Blind Size Calculator

Enter your window's measured width and height to get the correct blind order size for inside or outside mount, using the standard clearance deduction or overlap allowance.

Quick Facts

Inside mount
Deduct about 1/2 in (0.5") from the measured width
Leaves clearance so the headrail and brackets don't rub the frame; height is ordered as measured.
Outside mount
Add roughly 3-4 in of width (1.5-2 in per side)
Extra width and length overlap the frame so the blind blocks light gaps at the edges.
Measuring tip
Measure to the nearest 1/8 in (0.125")
Use the narrowest width and shortest height of the opening for inside mount.

Your Results

Calculated
Order width
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Width to specify when ordering
Order length (drop)
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Height to specify when ordering
Metric size
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Order size in centimeters
Approx. area
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Order width × length, in square feet

Ready

Enter your window's width and height, choose a mount type, then press Calculate.

About the Blind Size Calculator

Blinds and shades are ordered by two numbers — width, then length (drop) — but the size you order is almost never the same as the size you measure. Manufacturers build in a small deduction for inside mount so the headrail clears the frame, and retailers recommend adding extra width and length for outside mount so the blind fully covers the opening. This calculator applies the standard sizing rule for whichever mount type you choose.

The formulas

Let W and H be your actual measured window width and height, in inches.

  • Inside mount width: Order width = W − 0.5 in. The half-inch deduction gives the headrail and mounting brackets clearance so the blind can tilt and rotate without rubbing the sides of the frame.
  • Inside mount length: Order length = H, with no deduction — order the exact height you measured from the top of the opening to the sill.
  • Outside mount width: Order width = W + 2 × side overlap. The default overlap of 1.5 in per side (3 in total) is the commonly recommended minimum so the blind overlaps the frame edges and blocks light gaps when closed.
  • Outside mount length: Order length = H + top/bottom allowance. The default of 3 in accounts for mounting the headrail above the top of the frame and letting the blind extend slightly below the sill.

How to measure

For inside mount, measure the width at the top, middle, and bottom of the opening and use the narrowest figure; measure the height at the left, center, and right and use the shortest figure — window openings are rarely perfectly square. For outside mount, measure the width and height of the area you want the blind to cover, since the deduction rule does not apply and you are choosing your own overlap instead. Round every measurement to the nearest 1/8 inch (0.125 in), which is the precision most blind manufacturers expect.

Reading the results

The calculator returns the order width and order length in inches (the numbers to enter on a blind manufacturer's order form), the same size converted to centimeters for reference, and the approximate area in square feet, which many retailers use as the basis for per-square-foot pricing.

Frequently Asked Questions

What is the difference between inside mount and outside mount?
Inside mount fits the blind inside the window frame or recess for a clean, built-in look, and requires enough frame depth to hold the headrail. Outside mount attaches to the wall or trim around the opening, which hides an uneven frame and blocks more light because the blind extends past the edges of the window.
Why do you subtract 1/2 inch from the width for inside mount?
Manufacturers deduct roughly 1/2 inch from your measured width to leave clearance for the mounting brackets and headrail, so the blind can tilt and rotate without rubbing against the sides of the frame. Exact deductions vary slightly by product, typically from 3/8 inch to 5/8 inch.
How much extra width should I add for outside mount?
A common rule of thumb is to add at least 1.5 to 2 inches of overlap on each side of the window opening (3 to 4 inches total), so the blind fully covers the frame and there is no light gap at the edges when closed.
Should I measure to the nearest 1/8 inch?
Yes. Measure the width at the top, middle, and bottom of the opening and the height at the left, center, and right, using a steel tape measure. Record the narrowest width and the shortest height, rounded to the nearest 1/8 inch, for the most accurate order size.

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