Most fit problems with blinds and shades start with a bad measurement or the wrong mount choice. This guide explains inside and outside mounts and the habits that keep a measurement honest.
Before you pick up a tape measure, decide how the shade will attach, because the two methods are measured differently. An inside mount sits within the window opening, against the frame or jamb. An outside mount attaches to the wall, the trim or the area above the window and extends past the opening.
Neither is better in every case. An inside mount gives a clean, built-in look, but it needs enough usable depth in the frame. An outside mount hides an uneven opening, can cover trim and gives better light coverage at the edges, which matters for bedrooms. Our bedroom shade guide explains why.
A third option exists for some products: a ceiling or recessed mount for shades that sit above the window. That is rare for typical home windows, so most people choose between inside and outside. If your window frame is shallow, crooked or covered with decorative trim, an outside mount is often the practical answer, even though it costs more wall space.
A steel tape measure is the right tool because fabric or cloth tapes stretch and give false readings. Add a pencil, paper and a stable step stool for tall windows. Measure each window separately, because windows that look identical often differ by a fraction of an inch, and a shade built to the wrong size cannot be fixed.
Write down width first and height second every time, label each window by room and position, and note the mount type next to each entry. Keeping the same order helps avoid swaps when you enter the numbers on an order or a measure request form.
For an inside mount, measure the width of the opening at the top, middle and bottom. Windows are rarely perfectly square, and readings can differ. Manufacturer instructions vary, but a common approach is to use the narrowest of the three width readings so the shade does not bind in the opening. Then measure the height at the left, center and right, again following the maker's instructions for which reading to use.
Check the depth of the opening too. Each shade type has a minimum depth that allows the headrail and brackets to sit properly, and that figure varies by product, so read the specification before ordering. Check for obstructions like window cranks, handles, latches or locks that can stop a shade rolling or tilting.
Also decide whether you want the shade flush with the frame. A shade that sits in deeper may look neater but can leave more room for side light. If the opening is too shallow, switch to an outside mount, which avoids the depth issue.
Work in the same order every time. Start at one side and move across. Hold the tape flat against the surface and avoid letting it sag, because a sagging tape reads long. Round measurements to the nearest eighth of an inch, or follow the rounding rule of the manufacturer, since some makers specify rounding down and some specify exact measurements.
For an outside mount, measure the width and height of the area you want to cover, not the glass or opening. Add overlap on each side so the shade extends past the opening and blocks side light. Many guides suggest a few inches of overlap on each side, but the right amount depends on the product and the room, so follow the manufacturer's guidance.
Decide where the brackets will fasten. Mounting into a stud, solid trim or a header holds better than drywall alone, and a shade hung on weak anchors will sag or pull away. Also consider anything in the way, such as a light switch, casing, crown molding or a curtain rod.
Do not guess at the overlap. Too little overlap leaves side light and exposes trim, and too much can collide with a neighboring window, door or wall fixture. Hold a straightedge or a piece of cardboard against the wall at the planned line and step back to see how it looks before you commit to a size.
The first mistake is deducting an inch yourself. Many makers build a small allowance into inside-mount shades so they clear the frame, and subtracting by hand can leave a shade too small. Order the size the maker's instructions specify.
The second is mixing the two methods, for example measuring the outside edge of trim and ordering an inside mount. The third is trusting a single reading for a window that is out of square. Take at least three, record them and use the method the manufacturer states.
The fourth is forgetting how the shade will operate. A cordless pull, a chain, a motor and a battery wand all need clear space, and a motor should be planned with the power option in mind. Our motor power guide covers that.
A second set of eyes catches typos. Have someone read your numbers back to you, and compare them with the window. Transposed digits, such as 38 for 83, are a classic error, and a shade built to the wrong width is usually not returnable because it was made to order.
Bay windows, arched windows, corner windows, skylights and very wide patio doors need extra thought. Many require several shades or special hardware, and the instructions for one product often do not carry over to another. If a window looks unusual, ask before you order rather than after.
Sun Shades Pro offers a measure request so you can get help with tricky windows. See interior roller and solar shades for the product family where careful measuring matters most.
Measure the opening or the area to be covered, not just the glass. For an inside mount you measure the opening within the frame, and for an outside mount you measure the area including overlap.
Window openings are rarely perfectly square. Taking readings at the top, middle and bottom shows the variation so you can follow the maker's rule for which one to use.
It depends on frame depth, look and light control. Inside mounts look built-in, and outside mounts cover more area and reduce light at the edges.
Not for standard windows if you follow the maker's directions, but unusual shapes and very large windows are worth getting help with.
Tell us which kind of shade you are considering and we will reply about measuring and options.