Measuring for blinds and shades: inside mount, outside mount and the maker's rules

Measure three widths and three heights, record the right one for the mount, check the depth against the maker's minimum and add overlap for an outside mount. The factory takes its own deductions; the reader takes none.

Inside mount or outside mount Understand

Every blind and shade is ordered for one of two positions, and the position decides which measurements count. An inside mount sits inside the window frame, with its brackets fixed to the top of the opening. An outside mount sits on the trim or the wall around the window and covers the opening plus a margin, called the overlap. Curtains are measured differently again, from the wall beside the window; Measuring for curtains covers them.

The frame here means the opening the window sits in, lined by the jambs at the sides, the head at the top and the sill at the bottom. The trim or casing is the flat molding around it. The headrail is the top bar of a blind or shade, which holds the mechanism and clips into brackets.

A maker's measuring guide usually opens by asking which mount you want, because the two are measured in opposite ways. Inside, you measure the opening exactly and the factory makes the product slightly smaller so it fits. Outside, you measure the area you want covered and the factory makes it that size. Bali's guide for wood and faux wood blinds lists the reasons for each, and they are a fair summary of what the rest of the makers say Product-specific:

The source for the table is Bali's measuring guide for wood and faux wood blinds, page 3. Hunter Douglas makes the same two points in its buyer's guide, Inside vs. outside mount: an outside mount needs about two inches of level surface above the trim (or it goes on the trim itself), and a casing that is too shallow rules out an inside mount. Product-specific

Three questions usually settle the mount for one window:

• Does the window need darkness? If someone sleeps by day, or the room is a nursery, the side gaps of an inside mount matter. Read Light gaps and the stack before deciding.
• Is the frame deep enough? Measure it before you fall for a product. The depth section below says how.
• Can you make holes, and where? Both mounts take screws: an inside mount into the top of the frame, an outside mount into the trim or the wall. In a rental, the lease decides whether either is allowed, and the renters section of Hardware and installing lists what to ask and the no-holes options. Ask the authority

Either way, measure the window and not the blind that is already there. Levolor's guide puts this in its margin notes, and it saves a common error: an old blind may have been ordered with the wrong deductions, or for the other mount. Product-specific

Width and height, three times each

Windows are not perfect rectangles. Frames settle, jambs bow and sills slope, so the opening can be a little wider at the top than at the bottom, or taller on one side. Every maker's guide we read asks for the width and the height in three places for that reason. Convention

The tape and the precision

Use a steel tape measure. Bali's guide says not to use a cloth tape, and Levolor's says a cloth tape may stretch. Write every figure to the nearest 1/8 in, which is the precision Bali, Levolor and The Shade Store all ask for. In centimeters, 1/8 in is about 3 mm, and the blind and shade size box rounds metric results to 0.5 cm. Measure each window on its own, even when a row of windows looks identical; Levolor's guide says to. Product-specific

Label every figure as width or height. Bali's guide asks you to record them clearly, and a swapped pair is one of the easiest errors to make and one of the most expensive to receive. We write the width first and the height second, every time. Our practice

Inside mount: the narrowest width

Measure across the opening, from the inside face of one jamb to the inside face of the other, at the top, the middle and the bottom. Record the narrowest. A blind made to the narrowest width fits through every part of the opening; one made to a wider figure binds where the opening pinches. Bali, Levolor's 2022 cellular technical guide and The Shade Store's 2024 cellular guide all say to record the narrowest (The Shade Store says the smallest). Convention

Look at the spread as well as the minimum. If the three widths differ by 1/4 in or less, the frame is close to square for this purpose. If they differ by more, the window is out of square, and an inside mount will show uneven gaps at the sides. The blind and shade size box flags a spread over 1/4 in as an observation, not as an error. That threshold is ours, chosen as a prompt to look again; no maker we read publishes one. Our practice

Inside mount: which height

Measure from the inside of the head (the top of the opening) to the top of the sill, at the left, the center and the right. Here the makers split, and the split depends on the product.

• Record the longest. Bali's wood blind guide and its Classic Roman shade guide (July 2017), Levolor's cellular guide and The Shade Store's cellular guide all ask for the longest of the three. A blind or shade hangs free, so the longest figure makes sure the bottom rail reaches the sill everywhere. Product-specific
• Record the shortest. SelectBlinds' shutter measuring instructions ask for the smallest height and width. A shutter is a rigid frame that has to fit inside the opening, so the shortest figure is the one that fits. Product-specific

So the rule is the product's rule. The blind and shade size box lets you choose longest or shortest, and defaults to longest, which is what most of the makers we read ask for.

Take no deductions

A deduction is the small amount taken off the width (and sometimes the height) so the product clears the frame and operates. On an inside mount the factory makes it, and the guides are blunt that you do not. Levolor's 2022 guide says to take no deductions and lists its own: 3/8 in off the width of an inside-mount cellular shade, nothing off an outside mount. The Shade Store says it makes deductions of about 1/8 to 1/2 in. Bali says the manufacturer takes the deductions for a fit. Product-specific

If you take 1/4 in off yourself and the factory takes its 3/8 in as well, the shade arrives 5/8 in narrower than the opening. Write the opening as measured and let the order form say what will be made.

Outside mount: the area to cover

For an outside mount, the size you order is the size of the area to cover, and you choose it.

• Width. Measure the opening (or the outside edges of the trim, if the blind is to cover the trim) and add the overlap each side. Bali and Levolor give 1 1/2 in each side, 3 in in all, and Levolor says at least that much. The Shade Store suggests 1 to 1 1/2 in each side if space allows. The factory takes nothing off the width of an outside mount. Product-specific
• Where the headrail goes. Decide whether it sits on the trim or on the wall above, and mark the spot. Bali's guide asks for this before the height, because the height is measured from there.
• Height. Measure from the top of the headrail position to the top of the sill, or lower if you want overlap below for more light control. Bali's Classic Roman guide says the length typically runs from 1 in above the trim to the sill or the apron (the trim below the sill). Levolor says to deduct 1/4 in if you do not want the shade to touch the sill. Product-specific
• Room for the brackets. Bali's Roman shade guide and Hunter Douglas both say outside-mount brackets need at least 2 in of flat surface. Where the trim is narrow or the brackets have to clear a frame or molding, Bali says projection brackets or shims can be ordered. Product-specific

For an outside mount, the blind and shade size box takes the opening width you measure and adds the overlap each side (1 1/2 in unless you change it) and the overlap above. Enter your maker's figures if they differ.

Depth, and what sticks out of the frame

Depth is the distance from the glass (or the window sash, whichever is nearer the room) to the front edge of the frame, measured inside the opening. It decides whether an inside mount is possible, and whether the product will sit flush with the frame or stick out past it. Physical principle

Each product has two depth figures. The minimum mounting depth is what its brackets need to hold. The flush or full recess depth is what it needs to sit entirely inside the frame, with nothing past the front edge. Between the two, the product mounts and works but its front stands proud of the frame. Below the minimum, it needs an outside mount.

The figures range widely, and they depend on the product and on how it is operated:

The Levolor rows are from the depth table for one collection in its 2022 cellular technical guide; other collections in the same guide have their own tables. The pattern across makers holds: motorized products need more depth than corded or cordless ones, and wide slats need more than narrow ones. Product-specific

Measure the shortest depth, and what is in the way

Levolor's guide says to measure the exact depth, note anything that could get in the way (handles, tracks, cranks), and use the shortest depth. The things that stick out into the frame reduce the depth a blind can use, because the blind has to hang in front of them:

• the sash lock on the meeting rail of a double-hung window;
• the crank handle of a casement window;
• window handles and tilt latches;
• alarm sensors on the frame;
• a sloped or stepped sill that brings the glass closer at the bottom.

Measure the depth in front of the deepest of these, not the clear depth beside it. If a lock stands 3/4 in out from a sash in a frame 2 3/4 in deep, the blind has 2 in to hang in. A crank handle can often be removed or folded; a lock usually cannot. Test it

The blind and shade size box compares the depth you enter with the maker's minimum you type, and says whether it meets it. It leaves the minimum blank on purpose, with the placeholder "from the maker's guide", because no single figure is right for every product.

Light gaps and the stack

Two things about a blind or shade are fixed by the product and the mount, not by how carefully you measure: the gap at the sides and the height of the raised blind. Both matter most where the room has to be dark.

The light gap

A light gap is the strip of open space between the edge of a blind or shade and the frame. On an inside mount it is there by design: the factory deduction that lets the shade run up and down without rubbing leaves a gap each side. Levolor's 2022 guide states it as 3/16 in on the left and right of an inside-mount cellular shade. Bali's wood blind guide says the deduction causes a small amount of light to spill through on each side. Product-specific

Light comes through that gap even when the fabric itself blocks it. Hunter Douglas's light control and privacy page says opaque fabrics darken a room but do not entirely remove the light. For a room that has to be dark, such as a bedroom for someone who sleeps by day, the edges are where to work.

The options, in the order we would try them:

1. Outside mount with overlap. Bali names light at the sides as the first reason to choose an outside mount: the overlap covers the gap. A wider overlap covers more; Bali's vertical cellular guide asks for 3 in each side. Product-specific
2. A curtain or side panels over the shade. Hunter Douglas suggests side panels or drapery to help cover light gaps. Measuring for curtains sizes them, and the linings section of Fabric, linings and fading covers what stops light.
3. Side channels. Some makers sell U-shaped channels that fix to the frame and hold the edges of the shade. Hunter Douglas offers them on one product line, as part of that product rather than as an add-on for any shade. Product-specific

What opacity to choose (light filtering, room darkening, blackout) is a separate decision, made in Daylight, glare and darkness. This section is about the edges, which leak whatever fabric you choose.

The stack

The stack height is how much vertical space a blind or shade takes up when it is fully raised: the folded fabric or the gathered slats, plus the headrail and the bottom rail. On an inside mount the stack hangs inside the frame, so it covers the top of the glass. On an outside mount fixed above the opening, it can sit partly or wholly clear of the glass, which Bali gives as a reason to choose one. Physical principle

The stack grows with the height of the shade, and it differs a great deal by kind. The Shade Store's stacking chart gives these figures for its own products Product-specific:

Levolor's 2022 guide prints stacking charts for each collection, by shade length from 36 to 144 in, and says they include the fabric, the headrail and the bottom rail. If the view or the daylight at the top of the window matters, look up the stack for the product and the height you are ordering, and subtract it from the glass you will see with the shade raised.

See it See it

• Levolor custom cellular shades technical guide (2022), a maker's full ordering guide: measuring, factory deductions, the light gap and depth tables in one place.
• Bali measuring guide for wood and faux wood blinds, a short maker's guide with a worksheet and the reasons for each mount.
• The Shade Store stacking chart: how much of the window each kind of shade covers when raised, by height.
• Hunter Douglas, Inside vs. outside mount: a plain-language comparison from another maker.

Practice Practice

Minimum: one window measured in six places plus its depth, with a mount chosen and a size worked out. Time: about 30 minutes. Materials: a steel tape measure, a pencil and paper (or a phone), a step stool if the top of the frame is out of reach, and the measuring guide for the product you have in mind if you have one.

1. Clear the window. Raise any blind that is up, open the curtains, and look at what is in the frame: locks, cranks, sensors, a sloped sill. Write each one down with how far it stands out.
2. Measure the three widths. Inside the frame, jamb to jamb, at the top, the middle and the bottom, to the nearest 1/8 in. Write W next to each.
3. Measure the three heights. Head to sill, at the left, the center and the right. Write H next to each.
4. Measure the depth. From the sash or the glass to the front edge of the frame, at the shallowest point, and again in front of anything that sticks out. Keep the smaller figure.
5. Measure for an outside mount too. The width across the outside of the trim, the width of the trim itself, and the distance from the top of the trim to the ceiling. These are cheap to take now and save a second trip.
6. Enter the figures in the blind and shade size box. Choose the mount, the height rule from your product's guide, and type the maker's minimum depth and overlap if you have them. Read the spread and the depth result.
7. Write the row in the window survey: the six measurements, the depth, the trim width, the space to the ceiling, what is in the way, the mount and the size to order. Then measure once more, on another day if you can, and compare.

Your windows Your windows

Check your work Check your work

Criteria

• Are there three widths and three heights, each to the nearest 1/8 in, labeled W and H?
• For an inside mount, is the width the narrowest, and is the height the one your product's guide asks for?
• Is the depth the shortest, measured in front of anything that sticks out, and is it compared with the maker's minimum?
• Is the size written as measured, with no deductions of your own?
• For an outside mount, is the overlap added each side, and is there 2 in of flat surface for the brackets?
• Where darkness matters, have you decided what to do about the light gap?
• Is the row in the window survey, and have you measured a second time?

Worked example

Priya's row (fictional) meets each criterion. Her six figures are labeled and to the nearest 1/8 in. The narrowest width (34 1/4 in) and the longest height (58 5/8 in) are recorded for a shade, with the shortest height (58 3/8 in) noted in case she looks at shutters. Her depth is 2 in, measured in front of the sash lock, and compared with the product's minimum; she has the figure for one product and not yet for the one she will buy. She has taken no deductions. For darkness she has worked out an outside mount, 37 1/2 by 61 1/8 in, on trim wide enough for the brackets. The one criterion she cannot meet yet is the mount itself, because it waits on her landlord's written answer.

Common mistakes

• Measuring the old blind instead of the window. Levolor's guide warns against it. The old blind may have been made for the other mount.
• Taking a deduction yourself. The factory takes its own, and two deductions make a shade too narrow.
• Measuring the depth beside the lock instead of in front of it. The blind hangs in front of whatever sticks out.
• Swapping width and height. Write W and H next to every figure, and width first on every order.
• Using a cloth tape. It stretches. Use a steel one.
• Using one maker's figures for another maker's product. Overlap, depth and stack vary by product; the guide for the product you order is the one that applies.

What is still unresolved

• Which kind of covering suits the window: Curtains, shades, blinds and shutters.
• Which opacity to order for darkness or glare: Daylight, glare and darkness.
• Whether you may make holes, and how to fix brackets to the wall you have: Hardware and installing, and your landlord or association in writing.
• Checking the order against your measurements before you place it: Buying, ordering or making, with the orders sheet.
• Cords on the product you choose, where a child lives or visits: Cords, children and fire.
• A window that is badly out of square or damaged: the maker or an installer.

Knowledge check and scenario

Checklist

• Three widths and three heights measured with a steel tape, to the nearest 1/8 in, and labeled.
• The depth measured in front of anything that sticks out, and compared with the maker's minimum.
• Inside or outside mount chosen, with the light gap considered where darkness matters.
• The survey row has six measurements, the depth, the mount and the size to order.
• Patio door, shutter or bay measured by its own maker's rule. (Go deeper; optional)