Fitts' Law Movement Time Calculator
Calculate movement time to a target using Fitts' law.
Enter distance, target width, and motor constants a and b to find index of difficulty and movement time.
Fitts’ law (1954) predicts the time required to move to a target of width W at distance D:
MT = a + b * log2(2D / W)
The term log2(2D/W) is the index of difficulty (ID), measured in bits. It captures the information content of the movement: small targets far away are harder (high ID), large targets nearby are easy (low ID).
Constants a and b. These are empirically fit from human performance data. Typical values for mouse-based pointing: a = 50 ms, b = 100 ms/bit. Touch screens tend to have lower b due to larger effective target widths from finger spread. Stylus input falls between mouse and finger.
Why bits? Fitts drew an analogy to Shannon’s information theory. A movement with ID = 3 bits requires as much motor precision as distinguishing between 8 discrete positions. This is why Fitts’ law generalizes across limbs, speeds, and input devices.
Practical use in UI design. The law implies: (1) make frequently used buttons large, (2) put them close to where the cursor typically is, (3) screen edges are effectively infinitely wide — a button pinned to the screen edge is faster to hit than an equal-sized floating button. The Windows taskbar and macOS Dock exploit this.
Steering law. For curved paths (like navigating a menu tunnel), the analogous formula is MT = a + b * D/W. This predicts why narrow menu tunnels with long cascades are slow to navigate.
The chart shows predicted movement time across a range of distances for your target width and constants.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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