Measure Windows for Inside- or Outside-Mount Blinds

The three measurements you need before ordering

For an inside-mount blind, measure the window opening at the top, middle and bottom for width, and at the left, center and right for height. Most common inside-mount product instructions use the narrowest width because a wider blind could bind at a tight point. The correct height selection depends on the manufacturer's product instructions; many blind and shade guides use the longest height, but do not transfer a height rule from one product to another. Measure the usable depth of the recess, too. A narrow frame or an obstructing handle can prevent an inside mount even when your width and height measurements look perfect.

For an outside-mount blind, measure the entire area you want to cover, not just the exposed glass. Decide how far the headrail will extend above the window and how much coverage you want at each side. That overlap improves coverage but must fit around nearby walls, doors, handles and furniture. The first choice is therefore the mounting method and actual product, not an attractive number from a generic calculator.

This guide gives you a copyable measurement worksheet and two worked examples. Use our interactive Blind Measurement Worksheet to compare three inside-mount readings or add outside-mount coverage. Its outputs are planning values, not a certified custom order. The brand-specific instructions always control the final purchase. Our general measurement guide explains why consistent units and clearly recorded dimensions matter for other projects, while this article focuses on window treatment ordering.

Inside mount or outside mount: the practical decision

An inside-mount blind sits within the window casing. It can leave the surrounding trim visible and produce a clean recessed appearance. But the product needs enough mounting depth, suitable fixing surfaces and room to operate. A latch, window crank, tilt-in sash or projecting lock can interfere with the headrail or slats. A shallow frame may allow a partially projecting mount for one product but prohibit mounting another altogether.

An outside-mount blind attaches to a suitable surface beyond the opening, such as the wall or trim. It can cover more of the window and conceal a shallow or uneven recess, though it uses more visible wall space. It may collide with nearby cabinetry, a door swing, decorative molding or another window treatment. You also have to verify that the proposed fasteners and substrate can support the chosen product; a measurement worksheet cannot establish fastening safety.

Common error: people see a manufacturer statement that a blind will be made slightly narrower, then subtract that amount a second time from their order. If the instructions say to provide the exact measured opening and the factory makes the clearance deduction, enter the opening measurement without adjusting it yourself. A different product or retailer may handle deductions differently, so verify rather than generalize.

Both LEVOLOR's custom wood-blind technical guide and Bali's measuring guides emphasize measuring individual windows and following the product-specific mount method. The examples here are explanatory, not instructions for every available style.

Prepare the window and the measurement worksheet

Use a rigid steel tape measure, pencil and paper or a spreadsheet. Record width first, then height, as indicated by the product ordering system. Label each window with its room and position, such as Living Room — Left, rather than writing 'window one' on several unmarked pages. Measure every opening separately even when two windows appear identical. Trim thickness, settlement and frame construction can change the dimensions slightly.

Measure the physical inside surfaces of the casing if choosing an inside mount. Do not measure only the visible glass. Place the tape at three different vertical positions for width and three horizontal positions for height. Keep the tape level, and repeat suspicious readings. Record the measurement in inches using the precision your supplier specifies, often eighth-inch increments. Do not round up to make a narrower opening look wider. For a custom order, confirm exactly how the seller wants fractions and where height is measured to the sill.

Measure recess depth from the planned attachment surface toward the room, noting the shortest usable depth when obstructions intrude. Check locks, handles, window-opening movement and the distance needed for a headrail and full operating path. The minimum required depth varies by product, lift type, valance and installation hardware; a number from a shutter guide is not interchangeable with one for roller or faux-wood blinds.

Use this worksheet structure: Window ID | mount type | top/middle/bottom widths | left/center/right heights | shallowest usable depth | planned overlap | obstacles | selected product and guide version | order dimensions. Keep the raw measurements even after deciding on the order size. That lets a retailer or installer retrace the decision if a product arrives looking wrong.

Worked inside-mount example with three width readings

Suppose a hypothetical bedroom window opening has the following interior measurements. The widths, from top to bottom, are 35 7/8 inches, 35 13/16 inches and 35 3/4 inches. The measured heights, from left to right, are 59 7/8 inches, 60 1/8 inches and exactly 60 inches. These are fictional readings selected to illustrate why one measurement is insufficient. They are not measurements of a real window or a recommendation to order a particular blind.

The narrowest width is 35 3/4 inches because 3/4 equals 12/16, which is less than 13/16 and 14/16 (7/8). The longest height is 60 1/8 inches. If the specific selected blind's guide instructs you to order the narrowest width and longest height without making your own deductions, the order measurements would be 35 3/4 by 60 1/8 inches. That is an illustration of the selection rule, not an assurance of fit. Confirm mounting depth, frame squareness, product width increments and what its order fields mean.

If the product accepts only one-eighth-inch increments, the 35 13/16-inch middle reading is not itself a standard eighth. Keep the precise raw value in the worksheet; do not arbitrarily round a selected controlling dimension upward. In this example the controlling width is already 35 3/4 inches, a valid eighth increment. Rounding policy should come from the actual ordering instructions. A crooked or seriously out-of-square opening deserves a product-specific evaluation, not a forced conversion into a tidy rectangle.

Worked outside-mount example: plan coverage rather than clearances

Imagine the visible opening is 35 3/4 inches wide. For a fictional outside-mount project, you decide to cover 1 1/2 inches beyond the opening on each side, following a particular product's recommendation. The desired coverage width is 35 3/4 + 1 1/2 + 1 1/2 = 38 3/4 inches. This is simple geometry, but only a suitable manufacturer guide can tell you whether that is the correct order width for its construction and whether the headrail, fabric and slats have the same effective width.

For height, choose where the top of the headrail will be and where the blind should end. Suppose you place its top 3 inches above a 60-inch-tall opening and want 2 inches of coverage below it. The total selected height in this illustrative layout is 3 + 60 + 2 = 65 inches. Check how that brand defines height and whether the operating hardware or bottom rail alters the finished coverage. If a sill projects into the path, you may need a different lower endpoint.

The Bali Natural Shades measuring guide illustrates an outside-mount width overlap of 1 1/2 inches per side for that product. It is not a universal requirement for every blind. A small apartment window against a perpendicular wall may not even have enough space for symmetrical overlap. Measure the free area first and select a suitable mounting system accordingly.

Compare an actual inside-mount order and an outside-mount order only after you have chosen the exact material, lift mechanism and bracket configuration. A lower online price does not offset an unusable fit. Keep the retailer's return and custom-order policies alongside the measurements before you purchase, particularly when products are made to order.

Check depth, operation and child safety

A recessed frame can look deep enough until the window crank, latch or tilting sash is operated. Inspect all moving parts and identify the complete area used when the blind is raised, lowered and tilted. A blind that clears the handle when closed might collide when opened. The actual headrail size, fixing position and hardware options are given in the manufacturer's technical or installation guide, not by a general home improvement article.

For an outside mount, confirm the mounting surface and nearby obstructions. Do not assume decorative trim, drywall alone or a deteriorated frame provides suitable support for the fastening method supplied with a heavy blind. Follow installation instructions for the actual wall substrate and seek qualified help if you cannot determine a sound fixing method. Mark electrical lines or other concealed services as a potential risk before drilling; a fit calculation does not locate them.

Safety is part of product selection, not an optional final decoration detail. In homes where young children may be present, the U.S. Consumer Product Safety Commission recommends choosing cordless window coverings to avoid accessible-cord strangulation hazards. Do not treat improvised cord tie-ups as an equivalent guarantee. Check the product's current safety information and suitable operation method before purchasing. If the room's occupants or accessibility needs make a blind difficult to reach, that requirement should be included when choosing the control mechanism and installation height.

Moisture exposure, sunlight, cleaning needs and privacy also affect the appropriate style. Faux wood, natural material, fabric and roller products have different environmental and mounting specifications. The measurement worksheet helps order a selected product; it does not establish that a given material belongs in a damp bathroom or in front of a heat source.

Eight ordering checks to prevent an expensive return

1. Identify each window. Mark room, side and the exact opening; keep a separate worksheet for every opening.
2. Choose the product. Get the latest measuring guide for its specific blind, shade or shutter style, not just the brand's general landing page.
3. Choose mounting style. Verify inside depth and obstructions, or outside wall space and overlap.
4. Measure three ways. Record three widths and three heights; preserve actual fractions rather than replacing the raw record with one number.
5. Apply the documented selection rule. Use the controlling measurements and rounding method from the selected guide, without inventing clearance deductions.
6. Check function and safety. Confirm handles, headrail path, lifting method, fasteners and cordless suitability where children may be present.
7. Confirm what is being ordered. Is the entered width an opening measurement, blind width or fabric width? Are brackets or valances included? Ask before paying.
8. Keep a record. Save product identification, order fields, measurements, guide revision, prices and return conditions with the project budget.

If you are comparing several rooms, enter real material and installation quotes in the Project Budget Workspace. It can help organize user-supplied costs but cannot verify the fit or price of a custom blind. If you also plan to replace a damaged window, our window-frame materials comparison discusses a separate whole-window decision; replacement-window dimensions are not blind-order dimensions.

Questions homeowners ask before placing an order

Do I subtract half an inch from my inside-mount width?

Do not make that deduction automatically. Some manufacturers already subtract clearance when they fabricate a custom blind from the supplied opening measurement, and product rules differ. Check the measuring guide and ordering interface for the exact blind. Double-subtracting can leave a wider-than-expected gap.

Why measure every window if they look identical?

Two openings built on the same wall can differ by small fractions because of trim and installation tolerances. A custom blind often has little flexibility. Record and label each window individually; do not use the first opening as the order size for the others without verifying them.

Should I use the shortest or longest height?

Follow the selected product's instructions. Many blind and shade products use the longest inside height, but other types or mounting scenarios may require different handling. If a sill, crank or irregular frame changes the usable height, consult the actual technical guide before ordering.

What if my window frame is shallow?

Check the product's minimum and flush-mount recess requirements along with any handle interference. A partial projection may be supported by some products, while other systems require an outside mount. Never assume the dimensions for a different product establish compatibility.

Can an outside-mount blind cover more light at the sides?

Additional overlap can improve coverage, but the appropriate amount depends on the product and space available. Measure the desired finished coverage and confirm how the manufacturer translates order width into actual shade or blind coverage. Surrounding walls, molding and operating hardware may limit your options.

Keep the worksheet and the product instructions together

A correct order starts with actual dimensions, a chosen mounting method and the product's real installation limits. For inside mounts, three widths expose the tightest part of the opening; multiple heights and a depth measurement reveal other fit risks. For outside mounts, decide the finished coverage and check that there is space and support for the brackets. The two hypothetical examples above show how to select and calculate dimensions, not universal sizes to copy into a cart. Write down the controlling measurements, do not invent your own factory deductions, and review the complete order details before purchasing. You will have a defensible set of measurements to discuss with the retailer if a question arises.

Reference guides: LEVOLOR — Custom Wood Blinds Technical Guide; Bali — Measuring Instructions; U.S. CPSC — Window Covering Cords. Product manuals take precedence over generalized examples; all dimensions in the worked scenarios are illustrative.

An obstruction and fit worksheet that catches errors a tape measure misses

Before ordering a custom blind, add a simple installation-risk column beside your width, height and recess-depth measurements. For each window, record the window ID, the finished wall or trim surface where brackets would sit, the narrowest opening, the controlling height required by the selected product, the minimum usable recess depth, and the parts that project into the blind's path. Common obstructions include window cranks, locks, latches, uneven trim, protruding tile, alarm contacts and casement handles. Photograph the ruler beside each obstruction to preserve context. A blind can satisfy a supplier's width range and still collide with a handle during opening.

Here is a fictional acceptance exercise, with all dimensions in inches. An inside recess measures 35 7/8 at the top, 35 13/16 at the middle and 35 3/4 at the bottom. The narrowest recorded opening is 35 3/4 inches; it is not an instruction to subtract another universal allowance. Measured heights are 58 1/4, 58 1/8 and 58 3/8 inches. Write all three heights on the sheet and select the ordering height according to that exact blind's instructions. Suppose the tightest usable recess depth after an obstruction is 1 7/8 inches, while the chosen supplier explicitly requires 2 1/4 inches for a full recess installation. That combination fails the proposed inside-mount fit even if the width seems perfect. Change the product or mounting plan, then remeasure. Never assume the headrail will clear simply because the slats fit.

For an illustrative outside mount, suppose the homeowner wants to cover a 48-inch-wide visible opening and chooses 1 1/2 inches of coverage on each side. The proposed coverage width becomes 48 + 1.5 + 1.5 = 51 inches. This is a coverage target, not automatically the manufacturer's order entry: check whether the product measures fabric width, headrail width or finished overall width. Also inspect whether 51 inches will overlap wall switches, curtain rods or nearby doors. Repeat for top height and bottom clearance rather than treating width as the whole problem.

Mark the window ready to order only after the mounting surface, fasteners for that wall material, manufacturer-specific size deductions, window operation and control placement have all been checked. If any item is unknown, write hold—verify instead of guessing. A second five-minute visit with the correct product instructions is cheaper than replacing a made-to-measure blind.

Installation-day acceptance: check function before removing packaging

1. Match the box label to the window ID on your measurements; similar-looking windows can differ by fractions of an inch.
2. Hold brackets in their specified positions and confirm that screws will anchor in suitable material. Drywall by itself is not always an adequate fixing substrate for the load and operation of a blind.
3. Check the product's full operating sweep: can the window crank turn, will the sash open, and does the raised blind clear a door or handle?
4. Inspect side coverage with the lights on at night if privacy is the goal. Daylight photographs can hide a gap visible from outside after dark.
5. For homes with children, prioritize a cordless covering wherever practicable and follow the manufacturer and U.S. CPSC guidance; older accessible cords can present strangulation hazards.
6. Keep the order sheet, instructions and any product-specific tolerances. Record a final photo only after the blind moves freely without scraping or binding.

The key distinction is between what you measured, what you entered in the cart and what the factory produced. Keeping those as separate fields makes a customer-service discussion factual rather than speculative.