In short
Fitts's law, published by Paul Fitts in 1954, states that the time needed to move to a target is a function of the distance to it and its size: farther and smaller targets take longer to hit, and the relationship is logarithmic, so small increases in size help a lot for small targets. For interfaces it means primary actions should be large and close to where the user is, screen edges and corners are effectively infinite targets, touch targets need a minimum size of around 44 pixels, and menus and controls should be placed to minimise travel.
The law
In 1954 the psychologist Paul Fitts measured how long people took to tap between targets with a stylus and found a simple relationship: movement time increases with the distance to the target and decreases with the width of the target, logarithmically. The common formulation is
MT = a + b * log2(2D / W)where MT is movement time, D is the distance, W is the target width, and a and b are constants for the device and the person. The term inside the logarithm is the index of difficulty: doubling the distance or halving the width adds one bit of difficulty and a fixed increment of time.
The law has been confirmed for mouse pointers, trackpads, styluses, fingers on touch screens, and eye movements. It is one of the few genuinely predictive laws in human-computer interaction, which is why it keeps appearing in design guidance.
What it means for interfaces
Bigger targets are faster to hit, and the gain is largest for small ones. Going from 20 to 40 pixels helps far more than going from 200 to 220. This is the argument for generous buttons and for the minimum touch target sizes that platform guidelines specify.
Closer targets are faster to hit. The next action should be near the last one. A confirmation button far from the field that triggered it costs a measurable fraction of a second and a little frustration, every time.
Edges and corners are infinitely large. A pointer cannot overshoot the screen edge; it stops there. A target on the edge is, for Fitts's purposes, as deep as the user's movement. Corners are the intersection of two edges and the easiest points on the screen to reach. This is why macOS keeps its menu bar at the top edge, why Windows put the Start button in a corner, and why close and menu controls belong at edges.
Travel distance compounds. A task that requires moving between the top-left and bottom-right of a large screen many times is slow regardless of how big the targets are. Grouping related controls reduces the distance.
The practical rules
- Primary buttons: large, near the content they act on. The submit button at the end of the form, sized generously, not in a distant footer.
- Touch targets at least 44 by 44 points (Apple) or 48 by 48 density-independent pixels (Google), with spacing between adjacent targets so a finger cannot hit two. WCAG 2.2 sets a 24-pixel minimum for conformance and recommends larger.
- Make the whole element clickable, not just the text inside it. A card, a row, a menu item: the target is the box, not the label.
- Put persistent controls at edges where the screen has them: the top bar, the bottom bar, the sides. Floating controls in the middle of the screen are the hardest to hit.
- Context menus at the pointer. Right-click menus appear where the cursor is because zero distance is the best distance.
- Keep related actions adjacent. Undo next to the action it undoes; cancel next to confirm, with enough space and difference to avoid mistakes.
- Avoid tiny links in dense text on phones. Inline links in a paragraph are small targets; spacing lines and enlarging tap areas helps.
Touch screens change the geometry
On a phone, distance is measured from the thumb, not from a cursor that may already be nearby. The comfortable zone for a one-handed grip is the lower half of the screen, curving away from the thumb's base; the far top corner is the hardest place to reach. Fitts's law still holds, but the starting point is the thumb's resting position, which is why bottom navigation bars, bottom sheets and reachable primary actions have become the mobile convention. The top-left hamburger icon, inherited from desktop, is placed exactly where the law says it should not be.
Touch also lacks the edge advantage: a finger can slide off the screen, and the bezel does not stop it the way a monitor edge stops a cursor. Edge targets on phones need their own margin.
Where it gets ignored
Small close icons on modals and cookie banners. Icon-only toolbars with 24-pixel buttons packed together. Tables with tiny action links in each row. Sliders and resize handles a few pixels wide. Dropdown menus where the target is the text and not the row. Every one is a target the law says will be slow and error-prone, and user testing confirms it every time. The related thumb-zone thinking and touch-target guidance in accessibility standards exist because the law's predictions are so reliable.
Related laws
Hick's law concerns the time to choose among options, which grows with their number; it governs how many items a menu should offer. The steering law extends Fitts to moving through a constrained path, such as a nested menu, and explains why cascading menus are painful. Together they form most of the quantitative basis for interface layout. If you would like a product's key screens reviewed against them, book a call; it is a quick review with concrete size and placement changes.
Common questions
What does Fitts's law say in simple terms?
The time it takes to point at something depends on how far away it is and how big it is: farther and smaller targets take longer. The effect is logarithmic, so enlarging small targets helps a great deal while enlarging already-large ones helps little. It applies to mouse pointers, fingers on touch screens and styluses alike.
How is Fitts's law used in UI design?
To size and place controls: primary buttons large and close to the content they act on, touch targets at least 44 pixels with spacing, whole cards and rows clickable rather than just their labels, persistent controls at screen edges and corners where a cursor cannot overshoot, and related actions kept adjacent to reduce travel.
Why are screen corners the easiest targets?
Because a mouse pointer stops at the screen edge, an edge target is effectively infinitely deep and cannot be overshot. A corner is the meeting of two edges, so it can be hit by flinging the pointer without any precision. This is why operating systems place menus, start buttons and close controls at edges and corners.
Does Fitts's law apply to touch screens?
Yes, with adjustments. Distance is measured from the thumb's resting position rather than a cursor, so the lower part of a phone screen is the easy zone and the far top corner the hardest. Touch also lacks the edge advantage, since a finger can slide off the screen. Bottom navigation and reachable primary actions follow from this.
What is the minimum size for a touch target?
Apple recommends 44 by 44 points, Google 48 by 48 density-independent pixels, and WCAG 2.2 requires at least 24 by 24 CSS pixels for conformance while recommending larger. Adjacent targets also need spacing so a finger cannot hit two at once. Fitts's law explains why: below these sizes, hit time and error rates rise sharply.
