Fitts law

There is one very important law in the design of interfaces ... stop. Was it not said that there is no and there can not be any laws in the design? Well, you know, it depends on what you mean by design and by the law. It doesn't worth it to accept any statement literally. So, the Fitts law.

The Fitts law determines the speed of user interaction with interface elements. That is, roughly speaking, it shows how quickly the user will be able to move the mouse cursor, say, to the Close button and click on it. Obviously, the interaction time depends on the size of the control and the distance to be made by the mouse cursor. One of the mathematical formulations of the Fitts law — Shannon's formulation — is as follows:

Where T is the average time required to move the cursor,
a and b are experimentally established constants representing the response time and speed of the input device,
D is the distance, and W is the width of the control, measured along the line of cursor movement.

No formulas

There is one interesting nuance in the Fitts law. The screen is not endless and has certain boundaries, which, in particular, limit the movement of the mouse. If no special settings are set, the cursor does not fly away from the screen when it reaches the screen boundary and does not appear from the opposite side, but remains bounded to the edge.

For this reason, the controls pressed against the screen's borders are reached at a fantastic speed; sometimes they do not even need to be looked at. We do not need to go far beyond an example: remember how easy it is to click the Close button if the window is maximized to the full screen.

So what, it would seem? Does someone care about this? Perhaps, only Apple does it consciously. Microsoft follows this principle, but always forgets something. Google carefully applied the law in his Chrome browser. As for the products of other companies... sometimes everything is very bad.


Let's take a look at the evolution of the interface of the well-known Windows operating system. As you know, the first version of Windows, which gained considerable popularity, was released in 1992 and was named Windows 3.1.

Back then Windows did not have its main interface element — the Start button — there was only a parody on the desktop and window manager. Even the button for closing the window was not a cross, but a strip, and was located on the left. However, if you bring the mouse to the upper left corner of the screen to the stop, this button is pressed!

They say that the biggest breakthrough in the development of the Windows interface was made by version 95. It was this version where the Start button appeared for the first time, and the desktop and file explorer were radically reworked. The buttons for controlling the window got their used view and location.

Note: both the Start button and the Close button have small gaps from the edges of the screen and do not touch them closely, as was the case in 3.1. And if you take the mouse to the corner until it stops, you can find out that the Close button reacts to clicking in the corner, and the Start button does not. Either Microsoft forgot about its main button, or the behavior of the Close button - some internal feature of the operating system.

The drawback of the Start button lasted a long time and was kept in all versions of Windows up to XP. The newly created OS received a new design and fixed many errors in the interface.

Hurray, now the active click trigger area is adjacent to the screen borders! No more drawbacks... unless you do not pull the taskbar up:

Having fixed one, the developers forgot about the other. Many users push the task bar two positions up, and the Start button does not take into account the Fitts rule. It's far-fetched? Maybe, however, in Vista and Windows 7, this drawback is also fixed.

And, starting with Vista, the icons are selected with an area, and it's much easier to click them. Here, too, there is a nuance: in Vista, the computer icon is docked by the area to both screen borders, and in Windows 7 — only to the left. How it happened hard to imagine, but it's an obvious drawback of Windows 7.

It is also worth noting the button for minimizing windows is moved to the lower right corner in Windows 7 and fully accords to Fitts: on systems with two monitors the system does not allow the mouse to go to the second monitor if you move the cursor along the task bar. Thus, even on systems with multiple monitors, the button "Collapse all windows" can be clicked blindly.

Mac OS

Now let's take a look at the Mac. In Mac OS, the Fitts law is taken into account quite intelligently. About many subtleties of its interface tells Jeffrey Raskin in his book "The Human Interface".

The Mac OS menu is sticked to the top of the screen. It is much easier to click menu items. Because of this, however, it is more difficult to get into the window close button, but it is used much less often.


A personal story is deserved by tabs in browsers. The first browser that took them to the title of the window was Chrome.

And Google did it well, because they did not forget to apply the Fitts law. Bookmarks in Chrome stick to the edges of the screen, it's very easy to click them.

Chrome has well saved space by taking out tabs in the headline. His example was immediately followed by competitors. First we saw this technique in Opera, and then in Firefox.

Alas, the authors of these browsers did not take into account what Google did. In Opera, above the tabs there is a two-pixel gap, apparently, left for the double click. The authors of Firefox not only forgot about the law, but ruined the whole idea and wasted a lot of space (fixed in the final version).