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Curtain Width Calculator

Work out how wide your curtains must be for the right gathered fullness across your window.

How it is calculated

The window or rod width is in centimetres. The tool multiplies it by the fullness ratio to get the total finished fabric width: total = width × fullness. That is divided between the panels, per panel = total ÷ panels, and a fixed 20 cm is added to each panel for side hems and seams, cut width = per panel + 20.

If the fullness is left empty or is 0 or less, the tool uses 2.0; if the panel count is empty or below 1, it uses 2, and a fractional number of panels is cut down to a whole number. Results are rounded to whole centimetres. The 20 cm allowance is a flat amount per panel, not per side, and no heading, header-tape or pattern-repeat allowance is added. The tool gives widths only: the drop (length) has to be worked out separately.

Worked example

A rod is 240 cm wide, and you want a fullness ratio of 2.5 with two panels. The total fabric width is 240 × 2.5 = 600 cm. Each panel takes half, 600 ÷ 2 = 300 cm. Adding 20 cm for the hems gives 300 + 20 = 320 cm of fabric to cut for each panel. If you had left the fullness blank, the tool would have used 2.0 and shown 480 cm in total, 240 cm per panel and 260 cm to cut.

Try this example →

Common mistakes

  • Entering the width in metres or inches. The field is centimetres, so a 2.4 m rod is 240.
  • Measuring the window glass instead of the rod or track. The fullness should be applied to the length the curtain actually covers when closed.
  • Leaving the fullness blank without noticing, which silently uses 2.0. Sheers usually need more than that, and heavy fabric may need less.
  • Buying the result for each panel as the total fabric. The last figure is per panel, so multiply by the number of panels to get the whole order.
  • Forgetting that fabric is sold in fixed widths. A 300 cm panel may need to be joined from several drops, which the tool does not work out.

About this calculator

This calculator finds the total fabric width needed by multiplying window width by a fullness ratio — typically 1.5× to 3× the track or rod width, depending on fabric weight and gather style — then dividing across the chosen number of panels and adding a hem/seam allowance per panel. Fullness ratio is the drapery-industry term for how much extra fabric is gathered into folds rather than left flat: too little (under 1.5×) looks skimpy when closed, while sheers are often cut at 2.5–3× for a soft, dense gather.

Interior designers and drapery workrooms use this calculation to quote fabric yardage before cutting, and DIY sewers use it to buy the right amount of fabric the first time, since fullness ratio — not just window width — is what actually determines whether finished curtains look full and tailored or thin and flat.

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Comments (1)

  • Editorial team

    The hemmed figure is the width of cloth to cut for each panel, so check it against the roll width before ordering. With a 240 cm rod, fullness 2.5 and two panels, each panel is 320 cm; with a single panel the same rod needs 620 cm in one piece, which only works if you plan to join drops. Raising fullness to 3 moves the two-panel cut to 380 cm. Which fabric widths do you usually buy, and would a drop-count helper be useful?

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