Smallest button size on a touchscreen

Question

Hi all,

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?

Answer 3

Depends on so many factors...

All touchscreens degrade over time, and give less accurate, more noisy, and less linear results over time. Resistive touchscreens have vastly improved over time, but you'll still have issues, especially with such a large touchscreen.

Further, people have different sized fingers, and different levels of hand/eye coordination.

Lastly, the hardware and software that actually processes the touches before it even gets into your application needs to be calibrated and characterized.

So it's really much more difficult than merely asking how big the buttons need to be.

First, I'd talk to the manufacturer of the touchscreen itself, and the manufacturer of the hardware/software interface and find out their recommendations.

Second, I'd do some tests - make a few targets onscreen, and then record all the points that the touchscreen hardware sends you when you press them. Do a bunch of tests for quick jabbing presses, hard long presses, light presses, etc. See how much the input jumps around (you might be surprised...) and how fast it actually responds (heavy filtering might make the presses come late, which could reduce usability without speedy feedback).

Thirdly, I'd design the user interface for GREAT user feedback. Make the buttons wider than the finger so that when the user presses it, they still see a little of the button, and it appears to depress when the press it.

Fourthly, I'd consider adding adaptive learning. You'll have to have calibration run and rerun occasionally, but in-between you can adjust to the user slightly to decrease the likelihood of errors. It's not for the faint-hearted, though, and is easy to do wrong so take care if you want to consider it.

Answer 4

I would agree somewhat with James that a US Quarter is a better target, but often you can change that so it is about a US Quarter in one dimension, and a US Dime in the other. Of course, you want to keep the center of active UI elements separated from each other by at least the distance of a US Dime, depending on how "solvable" clicking on the wrong element is. (Clicking on the wrong element in a list is easily solvable if all that is being done is selecting it; clicking on the exit button of a dialog isn't so much.)

Be sure to make your most common cases really easy to do. This usually means making a really big button; or, sometimes, making a whole UI area "clickable" (but if you do that, it's often nicer to the user to also see some sort of button in the area to imply they can click on it.)

Depending on the quality of the touchscreen, you can often "cheat" around the edges. Not only are the edges and corners easiest to hit on a traditional mouse-based interface, but they can be for touchscreen-based interfaces as well, since the user can press their finger right up against the framing element and not worry about triggering something else. Make sure your UI elements accept clicks all the way to the edge of the screen if they are near the edge, as well. Using this, you could have a US dime-sized button in each corner that would still be very repeatably clickable (although possibly lacking feedback if your users have large fingers).

Answer 5

Bigger is better, as we know size matters.

It is important for users of touchscreen interface to get feedback when click somewhere on screen and because this cannot be mechanical as on normal keyboard or mouse button we use sound and/or changing color.

Playing sound only when control is clicked instead of each touch anywhere in screen to achieves good usability.

If working environment is noisy you must give color feedback too, and in this case button should be twice or triple bigger than finger. It is important that user can see feedback when his hand is on screen, and you know that hands are not from glass.

Ordering on screen is important too. For mouse driven interface using of drop downs combobxes, menus and etc is good, but for touch screen you will kill usability with them. So instead of placing controls in upper left part of screen put them in bottom right side. This will give your users free of hands information area on screen when they works.

Think about left/right hand layout, and day/night color scheme too.

Currently i am working to create set of .net UI components for touch screen and you can take a look for demos soon on my web site.

Atanas

Answer 6

hi please provide me some sample code for touch screen programming in java , it contains zoom,scroll when it zooms and keyboard while we focus textbox.....