Blinds Size Calculator
Ordering window blinds that do not fit is a common and costly mistake, because the right order size depends on how the blind is mounted, not just the raw window opening. This blinds size calculator takes your measured window width and height in inches and applies the standard allowances for an inside mount or an outside mount, then returns the dimensions you should order. For an inside mount the blind sits within the window recess, so a small deduction is taken from the width on each side to leave running clearance, while the height is kept at the full opening so the blind covers the gap. For an outside mount the blind is fixed to the wall or frame above the window, so an overlap is added to each side and to the top to block light leakage. Every figure here is computed deterministically from your measurements and the allowance you choose, so two people entering the same numbers always get the same order size. Measure the window in three places for both width and height and use the smallest reading, then enter it below to see the exact blind size to order, with a worked example that reconciles to the calculator defaults.
Order size depends on the mount: an inside mount deducts clearance from the width, an outside mount adds overlap to every edge. For a 36 in by 48 in window on an inside mount with a 0.25 in deduction per side, order a blind 35.50 in wide by 48.00 in tall.
Blinds size formula
Inside mount: order width = window width - (2 x allowance), order height = window height
Outside mount: order width = window width + (2 x allowance), order height = window height + (2 x allowance)
allowance = clearance (inside) or overlap (outside) per side, in inches
An inside mount removes a small clearance from each side so the blind runs freely inside the recess. An outside mount adds overlap on each side and the top so light does not leak around the edges.
Worked example
Take a window measured at 36 inches wide and 48 inches tall, ordered as an inside mount with a 0.25 inch clearance on each side.
- Width deduction: 2 x 0.25 = 0.50 in
- Order width: 36 - 0.50 = 35.50 in
- Inside mount keeps the full height: order height = 48.00 in
The order size is 35.50 in wide by 48.00 in tall, which matches the calculator's default inputs exactly.
Blinds Size Calculator: frequently asked questions
What is the difference between an inside and outside mount?
An inside mount fits the blind inside the window recess for a clean, built-in look, so a small clearance is deducted from the width on each side. An outside mount fixes the blind to the wall or frame above the window, adding overlap on each side and the top to block light and make the window look larger.
How much should I deduct for an inside mount?
Manufacturers commonly deduct between an eighth and a quarter of an inch from each side so the blind clears the recess and operates smoothly. This calculator uses an editable allowance so you can match the exact deduction your supplier specifies.
How much overlap should an outside mount have?
A typical outside mount adds two to three inches of overlap on each side and at the top to stop light leaking around the blind. Enter your chosen overlap per side and the calculator adds it to both edges of the width and to the top of the height.
Why measure the window in three places?
Window openings are rarely perfectly square. Measuring the width at the top, middle and bottom, and the height at left, middle and right, then using the smallest reading ensures the blind fits even at the tightest point of the opening.
Does this calculator place the order for me?
No. It returns the order dimensions in inches based on your measurements and mount choice. Always confirm the deduction and overlap rules with your specific blind supplier before ordering, as these can vary slightly by product line.
Official sources
- Measurement, units and dimensional standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.