Decorating
Curtain and Blind Size Calculator
Get curtain rod width, panel width, and fabric length for your window, or the right blind mount size.
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Always double-check blind measurements against manufacturer mounting instructions before ordering custom sizes.
Why 'window size' and 'curtain size' are different numbers
The single most common window-treatment mistake is ordering curtains or blinds sized exactly to the window opening. Curtains hung on a rod matched exactly to window width stack directly over the glass when open, blocking light even in the 'open' position. Blinds ordered to the wrong mount type either won't fit inside the frame or will show gaps of light around the edges.
Getting this right means separating two decisions: how the hardware relates to the window opening (mount type and rod extension), and how much fabric or slat material actually needs to be ordered to achieve the desired coverage and look.
Curtain rod width and mounting
For an outside-mount rod — the far more common choice for curtains — extend the rod 4-6 inches beyond each side of the window frame. This lets the stacked panels clear the glass entirely when opened, maximizing both light and the visual width of the window. A wider extension (up to 10-12 inches per side) makes a window look proportionally larger, a common trick in small rooms.
Curtain length styles
| Style | Where it ends | Best for |
|---|---|---|
| Sill length | At the windowsill | Kitchens, café curtains, casual rooms |
| Apron length | 4-6 in below the sill | Rooms with radiators or furniture below the window |
| Floor break | Just touching or half an inch above the floor | Most living and bedrooms, clean formal look |
| Puddle | 1-3 in of fabric pooling on the floor | Formal dining rooms, dramatic statement windows |
Fullness: the fabric width most people underestimate
Flat, skimpy curtain panels are almost always a fullness problem, not a fabric-quality problem. To look properly gathered when closed, total fabric width across both panels should be 2 to 2.5 times the rod width — a 48-inch rod needs roughly 96-120 inches of total fabric, split between two panels of 48-60 inches each.
Blind sizing: inside vs. outside mount
Inside-mount blinds sit within the window frame and need the frame to be at least 2.5 inches deep to clear the mounting brackets and any window crank hardware. Order inside-mount blinds slightly narrower than the exact frame opening — typically 1/4 to 1/2 inch under — to allow clearance for the mechanism to tilt and raise without binding.
Outside-mount blinds cover the entire window opening and mount to the wall or trim around it. Order these 2-3 inches wider than the window opening on each side and a few inches taller, so the blind fully overlaps the frame and blocks light leakage at the edges.
Steps to measure your own windows
- Decide mount type first — inside needs frame depth, outside covers the frame entirely.
- Measure window width and height in three places each, using the smallest width and tallest height.
- For curtains, add 4-6 inches of rod extension per side beyond the frame for an outside mount.
- Choose a curtain length style and measure floor-to-rod distance for floor-length styles.
- Multiply rod width by your target fullness ratio to get total fabric width needed across all panels.
How wide should curtain rods be compared to the window?
Extend the rod 4-6 inches beyond each side of the window frame, so panels can be pulled fully clear of the glass to let in maximum light when open, rather than stacking over the window edges.
How long should curtains hang?
Floor-length curtains should just brush the floor (break) or hover half an inch above it. A puddle style adds 1-3 inches of extra fabric pooling on the floor for a more formal look.
How much fullness do curtain panels need?
Most curtains look best with 2 to 2.5 times the rod width in total fabric width, so the panels look gathered rather than flat and stretched when closed.
Should blinds be inside-mount or outside-mount?
Inside-mount sits within the window frame for a clean, built-in look and needs at least 2.5 inches of frame depth. Outside-mount covers the frame entirely, blocks more light at the edges, and can make a window look larger.
Last reviewed February 2026