How the fabric is worked out
Fabric width is the track width multiplied by the fullness ratio. A 1.5 m track at twice fullness needs 3 m of fabric across.
Fabric is woven in fixed widths, usually about 1.37 m, so the required width is divided by that and rounded up. Three metres needs three widths, which is 4.11 m of fabric before cutting.
Each width is cut to the drop plus the heading and hem allowances, which is why the cut drop matters as much as the total width.
Worked example
A 1.5 m track at twice fullness, a 1.8 m finished drop, a 0.15 m heading and a 0.1 m hem needs 3 m of fabric width and a 2.05 m cut drop.
That is three fabric widths, so 6.15 m of fabric off the roll. At 2.5 times fullness it would be four widths and 8.2 m.
Choosing the fullness
Twice the track width is the usual starting point. Pencil pleat and eyelet headings look well at 2 to 2.5, and a sheer or a voile needs 2.5 to 3 to look full.
Below about 1.5 the curtain looks flat and hangs in straight folds. Above 3 you are buying fabric that cannot be seen.
Pleated headings have their own calculation, because the pleats are made from the fabric rather than gathered, and the tape states how much fabric each pleat needs.
Limits of this calculator
It assumes one window and a plain fabric. A patterned fabric with a large repeat needs extra for matching, which is not included.
It does not account for a bay window, which needs each section calculated and the returns added, or for a curtain that hangs past the track.
It does not include lining, interlining, tape, hooks or the cost of making up.
For the floor covering in the same room, use the carpet calculator.
Inputs for the curtain size calculator
Width to cover
Track or pole width, not the glass. Curtains cover the whole track.
Fullness ratio
How much fabric to the window width. Two times gives a gathered curtain, 2.5 a fuller one.
Finished drop
Finished length from the track or pole to the hem.
Heading allowance
Extra above the drop for the tape or pleats, usually 0.1 to 0.2 m.
Hem allowance
Extra below the drop for the hem, usually 0.1 m for a double hem.
CALCULATOR QUESTIONS
Curtain size calculator questions
How much fabric do I need for a window?
Twice the track width at minimum, plus the heading and hem on every drop. A 1.5 m track takes about 3 m of fabric across.
What fullness should I use?
Two times for a standard curtain, 2.5 for a fuller look, 3 for a sheer.
Should I measure the window or the track?
The track or pole, and add 15 to 20 cm each side if you want the curtains to close over the window edges.
Does this include lining?
No. Lining and interlining are separate quantities, usually the same width and drop.