How to Measure Windows for Custom Blinds (and Why a Free In-Home Consultation Beats DIY)

You’ve picked out your blinds. Now comes the part most homeowners dread: measuring the windows. Get it right and your custom window blinds hang beautifully, operate smoothly, and look like they were always meant to be there. Get it wrong  even by a quarter inch  and you’re looking at blinds that scrape the frame, leave light gaps, or simply won’t fit.

The good news: measuring for blinds for windows follows a clear set of rules. Once you understand inside vs. outside mount and know which three measurements to take (and which number to actually use), the process is manageable. The even better news: Bartlett Blinds offers free in-home consultations where our team handles every measurement professionally, so you never have to guess. This guide walks you through the full DIY process, then explains exactly why professional measuring is worth it for custom window treatments.

Step One: Decide Between Inside Mount and Outside Mount

Before you pick up a tape measure, you need to make one foundational decision: will your blinds mount inside the window frame, or on the wall outside it? This choice changes how you measure, what numbers matter, and what the finished installation will look like.

Inside mount blinds are installed within the window frame itself. The headrail sits inside the opening, and the blind drops straight down inside the casing. The result is a clean, built-in appearance that keeps decorative window trim fully visible and lets the architecture of the window frame speak. Inside mounts work best when the window frame has sufficient depth  most standard blinds need between 1.75 and 2.75 inches of clearance depending on the product. You also need to check for obstructions: window cranks, handles, and locks can interfere with the headrail or the blind’s travel if they protrude into the frame.

Outside mount blinds are mounted on the wall or trim above the window opening. This approach gives you more flexibility  it works regardless of frame depth, hides uneven or shallow frames, and lets you cover a larger area for better light control. Adding 1.5 to 3 inches of overlap on each side reduces light bleed at the edges. Mounting the headrail 2 to 3 inches above the frame makes the window appear taller and gives the blind room to clear the frame completely when raised.

If you’re not sure which mount is right for a particular window, outside mount is generally the safer choice. It’s more forgiving of measurement variations and gives you more control over the final look.

How to Measure for Inside Mount Blinds

Precision matters most for inside mounts. Windows are rarely perfectly square  small variations in the frame between the top and bottom are common even in newer construction, and a measurement taken in just one spot can miss those differences entirely.

Measure the width in three places: at the top of the opening, at the midpoint, and at the bottom. Record each measurement to the nearest 1/8 inch using a rigid steel tape measure. Fabric and flexible tapes stretch inside narrow frames and give unreliable readings.

Use the narrowest width. This is the number you submit when ordering. It guarantees the blind will clear both sides of the frame without binding or rubbing. Manufacturers make a small built-in deduction for clearance, don’t make your own deductions on top of theirs, or you’ll end up with unnecessary light gaps.

Measure the height in three places: on the left side, in the center, and on the right side of the opening. Measure from the top inside edge of the frame down to the sill.

Use the longest height. The manufacturer’s clearance deduction is calculated from the height you provide, so giving them the tallest measurement ensures the blind reaches the sill without leaving a gap at the bottom.

Check the depth. Measure from the front of the window opening back to the glass in several spots and use the smallest number. If it’s less than the minimum mounting depth for your chosen blind style, you’ll need to switch to an outside mount. That minimum depth requirement varies by product type and is listed in the product specifications.

How to Measure for Outside Mount Blinds

Outside mounts are less punishing for small errors, but you still want to be deliberate about where your coverage starts and ends.

Decide how much coverage you want. For good light control, plan to extend the blind 1.5 to 3 inches beyond the window frame on each side. This overlap prevents light from bleeding around the edges and gives the installation a finished, intentional look.

Measure the total width you want covered. This is the full width of the window frame plus your planned overlap on each side. Take three measurements across the area just as you would for an inside mount  top, middle, bottom  and use the widest measurement to ensure full coverage with no gaps.

Decide where the headrail will mount. Mark a point 2 to 3 inches above the top of the window frame. This gives the blind room to clear the frame completely when raised and creates a more proportional look. From that mark, measure straight down to where you want the blind to end, typically the bottom of the sill or slightly below.

Measure the height at three points: left, center, and right. Use the longest measurement so the blind covers the full opening without falling short at any point.

Write every measurement down immediately. It sounds obvious, but transposing numbers from memory between windows is one of the most common sources of ordering errors.

Common DIY Measuring Mistakes (and How to Avoid Them)

Even careful homeowners run into the same pitfalls when measuring windows for custom blinds. Here are the most frequent mistakes and what to do instead.

•       Measuring in one spot only. Windows shift, settle, and vary. A single measurement is a single data point. Take three for every dimension, every time.
•       Using a flexible tape measure. Cloth and plastic tapes bend and stretch inside narrow frames. A rigid steel tape measure is the only reliable tool for this job.
•       Measuring the old blinds instead of the window. Old blinds may have been installed with incorrect measurements, may have stretched or warped, or may simply not match what you’re ordering. Always measure the actual window opening.
•       Making your own deductions for inside mounts. Blind manufacturers build in a standard clearance deduction from the dimensions you provide. If you also reduce the numbers before submitting, you’ll over-correct and create light gaps that defeat the purpose of an inside mount.
•       Ignoring obstructions. Window cranks, locks, and handles that protrude into the frame can block the headrail or interfere with the blind’s operation. Account for them before choosing the inside mount.
•       Forgetting to check if windows are square. If your width measurements at the top and bottom differ by more than half an inch, the window is out of square. Inside mount blinds won’t sit or operate well in a significantly out-of-square frame  outside mount is the right call.

Why Professional Measuring Beats DIY for Custom Window Treatments

A DIY measuring guide gives you the rules. A professional consultation gives you the result. For custom window blinds, those are two very different things  and the gap between them matters more than most people expect before they’ve been through the process.

Here’s what changes when Bartlett Blinds measures your windows instead of you:

•       Every window is measured in person with professional tools. No guessing about which measurement to use, no checking dimensions against a written guide mid-tape-measure.
•       Mount type decisions are made on the spot with real context. A consultant can see the depth of your frame, spot an obstructing window crank, and tell you immediately whether inside or outside mount is the better call for each window.
•       Out-of-square frames are identified before ordering. Catching a window that’s more than half an inch out of square before ordering custom blinds saves a return and a reorder.
•       You see real fabric samples in your actual light. Colors and textures look different on a screen versus in the room where they’ll live. In-home consultations let you make that comparison with real materials.
•       The measurements go directly to the custom order. No transcription errors, no unit confusion. What gets measured is what gets ordered.

For window blind installation near me searches in the Collegeville and Montgomery County area, Bartlett Blinds offers free in-home consultations that include professional measuring and installation with every order. The consultation takes no more than an hour for most homes and removes every variable that causes DIY measuring to go wrong.

What Happens If You Get the Measurements Wrong?

This is the part of the conversation most measuring guides skip. For standard off-the-shelf blinds, a wrong measurement might mean returning a product and trying again. For custom window treatments, it’s more complicated. Custom blinds are made to your dimensions; they can’t be returned and re-cut like a piece of lumber. If the width is too narrow for an inside mount, you’re looking at a reorder. If the height is too short, the blind won’t reach the sill.

The costs add up fast, both in money and time. Professional measuring eliminates this risk. It’s one of the clearest cases where the free service attached to a purchase is genuinely worth more than the convenience of skipping it.

Ready for Custom Window Treatments Near Me? Start with a Free Consultation

Whether you’re measuring one window or an entire home, Bartlett Blinds makes it easy. We’re a veteran-owned, locally operated window treatment company based in Collegeville, PA, serving Montgomery County and the surrounding area. Every consultation is free, conducted in your home with real samples, and backed by professional installation. Browse our full range of blinds for windows, custom window shades, and shutters to get a sense of what’s available, then let our team handle the measuring. You focus on the style, we’ll make sure everything fits perfectly.

Schedule your free in-home consultation today and let Bartlett Blinds bring the showroom to you.

 

Frequently Asked Questions

Q: What is the difference between inside mount and outside mount blinds?

A: Inside mount blinds are installed within the window frame for a clean, built-in look that keeps decorative trim visible. Outside mount blinds are installed on the wall or trim above the window opening, which works for any frame depth and allows for more light-blocking coverage. The right choice depends on your window’s depth, whether the frame is square, and your preferred aesthetic.

Q: How many measurements do I need to take for custom window blinds?

A: You need at least three width measurements and three height measurements for each window  taken at the top, middle, and bottom (for width) and at the left, center, and right (for height). Windows are rarely perfectly square, and a single measurement can miss variations that cause blinds to scrape or leave gaps. For inside mounts, use the narrowest width and longest height. For outside mounts, use the widest width and longest height.

Q: How much overlap should outside mount blinds have?

A: Plan on extending outside mount blinds 1.5 to 3 inches beyond the window frame on each side, and mounting the headrail 2 to 3 inches above the top of the frame. This overlap blocks light at the edges, gives the blind room to clear the frame when raised, and creates a proportional, professionally finished look.

Q: What tools do I need to measure windows for blinds?

A: A rigid steel tape measure is essential. Flexible or cloth tape measures bend inside window frames and give inaccurate readings that are especially problematic for inside mounts where precision is critical. You also need a notepad to record measurements immediately, and a pencil to mark headrail positions if measuring for outside mounts.

Q: Can I measure windows for custom blinds myself, or do I need a professional?

A: DIY measuring is possible if you follow the correct process carefully. However, professionals catch issues  like out-of-square frames, insufficient depth, or obstructions  that most homeowners miss on the first pass. For custom window treatments that can’t be returned and re-cut, a professional measurement eliminates the risk of a costly ordering error. Bartlett Blinds offers free in-home measuring with every consultation.

Q: What happens if my window frame isn’t square?

A: If your width measurements vary by more than half an inch between the top and bottom of the frame, the window is considered out of square. Inside mount blinds won’t fit or operate correctly in significantly out-of-square frames, so an outside mount is the recommended solution. A professional consultant will identify this during an in-home visit and recommend the right approach before you order.

Q: Where can I get a professional window blind installation near me in Collegeville, PA?

A: Bartlett Blinds and Shutters provides free in-home consultations with professional measuring and installation throughout Collegeville, Montgomery County, Bucks County, Chester County, Delaware County, and Berks County, PA. Contact us to schedule your appointment  there’s no obligation, and installation is included with every custom blind order.