Curtain fabric calculator: widths, cut drops and length to buy

What this planner works out

For each curtain the planner counts the fabric widths, the cut drop of every width (rounded up to the pattern repeat) and the fabric for that window. It adds up every curtain of the same fabric, rounds up to your store’s cutting step (for example 0.1 m or a quarter yard) and shows the length to buy, the leftover and the cost. A lining is a fabric of its own.

It is a quantity estimate from your measurements and the values on the bolt label.

Measuring, step by step

1. Measure each window in step 1, its width and height, like the other openings of the room in how to measure a room. Identical windows share one curtain line.
2. Measure the track or rod, or decide how far it will reach past the window on each side. A curtain picks a window, and the window width plus that overhang on each side fills in the track width; type over it with the real length once the track is up.
3. Measure the returns, the distance from the track to the wall at each end, if the curtain wraps back to the wall. Add the overlap where a pair meets in the middle (0 for a single panel).
4. Measure the finished drop from the point where the curtain hangs on the track or rod to where it should end: the sill, below it or the floor. Measure at both ends and in the middle and use the longest, because floors and sills are not always level.
5. Add your sewing values: fullness, side hems per panel, and the hem and heading allowances.
6. Read the bolt label and ask the store for the fabric width, the fabric lost per width at the seams, the pattern repeat, the cutting step and its price.

The method

1. Finished width = track + 2 × return + overlap.
2. Flat width = finished width × fullness + panels × side hems.
3. Widths = flat width ÷ usable fabric width (fabric width − seam loss), rounded up. A pair with an odd number of widths shares one width cut in half lengthwise.
4. Cut drop = finished drop + hem + heading; for a straight match, rounded up to whole repeats, the same rounding the wallpaper calculator uses for its drops.
5. Fabric = windows × widths × cut drop, added up over every curtain of that fabric, plus your extra length, then rounded up to whole sales units. Nothing is ever rounded down; how pack rounding works explains why the adding comes before the rounding.

Fullness is applied to the returns and the overlap too. If you want them flat, add them to the side hems instead.

Worked example: a plain pair

Track 2.00 m, a pair, fullness 2, fabric 1.40 m wide, drop 2.30 m, hem 0.15 m, heading 0.08 m, sold by 0.1 m.

• Flat width 2.00 × 2 = 4.00 m; 4.00 ÷ 1.40 = 2.86, so 3 widths, 1.5 per panel (one width is halved).
• Cut drop 2.30 + 0.15 + 0.08 = 2.53 m; 3 × 2.53 = 7.59 m.
• 7.59 ÷ 0.1 = 75.9, so 76 units = 7.6 m, 0.01 m left over.

Worked example: the same pair with a 68 cm repeat

2.53 ÷ 0.68 = 3.72, so each cut is 4 repeats = 2.72 m. 3 × 2.72 = 8.16 m, so 8.2 m, 0.04 m left over. For a half-drop pattern you would add one repeat as extra length: 8.16 + 0.68 = 8.84 m, so 8.9 m.

Worked example: lining

A lining 1.37 m wide with a 0.15 m lining allowance behind the plain pair: still 3 widths, each cut 2.30 + 0.15 = 2.45 m, so 7.35 m and 7.4 m to buy. The planner notes that 1.37 m is narrower than the fabric; the face fabric stays at 7.6 m.

Worked example: two windows, one fabric

Add a second window with a 1.00 m track, fullness 2 and a 1.40 m drop: 2.00 ÷ 1.40 = 1.43, so 2 widths of 1.63 m, 3.26 m. Both together: 7.59 + 3.26 = 10.85 m. If the store cuts in half meters, that is 22 × 0.5 m = 11.0 m, 0.15 m left over. Window by window it would be 8.0 + 3.5 = 11.5 m.

Worked example in inches and yards

The planner opens with this example. A 48 in window with the rod reaching 12 in past each side gives a 72 in rod. Returns 3 1/2 in, overlap 7 in, fullness 2.5, a pair with 6 in of side hems per panel; fabric 54 in wide, 1 in lost at the seams, a 25 in repeat; drop 84 in, hem 8 in, heading 8 in; sold by the quarter yard (9 in).

• Finished width 72 + 7 + 7 = 86 in; flat width 86 × 2.5 + 12 = 227 in; 227 ÷ 53 = 4.28, so 5 widths (2 1/2 per panel).
• Cut drop 84 + 8 + 8 = 100 in, exactly 4 repeats, so it stays 100 in.
• 5 × 100 = 500 in (13.89 yd); 500 ÷ 9 = 55.6, so 56 quarter yards = 14 yd, 4 in left over.

One yard is exactly 0.9144 m, so metric and imperial results match. Every fabric gets one row on the shopping list, the CSV and the printable plan. Ask the store whether the length comes in one piece and from one batch (dye lot).

Common mistakes

• Taking the window width as the finished width. In the inch example, typing the 48 in window as the track, with no returns and no overlap, gives a flat width of 48 × 2.5 + 12 = 132 in: 3 widths and 8.5 yd. The real finished width of 86 in needs 5 widths and 14 yd, so the curtains would be 5.5 yd short.
• Measuring the drop from the window instead of the track. If the track of the plain pair sits 15 cm above the window, a drop measured from the top of the window is 2.15 m instead of 2.30 m. The cut drop becomes 2.38 m and the fabric 3 × 2.38 = 7.14 m, so 7.2 m instead of 7.6 m, and every curtain ends 15 cm too high.

Limits

• No sewing instructions, and no advice on headings, fullness, linings or interlinings beyond labeled examples.
• No claims about blackout, thermal, flame-retardant or shrinkage properties: the label and the store are the source.
• Not a track or rod assessment. Choosing the track or rod, the wall fixings and the load it can carry are not covered.
• Not covered either: half-drop and railroaded fabric, reusing offcuts between cuts, heading tape, hooks and weights, blinds, and lining narrowing.

To paint or paper the same room, tick Keep my projects on this device (or download a backup), then use Open project in the room refresh planner (or restore the backup there) and add paint for the same rooms.