FittsLaw [fact] v1.0.0

Fitts's Law (1954): movement time to a target is a logarithmic function of the ratio of distance to target size, approximately MT = a + b · log2(D/W + 1).

The time to acquire a target with a pointer is approximately MT = a + b · log2(D/W + 1), where D is the distance to the target and W is the target's width along the movement axis — bigger targets at shorter distances are faster, and the log scaling means small targets penalise heavily while small distance gains return little.

Confidence

proven

Applies To

• button size + spacing on touch UIs
• edge / corner targeting (infinite effective size — see Mac menu bar)
• icon-button hit areas
• drag-and-drop zone sizing
• menu item spacing

Quantitative

• Formula: MT = a + b · log2(D/W + 1)
• Practical Implication: halving distance saves less time than doubling target size
• Edge Targets: screen-edge targets behave as if W = ∞ (cursor stops at the edge)
• Minimum Mobile Target: 44 × 44 CSS px (Apple HIG) / 48 × 48 dp (Material)

Counter Conditions

• Touch input (no cursor-overshoot correction) follows Fitts's Law with different constants from mouse input.
• Multi-touch / gesture targets behave differently — Fitts's Law assumes a single point of acquisition.
• Fitts's Law concerns acquisition time, not perception or decision time — Hick's Law covers the latter.

FittsLaw [fact] v1.0.0

Fitts's Law (1954): movement time to a target is a logarithmic function of the ratio of distance to target size, approximately MT = a + b · log2(D/W + 1).

The time to acquire a target with a pointer is approximately MT = a + b · log2(D/W + 1), where D is the distance to the target and W is the target's width along the movement axis — bigger targets at shorter distances are faster, and the log scaling means small targets penalise heavily while small distance gains return little.

Confidence

proven

Applies To

• button size + spacing on touch UIs
• edge / corner targeting (infinite effective size — see Mac menu bar)
• icon-button hit areas
• drag-and-drop zone sizing
• menu item spacing

Quantitative

• Formula: MT = a + b · log2(D/W + 1)
• Practical Implication: halving distance saves less time than doubling target size
• Edge Targets: screen-edge targets behave as if W = ∞ (cursor stops at the edge)
• Minimum Mobile Target: 44 × 44 CSS px (Apple HIG) / 48 × 48 dp (Material)

Counter Conditions

• Touch input (no cursor-overshoot correction) follows Fitts's Law with different constants from mouse input.
• Multi-touch / gesture targets behave differently — Fitts's Law assumes a single point of acquisition.
• Fitts's Law concerns acquisition time, not perception or decision time — Hick's Law covers the latter.

Sources

Confidence

proven

Source

• Paul M. Fitts, 'The information capacity of the human motor system in controlling the amplitude of movement', Journal of Experimental Psychology (1954)
• MacKenzie, I. S., 'Fitts' law as a research and design tool in human-computer interaction', HCI (1992)
• Bruce Tognazzini, 'A Quiz Designed to Give You Fitts' (askTog, 1999)

Applies To

• button size + spacing on touch UIs
• edge / corner targeting (infinite effective size — see Mac menu bar)
• icon-button hit areas
• drag-and-drop zone sizing
• menu item spacing

Quantitative

• Formula: MT = a + b · log2(D/W + 1)
• Practical Implication: halving distance saves less time than doubling target size
• Edge Targets: screen-edge targets behave as if W = ∞ (cursor stops at the edge)
• Minimum Mobile Target: 44 × 44 CSS px (Apple HIG) / 48 × 48 dp (Material)

Counter Conditions

• Touch input (no cursor-overshoot correction) follows Fitts's Law with different constants from mouse input.
• Multi-touch / gesture targets behave differently — Fitts's Law assumes a single point of acquisition.
• Fitts's Law concerns acquisition time, not perception or decision time — Hick's Law covers the latter.