How to Measure Windows for Blinds: Step-by-Step Measuring Guide (2026)

To measure windows for blinds accurately: for inside mount, measure the window casing width at the top, middle, and bottom and use the narrowest measurement, then measure height at the left, center, and right and use the longest (never make deductions yourself); for outside mount, measure the outer window trim and add at least 4 inches to both width and height for proper overlap and light blockage. Following this standardized 3-point measuring method with a rigid steel tape measure guarantees a flawless, custom factory fit for all horizontal blinds, roller shades, and cellular shades across the US, Canada, and Australia.

Quick Reference: Inside Mount vs. Outside Mount Measuring Rules

How to Measure for Blinds: Inside Mount Step-by-Step

When homeowners search for how to measure for blinds, inside mounting is the most popular choice because the headrail nestles flush within the window frame without protruding into the room. Because window jambs are rarely perfectly square, precision is critical.

Step 1: Verify Window Frame Depth

Before recording width, use a rigid steel tape measure to check the flat depth of your window casing from the glass pane to the front edge of the trim. Most horizontal faux wood blinds, cellular shades, and roller headrails require a minimum of 1.5 inches for basic bracket attachment, and 2 to 2.5 inches for a completely flush installation. If your frame is shallower than 1.5 inches, stop and switch to an outside mount. For depth specifications across different styles, consult our inside mount vs. outside mount blinds guide.

Step 2: Measure Window Width at Three Points

Measure the exact inside opening width horizontally across the window frame in three separate places:

1. Across the top of the window opening.
2. Across the exact middle of the window opening.
3. Across the bottom of the window opening directly above the sill.

Record the narrowest of the three measurements to the nearest 1/8 inch. If your narrowest reading falls on a 1/16" increment, always round down to the nearest 1/8". Using the smallest measurement ensures your blind will never bind or scrape against uneven window jambs as it lowers.

Step 3: Measure Window Height at Three Points

Measure the exact vertical distance from the top inside jamb down to the window sill in three places:

1. Down the far left side.
2. Down the center of the window.
3. Down the far right side.

Record the longest of the three measurements. Round up to the nearest 1/8 inch. Ordering the longest height ensures the shade fully touches the bottom window sill when lowered, eliminating horizontal light leaks at the bottom.

Step 4: Do Not Make Your Own Deductions!

The number one mistake in custom window ordering is attempting to subtract clearance margins at home. Custom manufacturers (including UAM Blinds) take precise factory deductions (typically 1/8 to 3/8 inch total width deduction) to ensure brackets, end caps, and fabric clear the casing smoothly. Submit your exact, unaltered window opening dimensions.

How to Measure for Outside Mount Blinds

Learning how to measure for outside mount blinds is the ideal solution if your window frames are shallow, if you have crank handles or alarm sensors obstructing the frame, or if you want 100% blackout coverage for a bedroom or nursery. Outside-mounted blinds sit over the window opening and overlap the drywall or exterior trim.

Step 1: Determine the Width of Coverage

Measure the outer width of the window frame casing where you want the blind to hang. To ensure complete privacy and eliminate side light gaps:

• Standard Living Spaces: Add at least 4 inches total (2 inches of overlap on each side).
• Bedrooms & Media Rooms: Add 6 to 8 inches total (3 to 4 inches of overlap on each side) to create an absolute blackout seal around the glass.

Step 2: Determine the Height of Coverage

Measure the vertical height of the window opening from where you plan to install the upper mounting brackets down to where you want the blind to terminate:

• Above the Window: Add at least 2 to 3 inches above the window frame or molding to provide solid structural wood backing for screws.
• Below the Window: If your window has an extended bottom sill, measure down to the top of the sill. If your window has no sill, add 2 to 4 inches below the bottom trim so the blind completely overlaps the lower opening.

Note: Outside-mounted custom blinds are manufactured to your exact submitted dimensions with zero factory deductions. What you order is exactly what is built.

Essential Tools You Need Before Measuring

Do not attempt to measure custom window treatments with makeshift tools:

• Rigid Steel Tape Measure: Essential. Never use flexible cloth sewing tapes or plastic craft rulers, which stretch over long spans and produce errors up to 1/2 inch.
• Step Stool or Small Ladder: Never guess top measurements from the floor. You must hold the steel tape flat and level against the upper frame.
• Pencil & Notepad: Record measurements immediately using the standard industry format: Width × Height (W × H). Swapping width and height is a common mistake that ruins custom orders.
• Official Worksheet: Download or review our Official Measuring & Installation Guide for printable worksheets, visual casing diagrams, and bracket specs.

Measuring Specialty Window Types

1. Bay Windows & Corner Windows

Bay windows require careful bracket clearance calculation because angled blinds can collide in the corners. Treat each glass panel as a separate window. For inside mounts, measure each pane individually and deduct the depth of the adjacent headrail so the brackets do not overlap. For full layout diagrams, review our blinds for bay windows complete guide.

2. Sliding Glass Doors & French Doors

Sliding patio doors are typically installed with outside mounts above the door trim. Measure the outer width and add 4 to 6 inches, and measure from the bracket mounting point down to 1/2 inch above the floor to prevent fabric dragging. For French doors with glass panes, measure the visible glass opening and add 2 inches to each side, checking for door handle clearance. For product recommendations, see our sliding glass door blinds guide and cellular shades for French doors. For dual-sheer fabrics on large patio openings, follow our detailed walkthrough on how to measure zebra shades for sliding doors.

Common Measuring Mistakes That Cause Sizing Returns

Ordering custom window treatments should be stress-free. Review these common pitfalls before hitting checkout:

• Measuring Old Blinds Instead of the Window: Never measure your old, worn-out blinds. Your previous blinds may have had custom deductions or an improper fit. Always measure the bare window frame directly.
• Confusing Width and Height: Window specifications are universally listed as Width first, Height second (W × H). Double-check your numbers before submitting.
• Overlooking Window Cranks & Latches: Casement window cranks, security sensors, or interior storm window latches can block inside-mounted blinds from lowering. Measure frame depth from the protruding obstruction, not the bare glass. For a complete list of ordering pitfalls, explore our comprehensive guide on 7 common blinds buying mistakes to avoid.
• What If Your Blinds Arrive Too Long? Even with careful measurement, standard windows in older homes can settle unevenly. If your horizontal slatted blinds pool at the bottom or hang slightly too long on your window sill, you can easily remove excess bottom slats at home; follow our step-by-step DIY tutorial on how to shorten mini blinds that are too wide or too long.

Frequently Asked Questions About Measuring Window Blinds

What is the most accurate way to measure windows for blinds?

The standard industry method is the 3-point measurement: measure width at the top, middle, and bottom of the casing and record the narrowest width; measure height at the left, center, and right and record the longest height. Always use a rigid steel tape measure rounded to the nearest 1/8 inch.

Do I need to deduct anything when ordering inside mount blinds?

No. Never deduct anything from your inside mount measurements. Submit your exact window opening dimensions. Custom blinds manufacturers apply precise automated deductions (usually 1/8" to 3/8") to allow clearance for brackets and smooth movement.

How do I know if my window depth is deep enough for inside mount?

Measure from the glass pane to the front face of the window trim. If you have at least 1.5 inches of flat depth, standard cordless blinds and cellular shades will fit. For a completely flush fit where the headrail does not protrude, 2 to 2.5 inches of depth is required.

How much should I add for outside mount window blinds?

Add a minimum of 4 inches total to the width (2 inches on each side) and 4 inches total to the height (2 to 3 inches above the trim, and 1 to 2 inches below the sill). For bedrooms, adding 6 to 8 inches of total width provides superior room-darkening performance.

What if my window is out of square?

Most windows in settled homes are slightly crooked. Measuring at three separate points catches frame variations. Ordering using the narrowest width guarantees the blind will lower without jamming against the tightest section of the casing.

Further Reading & Resource Guides

• Official Measuring & Installation Guide — Printable worksheets, visual casing diagrams, and bracket instructions
• Inside Mount vs. Outside Mount Blinds Guide — In-depth breakdown of aesthetics, light control, and hardware
• How to Measure Zebra Shades for Windows & Sliding Doors — Inside, outside, and split-shade sliding door instructions
• Blinds vs Shades: What's the Real Difference? — Compare hard slatted blinds with soft fabric shades
• Honeycomb Cellular Shades Complete Guide — Cell counts, R-values, and room recommendations
• Cordless Blackout Roller Shades Guide — Complete room darkening and fabric selection walkthrough
• How to Install Blinds Yourself: A Step-by-Step DIY Guide — Easy 15-minute mounting instructions
• How to Shorten Mini Blinds: Step-by-Step DIY Guide — Safely trim excess horizontal slats at home
• 7 Common Honeycomb Shades Problems & Buying Mistakes — Prevent sizing and ordering errors

Need expert assistance measuring an unconventional window opening, curved arch, or multi-pane patio door? Contact our dedicated measurement specialists at support@uamblinds.com. Send us a quick photo of your window frame, and we will verify your mounting depth and dimensions before you order — with free shipping on custom orders $99+ to the US, Canada, and Australia.