Understanding the Curtain Size Calculator
This tool works out how much fabric a curtain project needs from four real measurements: the track (or pole) width, the heading style you're sewing, the number of panels, and the finished drop. It applies the same fullness-ratio method used by curtain makers and soft-furnishing suppliers to turn a window measurement into a fabric shopping list.
The formulas
- Total finished width = Track width × Fullness ratio. The fullness ratio sets how gathered the curtain looks: about 1.5x for tab-top, 2x for eyelet or pinch pleat, and up to 2.5x for pencil pleat or gathered headings.
- Fabric widths required = Total finished width ÷ Fabric roll width, rounded up to the next whole number, since fabric can only be joined in full widths.
- Cut length per drop = Finished drop + Heading allowance + Bottom hem allowance. This calculator assumes a standard 8in (4in double-turned) bottom hem, plus a heading allowance that varies from 2in (tab-top) to 6in (pencil pleat tape).
- Total fabric (yards) = (Fabric widths required × Cut length) ÷ 36.
How to get the best results
- Measure the track or pole itself, after it's mounted — not the window frame. Tracks are usually hung a few inches wider than the window so curtains can stack clear of the glass.
- Measure the drop from the exact point the curtain hangs (top of the track, or the ring/hook position) down to where you want the hem to finish — sill, below-sill, or floor.
- Check the fabric roll width on the bolt label; most home-decor cotton and linen runs 54in (137cm) wide, but some runs 45in or 118in-wide.
Practical context
These formulas cover standard, unpatterned fabric. Add extra yardage for a pattern repeat (patterned fabric usually needs one repeat added per fabric width so motifs line up), for interlining or blackout lining, and for very tall or bay windows, where a professional curtain maker can confirm the cut plan before you buy fabric.
Frequently Asked Questions
How do I calculate how much fabric I need for curtains?
Multiply the track or pole width by your chosen fullness ratio to get the total finished width, then divide by your fabric's roll width and round up to a whole number of fabric widths. Multiply that count by the cut length (finished drop plus heading and hem allowances) and divide by 36 to get yards of fabric.
What fullness ratio should I use for curtains?
Fullness ratio depends on the heading style: about 1.5x for tab-top, 2x for eyelet or pinch pleat, and up to 2.5x for pencil pleat or gathered headers. A higher ratio produces fuller, more tightly gathered curtains but uses more fabric.
How much extra should I add for hems and heading?
Add a bottom hem allowance — commonly 8 inches for a standard double 4-inch turned hem — plus a heading allowance that varies by style, from about 2 inches for tab-top to 6 inches for pencil pleat tape, on top of your finished drop measurement.
Should I measure the window or the track for curtain width?
Always measure the track or pole after it is mounted, not the window frame. Tracks typically extend a few inches beyond the window on each side so open curtains can stack off the glass, and that extended width is what the fullness ratio should be applied to.