and b, but what about D ? How can I express some kind of minimum for the interaction time?

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Don't use Fitts law to settle your minimum target size guidelines. Use real users instead. –  Jørn E. Angeltveit Aug 9 '12 at 14:46
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The constants in the expression above are "to be determined empericaly". So, use the equation as a way to understand how distance and size influence the time it takes to aquire a target, but don't try to use it as a way to calculate times or sizes beforehand. –  André Aug 10 '12 at 11:36
    
@JørnE.Angeltveit I think the idea of the law is that it will work for real users otherwise it would be pointless –  joekr Aug 13 '12 at 8:54
    
@André a and b are to be determined empirically since they depend on your input modality but T/D/W are really the things you are trying to determine as i understand it. –  joekr Aug 13 '12 at 8:56
    
@joekr. No! The idea of the law is not that it should work with real users! And no, the work Fitts did wasn't pointless - of course it wasn't! Fitts did some basic and elementary research on human movement. One of the outcomes from that research was the popular formula, but that's not the important part of his work. The formula is a model. And just like any other model, it's a representation of the real world - it is not the real world! Have you ever seen any of the scatter plots that "makes" this law? It's mess. Real users are so unlike the formula. –  Jørn E. Angeltveit Aug 13 '12 at 9:49
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1 Answer 1

When it comes to touch UIs I have heard 40-45 points squared or the equivalent size of a finger on a surface is a good idea.

You could take a look at this discussion.

If you trust Apple to tell you what to do, this article has an answer to your question:

The comfortable minimum size of tappable UI elements is 44 x 44 points.

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That only goes for touch-ui's, and I don't read that joekr was specifically asking that. –  André Aug 10 '12 at 11:33
    
Thanks for the feedback. I have edited my answer to respond to your comment. He was not specifically asking for touch UIs, but maybe this can help him or others answer the question in full. –  Jonas G. Drange Aug 10 '12 at 12:04
    
I'm really looking for a generic solution so guidelines for a specific modality or even task/application environment are not what i'm looking for. I would however be interested how people came up with those guidelines as they should have done exactly the calculation i'm looking for using their application and modality parameters. –  joekr Aug 13 '12 at 8:58
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