Blind Size Calculator

A window blind is a type of window covering made of horizontal or vertical slats of hard material such as wood, plastic, or metal. A cord or wand on the side lets you tilt the slats open or closed to control light and privacy. Blinds differ from curtains in that they sit flush with the window frame rather than draping around it.

Measure the width at three points — top, middle, and bottom — and the height at three points — left, center, and right. Always use the smallest width and smallest height measurement as your baseline. For inside mounts, a small deduction is applied so the blind fits inside the frame without binding; for outside mounts, extra inches are added beyond the frame for better light blocking.

Most manufacturers recommend deducting 1/4 inch (about 0.6 cm) from the narrowest width so the blind slides freely inside the frame. Some brands specify up to 1/2 inch. Always check the manufacturer's specific deduction requirement before ordering, as it can vary by blind type and material.

For an inside mount on a 34.5-inch window, you would order a blind approximately 34.25 inches wide (34.5 minus the standard 0.25-inch deduction). For an outside mount, you would typically add 3 inches per side, giving you a 40.5-inch blind. Always confirm the exact deduction with the blind manufacturer.

An outside mount blind is installed on the wall or window trim surrounding the window frame rather than inside the recess. It covers the frame completely, which blocks more light and can make a window appear larger. Outside mounts are ideal when the window recess is too shallow for an inside mount or when you want maximum blackout coverage.

Common off-the-shelf blind widths range from 23 to 72 inches in increments of roughly 1–2 inches. Standard heights are typically 48, 60, or 72 inches. If your window doesn't match a standard size, most retailers offer custom-cut blinds to your exact dimensions, often at a similar price point.

Custom size blinds are made to your exact window measurements rather than standard retail dimensions. They are ideal for non-standard or uniquely shaped windows. Many online and in-store retailers offer cut-to-size or made-to-order services, and they generally cost only slightly more than stock sizes.

First decide on inside or outside mount. For inside mounts, take the smallest of your three width readings and subtract 1/4 inch; use the smallest height reading as-is or reduce slightly per manufacturer guidance. For outside mounts, take the smallest width and add 3 inches per side (6 inches total), and add 3 inches above and below for height. This calculator handles those adjustments automatically.