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.