Chapter 2 · The Formula
Fitts expressed movement time as a function of distance and target width:
$$MT = a + b \log_2\left(1 + \frac{D}{W}\right)$$
- MT
- — Movement time — how long the reach takes
- D
- — Distance from the start point to the target
- W
- — Width of the target along the direction of movement
- a
- — A constant reflecting start/stop overhead
- b
- — A constant reflecting sensitivity to difficulty
Takeaway: Small targets that are farther away take longer to reach.