Why the "OK" button is big and close, and the fine print is small and far: the time it takes to hit a target depends on its size and its distance.
Fitts's Law says the time it takes to move to and successfully select a target is predictable from two things: how far away the target is, and how big it is. Closer and bigger targets get hit faster and more accurately. Farther and smaller targets take longer and get missed more often.
That one relationship quietly shapes a huge amount of the designed world: why the default button in a dialog box is the largest one, why your phone's keyboard keys are bigger than they look like they need to be, why the Mac menu bar lives glued to the very top edge of the screen, and why the items next to the checkout line are small and impulse-sized while the things you came in for are a full aisle away.
Picture two buttons you need to click as fast as possible. One is big and sits right next to your cursor. The other is tiny and sits across the screen. You already know, intuitively, which one you'll hit faster and more reliably. Fitts's Law just makes that intuition precise and measurable.
Try it: which target below would be faster and safer to click?
Fitts's original 1954 formulation, later refined by Scott MacKenzie, is usually written:
MT = a + b · log₂(2A/W + 1)
MT is movement time — what we're predicting. A is the amplitude, the distance from the starting point to the center of the target. W is the target's width along the direction of travel. a and b are constants fitted from real measurements (start-up time and the device's raw speed). The part inside the logarithm, log₂(2A/W + 1), is called the Index of Difficulty — a single number that captures "how hard is this target to hit," and it climbs as distance goes up or width goes down.
Play with it: drag the sliders and watch the predicted difficulty change.
If you want something selected quickly and accurately, make it bigger, move it closer to where attention already is, or both. This is why the primary action in a dialog box (like "Save" or "Continue") is visually the largest button, and why it's usually placed where the cursor or thumb naturally lands next.
Guess: a checkout form has a 40px-wide "Place Order" button 300px below the last input field. Support tickets mention accidental mis-taps. What's the single highest-leverage fix, per Fitts's Law?
A cursor can't overshoot past the edge of the screen — it just stops there. That means the very edges and corners of a screen behave like infinitely wide targets in one dimension: you can slam the mouse toward them as fast as you want without worrying about overshooting. Interface designers lean on this constantly: the Mac menu bar sits glued to the top edge, Windows docks the Start button in a corner, and right-click menus appear exactly where the cursor already is instead of somewhere you'd have to travel to.
Guess: which screen location is, functionally, the "easiest to hit" target on a desktop, per Fitts's Law?
Fitts's Law was derived from physical hand movements in Paul Fitts's original 1954 lab studies, long before computers had graphical interfaces — so it applies just as much to a hand reaching for a wrench on a pegboard, a thumb stretching across a phone screen, or a shopper's arm reaching for a product on a shelf. Anywhere something needs to be physically reached and selected, the same size-and-distance trade-off governs how fast and how accurately it happens.
Dialog boxes make the recommended action (e.g. "Save") visually larger than secondary options like "Cancel," and place it where the cursor naturally rests after the last input.
Apple's and Google's interface guidelines set minimum touch-target sizes (roughly 44×44pt and 48×48dp) because fingertips are imprecise "cursors" — small targets that work fine with a mouse fail constantly with a thumb.
Workbench and cockpit designers put the tools and switches used most often within the shortest, most direct reach, and move rarely-used controls further away or behind a panel.
Checkout-line shelves hold small, impulse-sized items within easy one-handed reach, while the staples that pull people into the store sit a full aisle away, trading speed for forced browsing.
Frequently used controls (volume, hazard lights) sit large and close to the driver's hands on the wheel or center stack, while rarely used settings are tucked into smaller, farther menus.
Console game UIs put the most-used ability on the largest, nearest-to-thumb face button, and radial "quick-select" wheels shrink travel distance for items chosen mid-combat.
Don't reach for Fitts's Law when the real problem is how many choices someone faces (that's Hick's Law), when the task is a continuous path rather than a discrete point-to-point selection (that's the Steering Law), when the delay is caused by system latency or the user's own thinking rather than physical movement, or when you haven't yet decided what the target should be in the first place — Fitts's Law optimizes placement and size, not the underlying decision of what belongs on the screen at all.
Describe a real control, button, tool, or shelf item you're trying to place or size, and get a ready-to-use prompt for thinking it through with Fitts's Law.
This packages Fitts's Law into a reusable instruction set you can hand to an AI assistant — so it applies this thinking automatically whenever you're reviewing a design, layout, or interface.
Describe what you're designing or deciding right now, and get a prompt that bundles your situation together with the Fitts's Law skill above — ready to paste into any AI assistant.