Fitts's Law, explained.
Fitts's Law relates pointing movement time to the distance to a target and its width: farther or narrower targets generally require more precise movement.
Why it happens
The pointer must cover distance while landing inside an acceptable region. A larger target tolerates a wider range of endpoints. The difficulty index captures the ratio between distance and width; actual time requires coefficients fitted to a particular task and device.
Fitts's Law relates movement time to target distance and width. A target can be visually prominent yet difficult to hit if it is narrow or far from the pointer.
Read the result
Start from the same button and hit the target several times. Compare size and distance separately. The difficulty index is not a predicted number of milliseconds, and keyboard selection does not measure pointing distance.
A worked example
A frequently used control
An editor's tiny confirmation button sits far from the area where a user is working.
A wider control closer to the relevant interaction reduces its pointing difficulty under comparable conditions.
The improvement concerns reaching the control, not whether the action is understandable or correct.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
The task places the start at 10% of the arena width and the target at 10% plus the selected distance. Width is a percentage of the same arena. The comparison uses the Shannon-form difficulty index log2(1 + distance/width), not a fitted prediction in milliseconds.
Where this idea is useful
A practical use
Enlarging frequently used buttons and placing them near the relevant action can reduce pointing effort in an interface.
A common misconception
“The largest possible button always creates the best interface.”
Targeting effort is only one design concern. Hierarchy, accidental activation, screen space and accessibility also matter.
What this explanation leaves out
- This browser task includes thinking and device effects as well as movement. Keyboard navigation bypasses pointer distance. The original law and later variants require empirical coefficients to predict time.
Why is target width relative to distance important?
Doubling both produces the same ratio in this difficulty formula. Changing one without the other changes how precise the movement must be relative to its travel.
Which frequent action asks for unnecessary travel or precision?
Associated thinkers
Further reading
Explore the original research or the teaching reference behind this experiment.