How to Measure for Outside-Mount Window Blinds

Outside-mount window with wicker blinds over white couch

The Spruce / Sanja Kostic

Project Overview
  • Total Time: 20 mins
  • Skill Level: Beginner

Window treatments are one of the great unsung heroes of contemporary home décor. Yet for all that they can do from an aesthetic standpoint, their main benefit is still the functional service that they bring to a room—providing privacy and blocking sun. There are two main categories of blinds: inside-mounted, in which the window treatment is set fully inside the recessed between the jambs; and outside-mounted blinds, which overlap the entire opening and a portion of the exposed case moldings. All blinds serve to provide privacy and block light, but outside-mount blinds have a large surface area that makes them a key element of room decor.

Still, for all of their benefit, these blinds can be a little tricky to install correctly. Of particular importance is correctly measuring the size for your blinds. But don't worry; with the right information, measuring and installing these blinds can be as simple as any other home improvement.

What You'll Need

Equipment / Tools

  • Step ladder
  • Tape measure
  • Pencil and note pad


  • Chosen window blinds style


Materials and tools to install outside-mount window blinds

The Spruce / Sanja Kostic

  1. Choose a Blind Style

    An outside-mount blind is a good choice if your window opening is not recessed within the jambs, or if the opening is out of square, as is often the case in older homes. An outside-mount is more easily adjusted to accommodate irregular sizes. 

    First, do your research and select a manufacturer and style for your outside-mount blinds. Review the manufacturer's instructions for measuring, as it may vary from manufacturer to manufacturer. Some blinds have a top valance that is designed for installation on the wall above the window, while others have brackets that can be attached to the top case molding. Some can be adapted for either method.

    Laptop screen with window blind selections next to houseplant

    The Spruce / Sanja Kostic

  2. Measure Width

    Measure the width of the window open and case moldings to determine how wide you want the blinds. An outside-mount blind can overlap the window opening by as much as you wish, but it is most typical to size blinds so they cover most or all of the side case moldings, without extending past them over the wall. Too wide, and the blinds will look strange when they are open. And you don't want to see too much of the wall through the slats of the blinds. Round off your measurements to the nearest 1/8 inch.

    As a rule, the blinds will look nice if they extend about 3 inches beyond each side of the window opening. So a 42-inch-wide window calls for a 48-inch-wide blind that allows for a 3-inch overlap on each side. This rule can be adapted depending on the type of case moldings you have. Windows with very narrow ranch moldings might call for less overlap, while wide colonial case moldings might call for a greater overlap.

    Window width being measured with yellow tape measure for blinds

    The Spruce / Sanja Kostic

  3. Measure Height

    Measure for the height of the blind, including the mounting bracket above the window opening. At the bottom, measure to a point that is slightly below the bottom of the window sill and apron. Again, round off the measurement to the nearest 1/8 inch.

    Window height being measured with yellow tape measure for blinds

    The Spruce / Sanja Kostic

  4. Order Blinds

    Fill out the online order form or visit the window-treatment showroom to place your order for blinds. When specifying dimensions for blinds, the convention is to list width, then height. For example, if you want blinds that are 42 inches wide and 65 inches long, you will specify a 42 x 65 blind.

    Double-check to make sure you've included any extra height and width for the mounting hardware and any extra side and bottom overhang desired. Remember that you are not listing the size of the window opening, but the window opening plus whatever overhang you desire. Blinds are custom manufactured, and if you get the measurements wrong, it is you who absorbs the costs of reordering them.

    Wicker blinds held in hands

    The Spruce / Sanja Kostic