A window-sign percentage has two inputs: the glass area that belongs in the denominator and the sign area that belongs in the numerator. Confirm both before resizing the artwork. Adding up every window on the storefront can conceal an overloaded pane if the applicable rule tests each opening separately.
Confirm the unit being regulated
Ask whether the rule applies per pane, opening, storefront, tenant, or building elevation. Then identify what counts as eligible glass: door glazing, transoms, sidelights, frames, opaque panels, and shared entrance areas each need deliberate treatment. Use a marked photograph or elevation so the reviewer can see the proposed boundary of the calculation.
Read the sign definition too. Ask how it treats interior-facing graphics visible from outside, temporary posters, illuminated devices, and patterned or perforated film. A product sold as see-through film does not tell you how a reviewer will count its area, so ask. Keep any required visibility or clear-view restriction as a separate check.
Measure first, then calculate
- Assign each pane or opening an identifier on the drawing.
- Record eligible width and height in the same units and calculate glass area.
- Outline existing and proposed graphics using the required sign-area definition.
- Multiply eligible glass by the allowed decimal percentage.
- Subtract retained counted signs from that allowance and compare the proposal.
For a rectangular area, width × height gives square feet when both dimensions are in feet. Convert inches first: a 30-inch width is 2.5 feet. If dimensions are both measured in inches, divide their product by 144 to obtain square feet. Use the approved method for irregular openings rather than counting their surrounding wall.
How one real code words it
San Luis Obispo, California sets its window-sign limit in section 15.40.470(B):
“Window signs are limited to a maximum of twenty-four square feet or fifteen percent of the window area, whichever is less.”
The same subsection decides what counts as a window sign at all: signs are “located within thirty-six inches of the face of a window,” and displays, graphics and text more than thirty-six inches behind the glass “are not considered signs.” So a poster hung four feet back from the glass is outside the count, and one taped to the glass is inside it. On a storefront with 200 square feet of glass, 15 percent is 30 square feet, and the 24-square-foot cap wins. What the section does not say is whether “the window area” means each pane or all the glass together. That is exactly the question the next example turns on, so ask before you print. (Ord. 1667, 2019. We read it on September 28, 2026. The 200 square feet is invented.)
Fictional example: aggregate success, per-pane failure
All dimensions and the 20% limit below are hypothetical. Imagine two 4-by-6-foot panes, each containing 24 sq ft of eligible glass. The proposed layout puts 7 sq ft of graphics on Pane A and 1 sq ft on Pane B.
| Opening | Glass area | Fictional 20% allowance | Proposed graphic area |
|---|---|---|---|
| Pane A | 24 sq ft | 4.8 sq ft | 7 sq ft |
| Pane B | 24 sq ft | 4.8 sq ft | 1 sq ft |
| Combined | 48 sq ft | 9.6 sq ft | 8 sq ft |
If this fictional rule tests the combined glass, 8 ÷ 48 × 100 is about 16.7%, below 20%. If it applies per pane, Pane A uses 7 ÷ 24 × 100, about 29.2%, and exceeds its 4.8 sq ft allowance by 2.2 sq ft. The same total artwork produces different outcomes.
Moving at least 2.2 sq ft from A to B would address this hypothetical per-pane arithmetic because B has 3.8 sq ft available. But any redesign must still respect the actual measurement method, placement restrictions, and other existing graphics.
Count the retained material
Show posters, decals, logos, and other material whose inclusion needs confirmation. Mark each as retained, removed, or proposed. Avoid calling a wall of existing graphics “existing conditions” without deciding whether it consumes the same allowance. If a category is excluded, record the basis for that exclusion.
Do not automatically add the glass-door area to create a larger denominator. In a fictional variant, an eligible 12 sq ft door would add 2.4 sq ft at 20%, but only if that glass belongs in the same aggregate calculation. It would not cure Pane A if individual panes are tested.
Send a checkable layout
Deliver a labeled elevation, glass schedule, graphic schedule, calculation, and the applicable definitions. Ask the authority to resolve disputed inclusions before the graphics are printed. The worked examples use invented numbers; the San Luis Obispo rule is quoted to show real wording, not as the limit for your site. For letters mounted to a solid building face, use the separate wall-sign calculation guide.