Fitts's Law and Hick's Law: The 70-Year-Old Psychology Behind "Make the Button Bigger"

The two laws, in one sentence each

Fitts's Law says the time to move a pointer to a target depends on how far away the target is and how large it is. A big button close to the thumb is quick to reach; a small link far from where the eye already rests is slow. Paul Fitts published this in 1954 as a study of the human motor system, not of websites.

Hick's Law, from William Hick in 1952, says the time to make a choice grows as the number of equally likely options grows. Face one clear option and you act almost at once; face ten and the decision itself takes measurable time before any movement begins. Neither man was thinking about checkout pages. Both results have outlived their original context because the underlying capacity they describe, how fast a person can aim and how fast a person can decide, did not change when the screen did.

Fitts's Law: the mathematics of reaching a target

Fitts's contribution was to make aiming quantitative. He showed that movement time scales with the ratio of distance to target size, a quantity later formalized as an index of difficulty. Double the distance and movement gets slower; double the target width and it gets faster. The relationship is stable enough that it holds across hands, styluses, and, later, cursors.

The bridge to interface design was built deliberately. In 1992 the HCI researcher Scott MacKenzie re-derived Fitts's Law in a form suited to pointing at on-screen targets and argued for it as a research and design tool for human-computer interaction. That work is why Fitts's Law is treated as sound for cursor and touch targets specifically, rather than only for the physical stylus tasks Fitts studied. It is one of the better-supported transfers in this whole area, because the task, moving a pointer to a target, is almost the same task Fitts ran.

What Fitts’s Law says about CTA button design

Read literally, Fitts's Law makes three claims about a primary button. A larger target is faster and more reliable to hit, which is the actual mechanism behind the folk advice to enlarge the main call to action. A target closer to where attention already sits, near the last line of copy the visitor read, or inside the thumb's natural arc on a phone, is reached sooner than one stranded at the far edge of the layout. And a target with a generous clickable area, large text combined with a large hit region, forgives the imprecise tap that a cramped control punishes.

None of this promises a conversion. What Fitts's Law legitimately supports is a narrower statement: for the physical act of acquiring the control, size and position reduce the time and error of the tap itself. That is a real, measured effect on pointing. Whether an easier tap changes whether the person wanted to buy is a separate question the law does not answer.

Hick's Law and the cost of choice

Hick's Law addresses the step before the tap: deciding which target to aim at. His experiments showed choice reaction time rising with the number of alternatives, and rising in a specific, decelerating way rather than in proportion. The practical translation is that every additional option on a page taxes the decision, and the first few options cost more than the tenth.

This is the evidence practitioners reach for when they argue to remove competing calls to action, collapse a sprawling navigation, or cut a menu to its essentials. The instinct is sound in direction. The caution is that Hick's original task, choosing between simple stimuli of equal likelihood, is a long way from a person weighing three genuinely different service packages where the options are not equally likely and carry different meaning. Hick's Law explains why a wall of identical choices slows people down. It does not by itself prove that any given option on your page should be removed.

From Hick’s Law to fewer form fields

The place where the choice-reduction idea has the strongest web-measured backing is the form, and the evidence there does not come from Hick at all. It comes from usability testing.

Baymard Institute, which has run large-scale checkout usability testing since 2011, finds that an average large ecommerce site can gain a 35.26 percent conversion-rate increase from checkout-usability fixes, and that a well-designed checkout needs as few as 12 to 14 form elements, roughly 7 to 8 true form fields, against the 20 or more common on unaudited sites. Baymard is a commercially interested party publishing its own dataset, so those point estimates are best read as directional rather than exact. But the direction is consistent and independently reported: shorter, clearer forms convert better than long ones.

It is fair to say Hick's Law offers a plausible mechanism for that result, fewer fields means less to decide and process, and unfair to claim Hick proved it. The form evidence stands on its own usability testing, and the 1952 psychology is a compatible explanation, not the source of the number.

The caveat: a lab pointing task is not a full-page conversion

This is the part most marketing write-ups skip. Fitts and Hick measured tightly controlled motor and decision tasks: move to a lit target, choose between simple stimuli. A conversion page is nothing like that. The visitor is weighing price, trust, timing, and need, not just aiming a cursor. Extending laws about aiming and simple choice to explain whether someone books a med-spa appointment is a reasonable extrapolation, and it is still an extrapolation. In most marketing literature it has not been experimentally re-verified on the specific behavior it is used to justify.

That distinction matters for how confidently you should act. The mechanical claims sit on firm ground: a bigger, closer, larger-hit-area button is genuinely faster and more reliable to tap, and a shorter form is measurably easier to complete. The behavioral claim, that these changes lift conversion for your offer, is the inference. It is often correct, but it is the kind of thing you verify on your own traffic rather than assert from a citation. Treating a seventy-year-old law as a license to skip the test is exactly the error this piece warns against.

Where the heuristic quietly becomes a dark pattern

"Reduce the choices" has a failure mode. Removing a legitimate option, hiding the decline, pre-selecting the expensive plan, or making the exit harder to find than the purchase is not applied Hick's Law; it is a manipulation the design literature and regulators now name explicitly.

The 2019 study Dark Patterns at Scale crawled roughly 11,000 shopping sites and catalogued 1,818 dark-pattern instances across 15 types, and in 2022 the US Federal Trade Commission's staff report Bringing Dark Patterns to Light named obstructed actions and buried choices as legally actionable under Section 5. The line is simple to state: simplifying a genuine decision serves the visitor; stripping or hiding a real choice to force an outcome serves you at their expense. Fitts's Law and Hick's Law describe how people aim and decide. They are not a warrant to remove the decision.

How to use these laws without overclaiming

The defensible way to apply this research is to treat the laws as a source of good hypotheses and your own funnel as the thing that decides. Enlarge the primary call to action, give it a generous hit area, place it where attention already rests, and cut the form to the fields you truly need. Those changes are cheap, they improve the mechanical experience in ways Fitts and Hick genuinely support, and none of them harm the visitor.

Then measure whether they moved anything. A change grounded in Fitts's Law can still fail to lift conversion, because conversion depends on far more than tap speed, and the only way to know for your business is a properly run test rather than a redesign shipped on faith. The laws tell you where to look. The evidence that a specific change worked has to come from your traffic, read with rigorous statistics, not from a paper written in 1954.