Blind & Shade Size Calculator
Enter your measured opening, pick an inside or outside mount, and get the width and drop to order — with the clearance or overlap already worked in.
Size to order
Inside mount: 0.25in deducted from the width for bracket clearance; order the exact drop and let the factory deduct if its chart requires it.
How to measure and order
Getting a blind to fit is almost entirely about the mount. For an inside mount, you measure the exact opening and the factory (or this calculator) trims a small amount off the width so the finished blind slides in without binding. The drop is usually left as measured. For an outside mount, there is no recess to fit into, so instead you add coverage beyond the opening on every side to stop daylight leaking around the edges.
Always measure with a steel tape, record to the nearest eighth of an inch or a couple of millimetres, and take the width in three places. When an inside-mount opening is out of square, order to the narrowest width; when it is badly out of square, an outside mount is often the more forgiving choice.
Frequently Asked Questions
What's the difference between an inside and an outside mount?
An inside mount fits the blind inside the window recess for a clean, built-in look; it needs a small clearance so the brackets fit and the slats don't scrape the reveal. An outside mount fixes the blind to the wall or frame above the opening, overlapping it on the sides and top to block light gaps and make the window look larger.
How much is deducted for an inside mount?
This tool takes a small default clearance off the width — about a quarter inch (0.6 cm) — and leaves the drop as measured. Manufacturers vary, so always check the specific product's deduction chart: some take the deduction for you when you enter the exact opening, while others expect you to order the finished size.
How much overlap should an outside-mount blind have?
A common starting point is about 1.5–2 inches (4–5 cm) of extra width on each side and 2 inches (5 cm) above the opening, which hides light gaps and gives room for the brackets. Add more if you want a bolder frame or need to clear a deep sill or handle.
Why measure the width in three places?
Windows are rarely perfectly square. Measuring the width at the top, middle, and bottom — and the height on both sides and the centre — reveals how much the opening is out of true. For an inside mount you order to the narrowest width so the blind always fits.
Estimates only. Every manufacturer has its own deduction and overlap chart — confirm the numbers against the product you are ordering before you commit, especially for custom, non-returnable blinds.