How to measure a window for installing blinds?

Since windows may not be perfectly rectangular, measure the width at three points (top, middle, bottom) and the height at three points (left, middle, right). Order in-mouldings with the minimum width and out-mouldings with the maximum width plus overlap.

How much is deducted for built-in solutions?

Most manufacturers deduct about half an inch from the measured width to ensure that the blinds do not touch the window frame. Many manufacturers perform this deduction in the factory already. If your seller does, order the blinds with the exact measurements of the window and set the deduction value in the calculation tool to zero.

How much overlap should you plan for with external blinds?

As a general rule of thumb, allow for about 1.5 inches of overlap per side, which will add approximately three inches to the width. In the top area, additional overlap should be considered for the track guide. If you want to block out light from all sides, consider adding an overlap in the bottom as well.

Should you choose interior or exterior blinds?

Interior blinds are the better choice for rectangular niches with sufficient depth as they offer a clean, neat look. If your window frame is flat, if your window isn't rectangular, if you need maximum light blockage or if you want to create the impression of a larger window, then exterior blinds should be your choice.

Will the blinds be made exactly to my specified dimensions?

Custom made blinds are manufactured to your exact measurements however as ready-made blinds come in fixed grid sizes internal blinds will be cut to the next smallest available width and external blinds will be cut to the next largest. Be sure to check the specific measurement instructions for the product you wish to purchase.

Blind Size Calculator

Window measurements

Mounting style

Inside sits within the window recess; outside covers the wall around it.

Inside mount

Outside mount

Measured window width

Measured window height

Width clearance deduction

Height gap (optional)

Blind width to order

Blind height to order

Blind width to order

Blind height to order: in
Coverage area: ft²

Order a blind 24 in wide by 36 in tall.

Inside mount: we subtracted your clearance deduction from the width so the blind drops cleanly inside the recess. The height is left at the opening size unless you set a gap.

Check the recess is deep enough for the headrail and brackets before you commit to an inside mount. Shallow frames often need up to two and a half inches of depth.

How blind sizing works

This blind size calculator converts the measured dimensions of a window into the actual measurements to order. Custom blinds are not cut exactly to the dimensions of the original opening. Interior mount solutions will be slightly smaller to avoid the window frame while exterior mount solutions will be larger to cover the edge of the opening. Select your desired mounting type and enter the width and height of the window, then the tool will apply the appropriate tolerances.

Measure your window.

As windows are not always perfectly rectangular, measure the width at three points (top, middle, bottom), and the height also at three points (left, middle, right).

For internal installations use the smallest measured width to ensure that it will fit in any installation. For external installations use the largest measured width and then decide how much of the blind you want to show overhanging the opening.

Interior dimensions:

Interior mount solutions are installed within the window recess which creates a clean appearance that makes it appear as if the blind is part of the window. As the blind needs to be inserted into the window frame manufacturers take away some of the width in order to avoid friction and hanging up.

W_{order} = W_{measured} - d \qquad H_{order} = H_{measured} - g

Here "d" is the drop, which is usually about half an inch. "g" is an optional gap at the bottom to ensure that the blinds do not hit the window sill. Many retailers will have this value deducted already in the factory so it can be set to zero and the blinds ordered directly to the exact dimensions of the window. This calculator allows you to adjust the drop value yourself to comply with your retailer's rules.

Example: If the niche is 30 inches wide and 48 inches tall, you want to subtract half an inch from the width, and do not want any clearance from the window sill, then your order measurements for the blinds would be 29.5 inches by 48 inches.

External installation:

For external installation, the blinds are attached to the top of the window and surrounding walls or trim, covering the opening. This type of installation is suitable for shallow recesses, windows that are not rectangular in shape, or rooms where as much light as possible needs to be blocked out. The dimensions of the blinds are larger than those of the window.

W_{order} = W_{measured} + 2o \qquad H_{order} = H_{measured} + n \cdot o

'o' is the desired overhang per side and usually one and a half inch. This will add about three inches to the total width. For height: if the blind overlaps only at the top, then 'n' equals 1 (the bottom end ends on the window sill). If the blind overlaps both top and bottom, then 'n' equals 2. Make sure there is enough room for the guide bar and mounting brackets at the top edge.

Example: A window that is 30 inches wide by 48 inches high with an inch-and-a-half overhang on all four sides and installed outside would have the following dimensions for ordering shades: 33" x 51".

Comparison between built-in and freestanding solutions

Measured window	Inside mount order	Outside mount order
24 x 36 in	23.5 x 36 in	27 x 39 in
30 x 48 in	29.5 x 48 in	33 x 51 in
36 x 60 in	35.5 x 60 in	39 x 63 in

For inset solutions the width is reduced by half an inch to allow for a gap to the window sill. For external mount solutions there are 1½ inches of overhang added on each side. Adjust the clearance in the calculator tool to ensure that the product will fit your requirements.

What type of installation should you choose?

Built-in solutions are the most common style and emphasize decorative frames. However, they require a deep enough recess to accommodate the headrail system and work best with rectangular, flat openings.

Inserts can hide unsightly or uneven window frames and make the window appear taller or wider. They offer the best light control and privacy because they do not create a gap in the frame. Choose an insert if the difference between the diagonals of your window is more than one inch, or if the opening is too shallow.

Pre-order.

Please carefully check the required depth of the product you select for installation and round any measurements to the nearest eighth inch as instructed by your retailer. Also measure each window individually even if they appear visually similar. Even slight differences in width can result in needing to rework the blinds.

The amount of space required varies by manufacturer and product. Please note that this tool provides only a general estimate. Be sure to check the seller's sizing guide for exact deductions and overlap amounts.

Frequently asked questions

How to measure a window for installing blinds?: Since windows may not be perfectly rectangular, measure the width at three points (top, middle, bottom) and the height at three points (left, middle, right). Order in-mouldings with the minimum width and out-mouldings with the maximum width plus overlap.
How much is deducted for built-in solutions?: Most manufacturers deduct about half an inch from the measured width to ensure that the blinds do not touch the window frame. Many manufacturers perform this deduction in the factory already. If your seller does, order the blinds with the exact measurements of the window and set the deduction value in the calculation tool to zero.
How much overlap should you plan for with external blinds?: As a general rule of thumb, allow for about 1.5 inches of overlap per side, which will add approximately three inches to the width. In the top area, additional overlap should be considered for the track guide. If you want to block out light from all sides, consider adding an overlap in the bottom as well.
Should you choose interior or exterior blinds?: Interior blinds are the better choice for rectangular niches with sufficient depth as they offer a clean, neat look. If your window frame is flat, if your window isn't rectangular, if you need maximum light blockage or if you want to create the impression of a larger window, then exterior blinds should be your choice.
Will the blinds be made exactly to my specified dimensions?: Custom made blinds are manufactured to your exact measurements however as ready-made blinds come in fixed grid sizes internal blinds will be cut to the next smallest available width and external blinds will be cut to the next largest. Be sure to check the specific measurement instructions for the product you wish to purchase.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

Window blind — Wikipedia
Background on window blind types, mounting and construction.
The Home Depot — How to Measure for Blinds and Shades
Retailer measuring guide for inside and outside mount blinds.
The Spruce — How to Measure Windows for Blinds
Step-by-step measuring method and mount-choice guidance.