The curtain calculator

Curtains, with the numbers settled.

Work out the fabric width, panel count, and packages for a chosen fullness. Then check the product’s heading and drop instructions.

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Measure the span, then choose the drape

Four checks before ordering

Check what “width” means. Enter one flat, ungathered panel. Pre-pleated curtains, wave systems, and products sold by finished coverage need their own maker’s method.
Measure the support. Use the installed rod or track span, excluding decorative finials. Include any required returns or overlap in your covered span once.
Record the drop separately. Measure from the exact point specified for the heading. A ring eye and the top of a pole are different starting points.
Inspect obstacles. Check opening windows, handles, radiators, fabric care, light gaps, and whether the stacked curtains fit beside the glass.

Fullness is a styling input, not a guarantee that a particular header or fabric works on your hardware. Follow the curtain maker’s stated requirements.

John Lewis measuring guidance · Our complete curtain guide

A worked example

A 240 cm span at 2× fullness needs 480 cm of flat fabric. Panels 140 cm wide give a raw requirement of 480 ÷ 140 = 3.43 panels. Round up to four: two per side. Four panels provide 560 cm total width, or 2.33× actual fullness. If each package contains two panels, buy two packages.

A 180 cm span with the same panel width and fullness needs at least three panels mathematically. Equal center-opening sides require four. If that looks too full, compare wider panels or a different manufacturer-approved fullness instead of quietly accepting an uneven arrangement.

Formula: required flat width = covered span × fullness; minimum panels = round up(required width ÷ panel width); center-opening count = round up to the next even number; packages = round up(used panels ÷ panels per package).