Blinds used to get a bad rap. Compared with dramatic pooling curtains, the ubiquitous white metal variety just seems, well, sad. These days, though, blinds come in an impressive range of colors—even cobalt blue!—and materials (bamboo is a designer favorite). They wear all the hats drapery does, warming up a stark space, offering privacy, and giving you control over natural light. If you’ve just moved into a new home or are planning a room redo, you’ll quickly find that you need to know how to measure blinds before you do anything else to dress your windows. Not sure where to start? No problem. We caught up with Haley Weidenbaum, founder of Everhem, to go over the difference between inside mounts and outside mounts, and how to measure for each.

Inside Mounts vs. Outside Mounts

Inside Mount Blinds

An inside mount is what it sounds like: The blinds are installed inside the window frame. Typically the brackets are screwed into the top or sides of the casing. 

The pros: “Inside mounting allows your blinds to lay flush with the window frame, giving it a very minimal profile,” says Weidenbaum. 

The cons: “If your paramount concern is blacking out a room, inside mounting is not advised,” she notes. “Because of the deductions to the width, there is no way to avoid slivers of light seeping through around the sides.”

Outside Mount Blinds

With an outside mount, the blinds are installed on the wall above the window or directly on the trim.

The pros: In this case, the blinds are usually larger than the opening to completely cover the casement, which Weidenbaum says makes them ideal for achieving near total darkness. 

The cons: One might be the very fact that they are mounted outside of the window jamb. Headrails (the rectangular boxes that hold the mechanics of the blinds) can range from ¾ to 3 inches deep, making for a clunky appearance.

Common Mistakes People Make When Measuring for Blinds 

Not Using a Steel Measuring Tape

If you want to walk away with accurate measurements, do not use an inflexible yardstick or flimsy measuring tape intended for fabric.

Not Taking Multiple Measurements

Pretty much no one’s windows are perfectly symmetrical, especially if you live in an older house; Weidenbaum has seen discrepancies of up to half an inch in width and height. In other words, there’s no such thing as over-measuring. “For inside mounting, it’s absolutely crucial to know the narrowest width measurement to ensure the shade won’t be too wide at any point along the window,” she says. “The opposite is true for outside mount shades.”

A Refresher Course on How to Read a Tape Measure

The big, bold numbers call out inches. The little lines in between denote smaller measurements. The longest ones (found in the middle between each inch) indicate a ½ inch increment; the second longest lines mark ¼ inches; and so on.

How to Measure for Inside Mount Blinds 

The Supplies

  • Steel measuring tape 
  • A way to record your measurements (i.e., a notepad, iPhone, or order form)

Step 1: Measure the Width of Your Window 

Grab your tape measure and measure the inside width of the window frame in three places: the top, middle, and bottom. Take the narrowest of the three measurements and round down to the nearest 1/8 inch. “The industry standard is to take a ¼- to ⅜-inch deduction off of the narrowest width measurement provided and use that as the finished blind width,” says Weidenbaum.

Step 2: Measure the Length of Your Window

Now measure the length on the inside of the window frame in three places: the left side, the middle, and the right side. If you don’t want your blinds to rest on the windowsill, deduct a ¼ inch from your final number. If your window does not have a sill or you’d like the blinds to hang lower, measure to the point where you like them to fall.

Step 3: Measure the Depth of Your Window

You’ll do this by measuring the upper window casing. For inside mounts, this measurement is key as some blinds have a minimum requirement for mounting.

How to Measure for Outside Mount Blinds

The Supplies

Same as above!

Step 1: Measure the Width of Your Window

Run your tape measure across the window in three places (as stated above), from one outside edge of the trim to the other. Use the widest measurement. To minimize light seepage, you could add anywhere from a ½ inch to 3 inches on either side. 

Step 2: Measure the Length of Your Window

As you did in step 1, measure the length of your window in three places, from outside edge to outside edge. A good rule of thumb is to add 3.5 inches to your final number since you’re mounting the blinds above the window. Happy shopping!