Inside vs. Outside Mount Blinds: Casing Depth & Measurement Rules

Choosing between an inside mount and an outside mount is the foundational decision when ordering custom blinds or shades.

While an inside mount provides a clean, tailored architectural look that showcases your window moldings, it requires a specific minimum casing depth. If your window frame is too shallow, the blind’s headrail will protrude awkwardly past the trim or fail to anchor securely.

Here is how to measure window casing depth correctly and determine which mounting style your windows require.

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1. How to Measure Window Casing Depth

Window casing depth is the clear, unobstructed distance measured from the front face of the window trim back to the closest obstruction:

• If your window has plain glass, measure from the trim edge to the glass pane.
• If your window has crank handles, mullions, latch locks, or alarm contacts, measure to the frontmost projection of that obstacle.

Minimum Depth vs. Flush Depth

Every window blind manufacturer specifies two distinct depth metrics:

1. Minimum Mounting Depth: The bare minimum flat surface area required for the mounting bracket screws to catch solid wood. The headrail will hang securely, but its front edge will protrude slightly past the window frame.
2. Flush Mounting Depth: The full depth required for both the mounting bracket and the entire headrail/valance to be fully recessed flush with the surrounding wall or molding.

Standard Depth Guidelines by Blind Style

Blind / Shade Style	Minimum Mounting Depth	Flush Mounting Depth
Cellular / Honeycomb Shades (3/8″)	0.75 inches	1.75–2.0 inches
Roller & Solar Shades	1.25 inches	2.25–2.5 inches
2-Inch Faux Wood / Wood Blinds	1.5 inches	2.75–3.0 inches
Sheer Shadings (Silhouette Style)	1.75 inches	3.25–3.5 inches
Roman Fabric Shades	1.5 inches	2.5–3.0 inches

If your window casing depth is less than the Minimum Mounting Depth, an inside mount is physically unfeasible—you must choose an outside mount.

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2. Measuring for an Inside Mount

For an inside mount, windows must be measured in three distinct locations because window frames frequently twist or settle over time.

The 3-Point Width Measurement

1. Measure the inside width across the top of the window frame where the headrail will mount.
2. Measure the inside width across the middle.
3. Measure the inside width across the bottom near the sill.
4. Record the smallest of the three widths. If your window narrows by 1/4 inch in the middle, ordering to the top width will cause the blind to bind and scrape as it lowers.

The 3-Point Height Measurement

1. Measure the inside height at the left edge.
2. Measure down the center.
3. Measure down the right edge.
4. Record the largest of the three heights. This ensures the blind lowers completely to the window sill without leaving an uneven light gap.

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3. When and How to Measure for an Outside Mount

An outside mount is recommended or required under the following conditions:

• Window casing depth is less than 1.25–1.5 inches.
• Window handles, cranks, or alarm sensors block the internal frame.
• The window frame is significantly out of square (differing by more than 3/8 inch).
• Maximum light-blocking is required (e.g. nurseries or home theaters), as outside mounts eliminate side light seepage.

Sizing Rules for Outside Mounts

1. Width: Measure the full width of the window casing from outer edge to outer edge. Add 3 to 4 inches (1.5″ to 2.0″ per side) past the molding to eliminate edge light gaps.
2. Height: Measure from the top of the upper molding down to the window sill. Add 3 to 4 inches above the trim to anchor the brackets in solid wall studs or header framing.

Unlike inside mounts, no factory deductions are applied to outside mount orders. The size you specify is the exact finished width of the headrail.

Use our interactive Window Blinds & Shades Mount Planner to test your casing depth and verify order dimensions instantly.