Fitt's Law for Thumbs

Fitts' law is one of the most well known paradigms in Computer Science. Essentially: the time taken to acquire a target is proportional to the distance and the size of the target. What's this mean? Well, if you want to make a really easy to click button, either: make the button close to the pointer or make it really really really big.

So do we actually use it? Well, yeah. Things like right clicks and close buttons have it all to owe to Fitts. This video does a good job of explaining it.

Does Fitts' law work for thumbs? Kind of. The problem is, without being flippant, thumbs are really weird. They move funny. They're not like a traditional pointer, in the way that not every movement is equally difficult. Consider a mouse, where Fitts' law works perfectly. No matter what direction you move it in, it's pretty easy. But try to bend your thumb backwards or forwards and you'll notice some movements are difficult. We can't assume Fitts' law works (in it's current state) when applied to a mobile environment.

FFitts' Law

There have been previous attempts to map Fitts' law to a mobile environment. The most popular has been FFitts' Law, a paper authored by Google engineers in 2013. It's a good start, but it assumes people use fingers to use their mobile phone. Now just whip out your smartphone and use it for a minute. Did you use your thumb? If you did, you join close to 99% of the rest of us.

People use their thumbs to operate their mobile phones. So what's the point of mapping Fitts' law for a finger? That's like examining how people play table tennis, but asking them to play with their weak hand.


We developed a simple game for an iPhone in which users tap a square which spawns randomly across the screen. Participants played this game for around two minutes, and from this we could generate a 'thumb map'. This basically shows the slowest (and therefore, most difficult) locations of acquisition.


The experiment showed that Fitts' law cannot simply be mapped to a smart phone in it's current state. The paper produced three key findings in HCI.

Id isn't proportional to A

Fitts’ law states that the difficulty of acquisition is positively correlated with the distance required to move. I found this wasn’t always the case – otherwise, the graph below would be symmetrical.

Thumb map

From the data gathered, we managed to produce the (to the best of my knowledge) first thumb map that is based on experimental data. It shows discrepancies in what we consider to be easy to click in smart phone UI.

Occlusion is an issue

Using various thumb maps, we spotted that occluded areas are slower to tap next, as expected (but never proved until this paper).

Fight for the cause..

Mobile phone UI is naff. This experiment shows we've been designing on a false positive all along. Why did Apple decide to place commonly used apps at the bottom, where we found it was most difficult to tap? The HCI community has put 20 years+ worth of GUI research effort into desktop computers - why are we just ignoring the striking problems which persist in mobile phone design?

The main problem is occlusion. Not seeing where our fat thumbs strike is a problem, and the way we're solving it is by using bigger - not better - phones. What a rubbish and uncreative solution.

But we have a chance to change it. If the HCI community starts taking notice of these problems, we can start to make designers listen. We can begin to design smart phone UI with users in mind, not just what looks pretty.

Fitts' Law for Thumbs

Authors Peter Stirrup, Paul Cairns
Year 2018
© Peter Stirrup, The University of York