What is the difference between inside mount and outside mount?

Inside mount (also called recess fit) means the blind sits inside the window frame recess. The blind must be slightly narrower than the opening — typically 3/8 in (1 cm) narrower — so it moves freely. Outside mount means the blind is mounted on the wall or trim above and outside the frame, covering it completely. It requires overlap on each side and above to block light.

How much deduction should I use for an inside mount blind?

Standard deduction is 3/8 in (about 1 cm) total width. Some manufacturers specify 1/4 in to 1/2 in — check your supplier's guide. For the drop, order the full recess depth or your desired length; no deduction is needed unless the window sill protrudes.

How much overlap should an outside mount blind have?

A minimum of 1.5 in (3.5 cm) overlap on each side of the opening is recommended for light control. For bedrooms or blackout blinds, 2 in (5 cm) per side is better. Above the opening, 2–3 in (5–7.5 cm) hides the headrail and blocks top light.

Where do I measure the window width?

Measure at the top, middle, and bottom of the opening. Use the narrowest measurement. This matters most for inside mounts where even a few millimeters can make the blind bind in the frame.

Do I need to check recess depth for an inside mount?

Yes. Most inside-mount blinds require a minimum recess depth of 1.5–3 in (4–7.5 cm) depending on the headrail type. Roller blinds are thinner; wood slat blinds are deeper. Check your blind type's spec before ordering.

Blind Size Calculator

Name: Blind Size Calculator
Availability: InStock
Author: Nham Vu

Enter your window dimensions and mount type to get the exact blind width and drop to order.

Window Measurements

Units:

Mount type

Blind fits inside the window recess.

Window opening width (in)

Measure at top, middle, and bottom — use the narrowest.

Window opening height (in)

Measure from top of opening to window sill.

Width deduction (in)

Standard is 3/8 in (0.375 in). Check your supplier spec.

Enter window dimensions on the left and click Calculate.

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Summary

Enter your window dimensions and mount type to get the exact blind width and drop to order.

How it works

Measure your window opening width and height at three points each and use the smallest measurement.
Choose inside mount (blind fits inside the window frame) or outside mount (blind covers the frame).
For inside mounts the calculator deducts a small clearance so the blind slides freely.
For outside mounts the calculator adds overlap on each side and above the opening to block light.
The result is the exact width and drop to specify when ordering your blind.

Use cases

Order roller, venetian, or cellular blinds at the correct size.
Determine how much frame coverage an outside-mount blind will add.
Compare inside-mount vs outside-mount light-blocking for a bedroom.
Check whether your window recess is deep enough for an inside mount.
Calculate blind sizes for multiple windows in a room remodel.
Verify a supplier quote uses the right deductions or overlaps.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu