Curtain Calculator
Calculate fabric width, panel size and rod length for curtains with any fullness.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Curtain fabric is measured at the flat width, and the fullness ratio decides how much of it gathers up — twice the window width is a standard drape, while two and a half times is generous. Curtains should also extend past the window frame so they do not block the light.
Worked example
calculated with the default values below
Window or opening width 72 in · Window or drop height 84 in · Fullness ratio 2x — standard drape · Number of panels 2 · Extension past each side 4 in · Hem and heading allowance 10 in · Fabric bolt width 54 in Curtain rod length 80.0 in · Total fabric width needed 160.0 in · Width per panel 80.0 in · Cut length per panel 94.0 in · Fabric to buy 10.44 yards · Fabric widths per panel 2
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
Y = p × ceil(L × f ÷ (p × b)) × (H + h) ÷ 36
- L
- Rod length = window width + 2 × overlap
- f
- Fullness factor
- p
- Number of panels
- b
- Fabric width (in)
- H
- Window height (in)
- h
- Hem allowance (in)
All units in inches; widths per panel are rounded up; yards = total inches ÷ 36.
Frequently asked questions
Why does the fullness ratio multiply the curtain fabric width?
The tool multiplies window width plus twice the extension by the fullness ratio before splitting into panels, so a 2.0 ratio on a 72-inch window with 4-inch extensions needs 160 inches of flat fabric. Each panel then needs enough 54-inch bolt widths to cover its share, and the cut length adds the 10-inch hem allowance. More fullness means more fabric and more seams.