How to Measure Windows for Custom Blinds: Inside Mount vs. Outside Mount Guide

Getting the measurements right is the single most important step when ordering custom made-to-measure window blinds. Unlike off-the-shelf curtains that can drape loosely across a wall, custom blinds are precision-engineered to fit your exact window frame down to the millimeter. Measure accurately, and your blinds will operate smoothly with zero light gaps and flawless symmetry.

Follow this expert guide from Parker Window Blinds to choose between Inside Mount and Outside Mount and measure your windows like an architectural professional.

Golden Rules Before You Begin Measuring

• Always Use a Steel Tape Measure: Never use a flexible cloth tailor’s tape. Cloth tapes stretch over time and can introduce an error of up to 15mm.
• Measure to the Millimeter or 1/8th of an Inch: Do not round up or down to the nearest whole inch.
• Measure Every Single Window Individually: Even if two windows in the same room look identical, builder tolerances on brickwork and UPVC jambs often vary by 5mm to 15mm.

Option 1: Inside Mount (Recessed Within the Window Casing)

An inside mount installs the blind inside the window frame recess. It provides a clean, modern, architectural look that showcases your window sill and woodwork.

Requirements for Inside Mount:

• Your window frame must have at least 2.5 inches (65mm) of clear depth for roller blinds, and 3 inches (75mm) for cassette zebra blinds.
• There must be no obstructions like window crank handles, alarm sensors, or burglar bars protruding inside the recess.

How to Measure for Inside Mount:

1. Measure Width in 3 Places: Measure across the top, middle, and bottom of the window recess. Record the smallest width. (Windows are rarely perfectly square!).
2. Measure Height (Drop) in 3 Places: Measure from the top ceiling of the recess down to the sill on the left, center, and right. Record the longest height.
3. Do NOT Take Any Deductions Yourself: Provide us the exact window opening dimensions. Our automated factory cutters automatically deduct the standard 10mm (3/8″) clearance so your fabric rolls down effortlessly without scraping against the side walls.

Option 2: Outside Mount (Mounted on the Wall or Ceiling Above the Frame)

An outside mount is installed onto the wall surface above the window casing, or onto the ceiling. The blind hangs over the window opening like a frame.

When to Choose Outside Mount:

• Your window frame is shallow (less than 2 inches deep).
• You want to achieve 100% total room blackout by eliminating side light halos around the edges.
• Your window opens inward, or has obstructive handles.
• You want to make a small window look dramatically larger and taller.

How to Measure for Outside Mount:

1. Measure the Width of the Window Opening: Then add 2 to 3 inches (50mm–75mm) to each side (total 4 to 6 inches wider). This overlap prevents annoying light leaks along the left and right edges.
2. Measure the Height of the Window Opening: Then add 3 inches (75mm) above the top frame (for the mounting brackets) and 2 to 3 inches below the sill (if space allows).
3. Specify These Exact Finished Dimensions: For outside mount, we fabricate the blind to the exact total width and height you submit.

Measuring for Different Frame Materials in India

• UPVC Windows: Best suited for inside mount if the reveal is deep enough, or top ceiling mount using self-tapping UPVC screws.
• Powder-Coated Aluminum Sections: Mount into the masonry wall immediately above or beside the aluminum frame to avoid drilling through thin aluminum sections.
• Teak Wood or Sal Wood Casings: Can be drilled into directly using wood screws for rock-solid bracket anchoring.
• Granite or Marble Sills: If your window has heavy stone lining, use masonry anchor plugs into the ceiling lintel for a secure fit.

Frequently Asked Questions