Smallest button size on a touchscreen

Question

I'm involved in writing a touchscreen application for a medical device. The program is kiosk-like, in that the start menu, etc, will not be accessible to the user, and the user will use an onscreen keyboard to type any text in the rare event that they need to. The spec'd screen size is 1280x1024.

The question is this: What's the minimum touchable button size for a reasonable interface? I'm thinking that an American dime is a reasonable minimum size in all directions, with the reasoning being that a dime is about as small as we can expect people to feel with their fingers (it's got a diameter of 17.91 mm, according to the almighty Wikipedia).

Or is a dime a bit on the large size?

EDIT: Some extra knowledge about our users. They have to have both hands, because of the nature of the device, and they will not be wearing gloves. They would have to be able to manipulate film cassettes of up to 14x17 inches in size (again, due to the nature of the device), so I feel reasonably confident that they have some manual dexterity.

Answer 1

There's a lot of factors that go into this that can require the buttons to be a lot larger.

Based on my experience, I would use something no smaller than a US Quarter--a dime is really too small for repeated use. If users will wear gloves or other things to "fatten" the finger, you will need even larger buttons. If you have users who are disabled or have poor motor control (more common than you would expect), a dime sized button is absolutely useless. Keep in mind that some touch screens are not particuarly accurate. I've worked with some that were up to 25 pixels off near the edges.

Also, how often are they pushing the buttons and how many buttons will be on the screen? Having to hit many small touch screen buttons in sequence will drive users crazy. That's among the reasons you'll notice that many touch screen systems such as ATM's are starting to have buttons that are in excess of 1 inch x 2 inch.

Also, how fault tolerant is the system if they accidently push the wrong button? Mabye because the screen was off or they were viewing from an angle? The less fault-tolerant it is, the bigger your buttons need to be. Mistakes happen. Smaller buttons means more mistakes.

Answer 2

That's going to depend a lot on the device itself as well as the UI that you're putting on it.

Many touchscreens (excluding multitouch) average the area touched, so chances are you don't want to put UI items any closer than fingertip area to cut down on mis-touches.

Touch screens need to be calibrated post installation, and even then there might be some parallax depending on the screen type or the natural angle from which the screen is being viewed.

You might also consider visual feedback on touch down and only let the event through on touch release. When you get a touch, display an x larger than a fingertip centered on the point of contact and let it track until the touch is released. That point is the click point.

Above all, user test, user test, user test.

If you're really worried about size - make it fluid so you can change it easily and test it on real users under real circumstances. Did I mention user testing?