How to Use the Blind & Curtain Size Calculator
Blinds and curtains need different order dimensions than your window's exact measurements, and the right adjustment depends on how they'll be mounted. This calculator takes your window's width and height along with the mount type — inside-mount blinds or outside-mount curtains — and calculates the size you should actually order.
Inside-mount blinds sit within the window frame itself, so they're typically made about 0.5 inch narrower and 0.25 inch shorter than the exact opening to raise and lower smoothly without rubbing the frame. Outside-mount curtains work the opposite way: the rod extends beyond the frame on both sides for full light blockage and easier operation, so curtains are usually sized about 8 inches wider and 6 inches taller than the window.
These figures reflect common installation practice, but the recommended allowance can vary slightly by brand, fabric weight, and hardware type. Always check the retailer's measuring guide before placing a final order.
Frequently Asked Questions
Inside-mount blinds need to fit inside the window frame, so they're typically made about 0.5 inch narrower and 0.25 inch shorter than the exact window opening.
Outside-mount curtain rods extend beyond the frame for full light coverage and easier operation, so curtains are typically sized about 8 inches wider and 6 inches taller than the window.