Touch Target Fitts's Law Simulator

Touch Target

Target size 44 px

Spacing between targets 8 px

Motor impairment (tremor) 30%

iOS 44pt

Android 48dp

Live Tap Statistics

Hit rate

— %

Taps so far

Mean move time

— ms

Throughput

— bit/s

Index of difficulty (current target)

—

How it works

Every tap is modelled with Fitts's Law, the standard human-motor-control equation used across UX/HCI research to predict how long a pointing movement takes:

ID = log2(D / W + 1)      (index of difficulty, bits)
MT = a + b * ID           (movement time)
TP = ID / MT              (throughput, bit/s)

D is the distance from the previous tap to the new target's centre, W is the target's width, and a≈150 ms / b≈120 ms/bit are typical constants fit from human pointing studies. Smaller, more distant targets have a higher index of difficulty and take longer to reach accurately.

The simulated finger does not land exactly on the target centre — its endpoint carries Gaussian aim error whose standard deviation grows with the motor impairment slider (modelling tremor / limited fine motor control, a real accessibility persona) and with the movement's index of difficulty. A tap is a hit only if the landing point falls inside the target's radius (W/2); otherwise it is a miss.

σ = (0.02 + tremor·0.16) · (1 + 0.15·ID)   [3D scene units]
landing = target_center + Gaussian(0, σ) in x and y
hit ⇔ |landing − target_center| < W/2

The green/red rings mark the platform minimum touch-target sizes referenced by real accessibility guidelines: 44×44 pt on iOS (Apple Human Interface Guidelines) and 48×48 dp on Android (Material Design), both aligned with WCAG 2.5.5 Target Size. Dragging the size slider below these thresholds visibly drops the hit rate — the mechanical reason those guidelines exist.

Current target Simulated finger tap

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