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How to Measure Your Windows Accurately for Perfect Window Covering Installation

July 1, 2025

Have you ever fallen in love with a beautiful set of blinds or curtains, only to discover after they arrive that they simply don’t fit? Few home-improvement moments are more frustrating—or more avoidable. Measuring your windows the right way is the single most important step toward a smooth, professional-looking window covering installation. In this guide, you’ll learn exactly how to take precise measurements, avoid common pitfalls, and feel confident ordering shades, blinds, or drapes that fit perfectly.

Understanding Window Measurements

Every window, even those that look identical, can vary by an eighth of an inch or more. That tiny difference can cause light gaps, scraping hardware, or coverings that simply will not snap into place. Because manufacturers fabricate blinds and shades to the exact sizes you supply, there is no room for guesswork.

Inside vs. Outside Mount

Before you pick up your tape measure, decide whether you want an inside mount (the covering sits within the window frame) or an outside mount (the covering overlaps the trim or wall). Inside mounts look built-in and save space, while outside mounts provide fuller light blockage and can visually enlarge a window.

Key Terminology

Width – The horizontal measurement taken at the top, middle, and bottom of the opening.
Height (or Drop) – The vertical measurement from the top of the opening to the sill—or as far down as you want your covering to hang.
Depth – The distance from the face of the window trim back to the glass, critical for inside mounts.

Tools You Will Need

Having the proper tools on hand not only speeds up the job but also improves accuracy. Gather these items before you begin:

A steel tape measure readable in 1⁄8-inch (or millimeter) increments. Avoid cloth sewing tapes—they stretch.
Pencil and notepad (or a notes app) to record each number instantly.
A step stool or ladder for tall windows.
Optional: laser distance measurer for high or hard-to-reach openings.
Masking tape to mark starting and end points if measuring solo.

How to Measure for an Inside Mount

Inside mounts are popular because they showcase existing trim and create a clean, streamlined look. Follow these steps:

Step 1: Measure the Width in Three Places

Measure at the top, middle, and bottom of the opening. Windows often bow slightly, so write down the narrowest width to ensure the blind will not rub.

Step 2: Measure the Height in Three Places

Again, take measurements on the left, center, and right sides. Record the longest height; most manufacturers deduct a small clearance so the blind doesn’t drag on the sill.

Step 3: Check the Depth

Use your tape to measure from the inside edge of the frame back to the glass. Compare that number with your blind or shade’s minimum depth requirement (usually listed on product pages). If your depth is shallow, you may need a low-profile headrail or an outside mount.

Pro Tips

Write width by height as W x H, and label each window (e.g., “Living room – east”).
Round down to the nearest 1⁄8 inch; never add “wiggle room.”
If your window is extremely out of square (more than 1⁄2-inch difference), consider an outside mount for better coverage.

How to Measure for an Outside Mount

Outside mounts attach to the face of the trim, the window frame, or even above the frame on the wall or ceiling. They are excellent for shallow depths, blackout needs, or hiding unattractive casings.

Step 1: Decide Where the Headrail Will Sit

The headrail can be flush with the top of the trim or positioned higher to create the illusion of taller windows. Mark that imaginary line lightly with a pencil.

Step 2: Determine Width Coverage

Measure the width of the window opening and add 2–3 inches on each side (4–6 inches total) to minimize light gaps. If mounting on drywall rather than trim, ensure you have solid anchors.

Step 3: Determine Height Coverage

Measure from the planned headrail position down to the desired bottom point. For blinds that stop at the sill, measure to the sill. For drapery or shades that extend below, measure to the floor or a few inches below the sill for full overlap.

Pro Tips

When in doubt, go wider and taller—outside mounts are forgiving.
Double-check there are no obstructions such as light switches or picture frames.
If you want blackout performance, overlap at least 3 inches on all sides.

Handling Uncommon Window Shapes

Arches, bay windows, corner windows, and sliding glass doors bring character to a home but require extra attention.

Tackling extra-tall or oversized windows? Here’s how to choose the right treatments for tall windows to ensure style, function, and full coverage.

Arched Windows

Manufacturers often need two numbers: the width of the base and the height at the tallest point of the arch. Some will also request the depth or a template.

Bay & Bow Windows

Measure each individual section separately, treating them as standalone windows. Remember to account for any angled headrails or shared corner brackets.

Sliding Glass & Patio Doors

For vertical blinds or panel tracks, measure the exact width of the opening and add at least 4 inches to ensure complete overlap. Height typically goes from the top of the track to ½ inch above the floor to prevent dragging.

Want to take your outdoor living area to the next level? Explore creative ways to use outdoor shades to enhance comfort, privacy, and style in your patio or deck spaces.

Double-Checking & Recording Your Measurements

Accuracy improves exponentially with verification. After finishing a window, step back, reread your notes, and measure once more. If the second reading differs, measure a third time and average—or accept the smaller value for inside mounts.

Creating a Window Schedule

Use a simple spreadsheet with columns for Room, Window ID, Mount Type, Width, Height, Depth, and Notes. This schedule becomes invaluable when you place orders or consult with installers weeks later.

Common Recording Mistakes

Mixing width and height figures.
Switching fractions (⅜ vs. ⅝).
Using different units (inches vs. millimeters) on the same sheet.

Preparing for Installation Day

Correct measurements set the stage, but a tidy, obstruction-free area makes installation faster and safer. Remove any existing treatments, fill stray screw holes, and vacuum debris inside the frame. Have a drill, level, and appropriate screws on standby. If you’ve measured accurately, brackets should align without forced adjustments.

Inspect Your Order on Arrival

Compare the manufacturer’s labels with your window schedule. Minor factory deductions are normal, but drastic discrepancies warrant a call before you mount anything.

Dry-Fit First

Place the blind or shade (without fully fastening) to confirm clearance. It’s easier to tweak bracket placement now than after everything is screwed tight.

Measuring correctly is step one—but don’t forget to consider easy-to-clean window treatments that simplify post-installation care.

The Power of Precision - Why Accurate Window Measurement Matters

Taking the time to measure your windows correctly isn’t just about fit—it’s about function, performance, and long-term satisfaction. Whether you’re aiming for better privacy, light control, or energy efficiency, the success of your window coverings starts with a tape measure. This guide arms you with the tools, terminology, and step-by-step process needed to get flawless results. With just a few careful measurements, you can avoid costly mistakes and ensure your window treatments perform beautifully for years to come.

Why Choose Us for Your Window Measuring and Treatment Needs

At Bloomin' Blinds, we don’t just sell window treatments—we guide you through every step, starting with the most critical one: accurate measurement. Our experienced team ensures your blinds, shades, or curtains are crafted to fit perfectly the first time, eliminating the hassle of returns or mismatches. Whether you're installing a sleek inside mount or customizing treatments for bay windows, we offer expert support, high-quality products, and personalized service. From consultation to installation, we’re here to make your window upgrade smooth, stress-free, and stylish.

Call us today at (855) 457-1022 or find your nearest location!

Frequently Asked Questions

Should I round my measurements up or down?
For inside mounts, round down to the nearest 1⁄8 inch so the product fits inside the frame. For outside mounts, round up to ensure full coverage.

What if my window frame isn’t perfectly square?
As long as the variance is less than ½ inch, most coverings can accommodate. Greater differences often call for an outside mount to hide the irregularity.

Can I reuse old hardware when replacing blinds?
Reusing brackets may seem convenient, but different brands often have unique headrail shapes. Using fresh, manufacturer-supplied hardware guarantees a secure fit.

How do I measure for layered treatments, like shades with curtains?
Measure the shade first (inside or outside mount), then measure the drapery rod width so panels extend 4–6 inches beyond the shade on each side for a balanced look.

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Warranty Info: All products offered by Bloomin’ Blinds are covered by a limited lifetime warranty for the original purchaser. The warranty protects against the following items: Warping, discoloration, manufacturer material defects and install issues. In the event of a warranty need, there will be no cost to the homeowner and Bloomin’ Blinds will service the repair at your home/office whenever possible. *** Warranty specifics varies by location, please contact for more information.

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How to Measure Your Windows Accurately for Perfect Window Covering Installation

Understanding Window Measurements

Inside vs. Outside Mount

Key Terminology

Tools You Will Need

How to Measure for an Inside Mount

Step 1: Measure the Width in Three Places

Step 2: Measure the Height in Three Places

Step 3: Check the Depth

Pro Tips

How to Measure for an Outside Mount

Step 1: Decide Where the Headrail Will Sit

Step 2: Determine Width Coverage

Step 3: Determine Height Coverage

Pro Tips

Handling Uncommon Window Shapes

Arched Windows

Bay & Bow Windows

Sliding Glass & Patio Doors

Double-Checking & Recording Your Measurements

Creating a Window Schedule

Common Recording Mistakes

Preparing for Installation Day

Inspect Your Order on Arrival

Dry-Fit First

The Power of Precision - Why Accurate Window Measurement Matters

Why Choose Us for Your Window Measuring and Treatment Needs

Frequently Asked Questions

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