Methodology

How the curtain calculators get their answers, so you can check them. The tools are plain arithmetic that runs in your browser. They do not use AI, and nothing you type is sent to a server.

Panels and packs

Inputs: usable rod span R, the width on the listing W, whether W is for one panel or the whole pack, panels per pack Q, the fullness you want F, and how the curtains open.

1. Width of one panel: P = W ÷ Q if the listing width is for the whole pack, otherwise P = W.
2. Flat fabric width needed: T = R × F.
3. Minimum panels: N = T ÷ P, rounded up.
4. Panels to hang: I = N for curtains opening to one side; for a middle opening, N rounded up to an even number so both sides match.
5. Packs: B = I ÷ Q, rounded up. Panels bought: U = B × Q. Spare panels: U − I. Spares are never counted as hung.
6. Fullness when hung: I × P ÷ R.

Worked example

R = 76 in, W = 52 in per panel, Q = 3, F = 2, middle opening: T = 152 in, N = 3, I = 4, B = 2, U = 6, 2 spare, fullness when hung 2.74×.

Length

Length needed L = D − O, where D is the drop from your reference point (usually the top of the rod) to where the curtain should end, and O is where the top of the curtain sits: negative above the reference, positive below it. For each length you enter, S − L shows how much longer (positive) or shorter (negative) it is. The header offset is never assumed; you measure it on your product.

Units and rounding

• All math is done in inches. Centimeters are divided by 2.54 (exact). Switching units converts the numbers without changing the result.
• Only displayed values are rounded: lengths to 0.1, fullness to 0.01. Counts are always rounded up.
• A value within a billionth of a whole number counts as that number, so unit conversion never adds a panel by accident.
• Accepted input: 76, 76.5, 76,5, 76 1/2, 76-1/2 and 76½. A comma before exactly three digits (1,500) is rejected as unclear.

Limits

• Rod span and listing width: 1 to 1200 in. Panels per pack: 1 to 12. Fullness: 1 to 4.
• More than 100 panels is treated as a probable unit or width mistake.
• Length check: drop 1 to 600 in, header offset up to 24 in above or below.
• Not covered: curved or bay tracks, corner windows, sewing your own curtains, pre-pleated curtains sold by finished width, hardware strength, light blocking and privacy.
• Fullness is a style choice. The results are planning estimates, not product or installation guarantees.

How the tools are checked

Every release runs automated tests before it can go live: the reference examples above, boundary and invalid inputs, thousands of random cases checking the rules (for example that inches and centimeters always agree), the wording of results, and automated accessibility checks. The worked examples in the guides are calculated by the same code.

Sources

• IKEA US, Window treatments buying guide (PDF), page 2 "Good to know" (E03), checked October 21, 2026.
• Home Depot product Q&A: shopper asks whether "52 in W" means one panel or both (E08), checked October 20, 2026.

Change log

• October 21, 2026: Added the guides on panel and pair widths, curtain length (with a length check) and measuring.
• October 21, 2026: The calculators accept comma decimals (132,5) and fractions (76 1/2). Limit messages now show only values that are accepted.
• October 21, 2026: With a middle opening, the spare-panel fullness only counts an even number of panels, so both sides stay equal.
• October 21, 2026: First version of the Curtain Panel Calculator.

Spotted a mistake? See how to report it. Try the tools: Curtain Panel Calculator.