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Fitts's Law

STR·01 · 1 of 11

The time to acquire a target is a function of its distance and its size.

T = a + b·log₂(2D/W)

documented A named, findable source says this. Why: Fitts 1954 is a published motor-control study, replicated for seventy years.

The same task, built twice

Both panes below are real interfaces. They respond, they can be got wrong, and each is timed from your first move in it. The times are not a study — two panes, one reader, no controls. They are here to make a difference you can feel into a number you can see, and nothing is recorded or sent anywhere.

Your task

Delete the file called quarterly-report.pdf, and confirm it.

Breaks it—
  • annual-accounts.xlsx
  • quarterly-report.pdf
  • notes.txt

The confirm button is 14px, sits in the far corner, and the safe option is the large one nearest your cursor. Every one of those is a Fitts decision, made backwards.

Keeps it—
  • annual-accounts.xlsx
  • quarterly-report.pdf
  • notes.txt

Confirm is 44px and lands where your cursor already is. Cancel is the same size — a destructive action should not be harder to escape than to commit.

What to watch Not that one is faster — one reader once is not a study. Watch where your cursor has to travel, and how many times you overshot the small target. That distance-over-width ratio is the whole law.

The principle

OriginPaul Fitts, 1954 — a motor-control study of pointing, decades before pointers; imported into HCI in the 1970s–80s (Card, English & Burr).
EvidenceOne of the most replicated results in human–machine studies; the reason screen edges and corners are "infinitely deep" targets, why the macOS menu bar lives at the edge, and why 44pt/48dp touch minimums exist.
Limits & misuseIt models pointing, not deciding, reading, or gesturing; it says nothing about which target is correct. "Make everything bigger" is not Fitts — it's a misreading with a real cost, since every enlarged target pushes another one farther away.