Ordering custom blinds can make an awkward window feel intentional, but the process starts well before fabric, slat color, or operating style. A measurement that is off by a small amount can create light gaps, prevent a shade from sitting level, or leave too little room for the hardware. The good news is that measuring is straightforward when you use a consistent method and make decisions about mounting before writing down any dimensions.
This guide focuses on the practical details that affect real installations: uneven walls, shallow frames, protruding handles, layered trim, and the difference between the opening you see and the space a blind actually needs. Take your time, measure more than once, and keep the notes for each window separate. Those habits are more useful than trying to estimate a size from a single quick measurement.
Decide whether the blind belongs inside or outside the frame
The first decision is the mounting style. An inside mount sits within the window opening, usually on the top surface or upper interior area of the frame. It has a neat, built-in appearance and leaves surrounding trim visible. It also depends on having enough unobstructed depth and an opening that is reasonably square. An outside mount attaches above or around the opening, covering part of the wall, casing, or trim.
Do not assume one choice is automatically better. Inside mounts are often appealing for deep, clean window frames, while outside mounts can be the practical answer for narrow frames, shallow recesses, or windows that need better light coverage. Before measuring, look for cranks, handles, alarm sensors, molding, and any object that occupies the area where brackets or fabric would sit. The mount type determines every number that follows.
Gather tools that make the numbers dependable
Use a steel tape measure rather than a cloth sewing tape, which can stretch or curve and make it harder to hold a straight line. A pencil, paper, and a level are helpful, and a step stool may be necessary for tall windows. A phone camera is useful as well: a quick photo of each opening can help you remember where a crank, trim detail, or unusual obstruction is located when you review measurements later.
Record dimensions in the format requested by the supplier, commonly width first and height second. Mark each window with a clear name, such as “kitchen sink,” “bedroom west,” or “living room left.” Avoid labels like “window one” unless you also have a sketch. If several windows look alike, draw a simple wall layout and place each label on it. This small organizational step prevents a correct size from being assigned to the wrong opening.
Take inside width measurements at three distinct points
For an inside mount, measure the width inside the opening at the top, middle, and bottom. Hold the tape measure against the surfaces that define the true opening, not against a decorative edge that projects inward. Write down all three dimensions rather than only the smallest one. Frames are frequently a little narrower in one area because of paint buildup, plaster, wood movement, or construction variation.
In most ordering systems, the narrowest width is the important measurement because it identifies the tightest point the blind must pass through. However, do not make your own deductions or reductions unless the ordering instructions specifically tell you to do so. Many custom blind providers make the fitting allowance themselves. When comparing styles or arranging a measurement visit for Boerne window treatments, it is sensible to ask whether the submitted dimension should be the exact opening size or the finished product size.
Measure inside height and depth with equal care
Measure the height from the top inside surface where the headrail will mount down to the sill or bottom of the opening. Take this measurement at the left, center, and right, especially if the sill is not perfectly level. Note the longest measurement as well as any obvious difference between sides. The manufacturer’s directions will indicate which figure governs, but the full set of notes gives an installer useful context.
Depth is often overlooked, yet it can decide whether an inside mount is possible. Measure from the front edge of the window opening to the nearest obstruction, such as the glass, a crank, a handle, or a screen frame. Some products need only enough room for their brackets, while others need more space to sit fully recessed. Record both total depth and clear depth if there is a protrusion, since a blind may fit but extend beyond the frame.
Check whether the opening is square before choosing a tight fit
A window can look rectangular while still being slightly out of square. To check, measure diagonally from the upper left corner to the lower right, then from the upper right corner to the lower left. If the two diagonal measurements differ noticeably, the opening is not square. This does not make custom blinds impossible, but it can affect how evenly the bottom rail meets the sill and how much light appears along the sides.
Also use a level to inspect the sill and the upper mounting surface if you suspect a problem. A tightly fitted inside mount may emphasize a sloping sill, whereas an outside mount can create a more visually balanced line. Keep these observations in your notes instead of trying to correct them by changing a measurement. The dimensions should describe the opening honestly; product recommendations can account for its quirks.
Map out obstacles before selecting the hardware position
Look closely at the upper part of the opening and the surrounding wall. Window cranks, tilt latches, security sensors, deep handles, and inward-opening shutters can interfere with brackets, headrails, or the movement of a shade. For each obstacle, measure how far it projects and how close it is to the mounting area. A photo with the tape measure in view can be especially helpful for unusual conditions.
Consider how the window will be used after installation. A blind that looks good when lowered may be frustrating if it blocks a handle you use every day or cannot clear an inward-opening sash. If you are exploring options described as custom window treatments Boerne, bring these functional details into the conversation alongside your measurements. Product type, bracket placement, and lift direction can matter as much as width and height.
Size an outside mount around coverage rather than the glass
For an outside mount, measure the width and height of the area you want the blind to cover. This is not necessarily the same as the glass size or even the interior opening size. Start by identifying where the brackets can attach securely, then determine how far the treatment can extend on each side without hitting trim, nearby walls, light switches, or another window.
Extra width beyond the opening helps reduce side light gaps and can make a narrow window appear more substantial. Extra height above the opening creates room for the headrail and allows the treatment to stack above the glass when raised, if the wall space permits. Rather than using a one-size-fits-all overlap, assess the trim, fabric or slat style, and nearby clearances. The finished width should be a deliberate coverage choice, not a guess.
Account for trim, casing, and walls that are not flat
Trim changes the available mounting surface. Some casing is broad and flat enough to support brackets, while other trim is narrow, angled, or highly decorative. Measure the flat portion rather than the overall visible width. If the outside mount will sit above the trim, measure the clear wall area above it and check whether the surface is level. A headrail mounted on a sloping or uneven surface will be noticeable.
Bay windows, arched tops, and closely grouped windows need an extra layer of planning. In a bay, each treatment needs room to operate without colliding with its neighbor, so record the angle and the distance between return walls. With side-by-side windows, note the amount of wall between openings and whether one continuous treatment or separate treatments makes more sense. A local showroom or service listing for window treatments Boerne, TX can be a useful starting point when a layout has enough unusual details that an in-person look is preferable.
Measure doors differently from ordinary windows
Doors introduce movement and clearance issues that stationary windows do not have. For a door, identify the swing direction, handle location, and how much open floor or wall space is available beside it. Measure the full glass area, the overall door area that needs privacy or light control, and the clearance above the door for mounting hardware. If there is a transom or adjacent sidelights, decide whether those pieces need separate coverage.
An outside mount is common for doors because many door frames lack the depth needed for an inside-mounted product. Still, confirm that the treatment can be raised, tilted, or moved aside without interfering with the handle or passageway. For sliding doors, measure the entire opening and note which panel moves. The appropriate configuration depends on the direction of travel, the available stack space, and whether daily access needs to remain especially easy.
Understand the difference between opening size and order size
One of the most common ordering mistakes is treating every recorded dimension as the exact finished blind size. For an inside mount, manufacturers often use the customer’s opening measurement and make a small allowance so the product operates within the frame. For an outside mount, the customer may instead be asked for the exact finished width and height desired. These are different instructions, even when the same tape measure was used.
Read the product-specific measuring guide before entering numbers. Roller shades, cellular shades, horizontal blinds, woven materials, and shutters can have different headrail dimensions, bracket requirements, and fabric coverage. Do not subtract clearance “just in case,” and do not add extra material to an inside measurement unless the instructions say to do it. When in doubt, preserve the raw measurements in your notes and get clarification before placing the order.
Use a final review to catch mistakes that are easy to miss
Before ordering, return to every window and repeat the measurements. A second pass often reveals a transposed number, a tape measure that was not held level, or an opening measured from trim rather than from the true frame. Compare the notes with your photos and sketch. Make sure every entry has a window label, mount type, width, height, depth where relevant, and any comments about obstructions or unusual trim.
Finally, think through the installed result rather than only the ordering form. Will the treatment clear a handle? Will an inside mount sit as deeply as you expect? Is there enough room above an outside-mounted shade for its hardware and stack? Careful measuring is less about producing a single perfect number and more about describing the full space accurately. That fuller picture gives you a far better chance of receiving blinds that fit, function smoothly, and look right from the first installation.