What is Fitts's Law?

Building a startup involves making a thousand decisions about how a user interacts with your product. You might spend hours debating the color of a button or the font in a menu. However, there is a fundamental law of human movement that often dictates whether your product feels intuitive or clunky. This is known as Fitts’s Law. It is a scientific model that helps us understand how long it takes for a person to move a pointer, like a mouse or a finger, to a specific target.

If you are an entrepreneur, you do not need to be a designer to appreciate this. You simply need to understand the relationship between distance and size. Fitts’s Law states that the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target. Essentially, the further away something is and the smaller it is, the longer it takes for a user to click or tap it.

This principle was first proposed by Paul Fitts in 1954. While he was originally looking at human motor skills in physical environments, his work has become a pillar of human-computer interaction. For a founder, this is a tool for reducing friction. Friction is the enemy of growth. If a user struggles to find the sign-up button or if the delete key is too small and far away, you are creating a barrier to success.

To understand the law deeply, we should look at the variables involved. The formula is often expressed as Time equals a plus b times the log base two of two times the distance divided by the width. In this equation, a and b represent constants that depend on the specific device being used. The core of the logic rests in the relationship between distance and width.

Distance refers to how far the user’s cursor or finger must travel to reach the goal.

Width refers to the size of the target itself along the axis of motion.

A larger target is easier to hit because it allows for a higher margin of error. A closer target is easier to hit because it requires less physical or digital travel. When you increase the size of a button, you are making it a more accessible target. When you place a frequently used tool near the user’s current cursor position, you are shortening the distance.

This leads to a concept called the Index of Difficulty. As the distance increases or the width decreases, the difficulty of the task rises. In a startup environment, your goal is often to keep the Index of Difficulty as low as possible for primary actions. You want the most important paths in your software to be the easiest to traverse.

How do we use this in a real business scenario? Consider a mobile application. On a phone, the user’s thumb has a specific reach. Fitts’s Law suggests that buttons placed in the middle of the screen or at the bottom near the thumb’s natural resting position will be reached faster.

Buttons placed at the very top corners require more time and effort. This is why many successful apps have moved their primary navigation to the bottom of the screen. They are minimizing the distance for the user.

Another interesting application involves the edges and corners of a computer screen. In a desktop environment, the edges of the screen provide what is known as infinite width. Since the cursor cannot move past the edge, the user can move the mouse rapidly toward the edge without fear of overshooting the target.

This makes the corners of a screen the most valuable real estate in an operating system. This is why the Start menu on Windows or the Apple icon on macOS are located in the corners. They are the easiest targets to hit because they effectively have infinite size in at least two directions.

• Make primary action buttons large.
• Group related functions close to each other.
• Utilize screen edges for frequently used menus.
• Avoid placing critical buttons in hard to reach areas.

Founders often confuse Fitts’s Law with Hick’s Law, but they address different aspects of the user experience. While Fitts’s Law is about the physical act of moving to a target, Hick’s Law is about the mental act of making a decision. Hick’s Law states that the time it takes to make a decision increases with the number and complexity of choices.

If your checkout page has twenty different buttons, Hick’s Law tells us the user will hesitate because they are overwhelmed by options. If those twenty buttons are tiny and scattered across the screen, Fitts’s Law tells us that even after the user decides which one to click, they will take a long time to actually move the mouse there.

You need both laws to build a solid product. Hick’s Law helps you simplify the choices. Fitts’s Law helps you make the physical execution of those choices effortless. If you ignore one, the other cannot save the user experience. A single, large button is the ultimate expression of both laws working in harmony.

Imagine you are building a dashboard for a logistics company. Your users are busy and often stressed. They need to approve shipments quickly. If the approval button is a tiny icon in the far right corner of a massive table, you are violating Fitts’s Law. You are forcing the user to perform a high precision movement under pressure.

In this scenario, you should enlarge the target area. You might even make the entire row of the table clickable to act as the button. This increases the width significantly and reduces the time required for the action.

Another scenario is the design of a mobile checkout flow. If the buy now button is at the top of a long scrolling page, the user has to scroll and then move their thumb a long distance. By pinning that button to the bottom of the screen, you keep the distance constant and minimal. This directly impacts conversion rates.

As we move into new types of technology, we have to ask how Fitts’s Law evolves. How does it apply to eye-tracking software? If a user only has to look at a target, does distance still matter in the same way?

In virtual reality, we are dealing with three dimensional space. The target has depth as well as height and width. We are still learning how depth affects the speed of movement.

There is also the question of gesture based interfaces. When there is no visible cursor, does the law change? In these environments, the user is moving their entire arm or hand through space. The physical fatigue of the user becomes a new variable that Paul Fitts might not have fully anticipated in a digital context.

As a founder, you should keep these unknowns in mind. While the core principle of distance and size remains a fact of human biology, the ways we interact with machines are changing. You must observe your users. Use tools to see where they struggle. If you see them missing a button or moving slowly, refer back to the relationship between distance and width. It is often the most straightforward way to fix a complex problem.