How this calculator works
A curtain panel is wider than the space it covers so that it can fold. The fullness ratio is how many times the rod length the flat fabric is: 1.5 gives light folds, 2 gives medium folds and 2.5 gives dense folds. Divide the required panel width by the fabric width to see how many widths must be joined. The cut length is the finished drop plus the header allowance for the chosen top and a bottom hem. The total fabric is the joined width multiplied by the cut length.
Formula
panel width = rod length x fullness ratio. fabric widths = panel width / fabric width, rounded up. cut length = drop + header allowance + hem. total fabric length = widths x cut length. area = total fabric length x fabric width.
Worked example
A 200 cm rod with medium fullness needs a panel 400 cm wide. On 150 cm fabric that is 3 widths, rounded up from 2.67. A 220 cm drop with a grommet top adds 10 cm and a 15 cm hem, so each width is cut 245 cm, and the total fabric is 3 x 245 = 735 cm, or 7.35 m.
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Frequently asked questions
How much fullness should curtains have?
A ratio of 2 is the common default for gathered curtains. Use 1.5 for a flat, modern look where the curtains will often be open, and 2.5 for a luxurious, deeply folded look. Sheer fabrics usually need more fullness than heavy ones.
Does this cover two panels or one?
One panel. Most windows use two panels meeting in the middle, so double the total fabric for a pair, or run the calculator for half the rod length if each panel covers half the window. Add an overlap at the centre if the curtains close fully.
Why is the header allowance different?
A rod pocket needs a casing wide enough for the rod, a grommet top needs a facing for the rings, and a tab top needs length for the tabs and the fold. The exact figure depends on the rod diameter and the pattern, so check your curtain instructions and adjust the allowance if needed.
Should I pre-wash the fabric?
Yes for washable cottons and linens. Shrinkage of 3 to 5 percent is common, which can shorten a long curtain noticeably. Pre-wash, dry and press before cutting, then use the fabric width as it is after finishing.