Hong Life CalculatorsUnderstanding the curtain fabric calculator — concept and principle
This calculator tells you how many widths of fabric to buy and how many metres that adds up to for a pair or panel of curtains. You enter the rail width, the finished drop, the fullness you want, the width of the fabric roll and the pattern repeat. It is used before ordering fabric for made-to-measure curtains, so you neither run short nor overbuy.
How it is calculated
- Widths of fabric = rail width × fullness ÷ fabric width, rounded up to a whole width.
- Cut drop = finished drop + 25 cm for the top heading and bottom hem. If the fabric has a pattern repeat, the cut drop is rounded up to the next whole multiple of the repeat.
- Fabric length (m) = widths × cut drop ÷ 100.
All inputs are in centimetres; the final length is in metres.
The principle
Curtains are gathered or pleated, so the flat fabric must be wider than the rail. Fullness is that ratio: 2 means twice the rail width of fabric. Because fabric comes in fixed roll widths, you must buy whole widths and join them with vertical seams, which is why the calculator rounds up. Each width is cut to the same length, and when a pattern repeats, every cut must start at the same point of the design so the panels match across seams; that is why the drop is rounded up to the repeat.
Worked example
- Inputs: rail 200 cm, drop 230 cm, fullness 2, fabric 140 cm wide, no pattern repeat.
- Fabric width needed: 200 × 2 = 400 cm; 400 ÷ 140 = 2.86, rounded up to 3 widths.
- Cut drop: 230 + 25 = 255 cm (no repeat, so no rounding).
- Fabric length: 3 × 255 = 765 cm = 7.65 m.
- For a pair, the 3 widths are usually split as 1.5 widths per curtain.
| Fabric width (cm) | Widths of fabric |
|---|---|
| 35 | 12 |
| 70 | 6 |
| 105 | 4 |
| 140 | 3 |
| 175 | 3 |
| 210 | 2 |
| 280 | 2 |
How to read the result
The fabric length is the minimum to order. With a 30 cm pattern repeat the cut drop would become 270 cm and the total 8.10 m, so patterned fabric always needs more. Typical fullness is about 1.5 for flat or eyelet styles and 2–2.5 for pencil and pinch pleats; sheers often look best at the high end. Heavier fabric looks fuller at a lower ratio.
Common mistakes
- Measuring the window instead of the rail; the rail usually extends past the frame on both sides.
- Forgetting overlaps and returns at the ends of the rail.
- Not checking whether the 25 cm allowance suits your heading; deep headings or double hems may need more.
Frequently asked questions
How do I measure the drop?
Measure from the top of the rail or the hook position down to where the curtain should end: sill, below the sill or floor.
Can I use half a width?
Yes. Splitting one width down the middle is common for a pair of curtains.
Should lining use the same amount?
Lining is usually the same number of widths, cut a little shorter, and has no pattern repeat.
Sources: Reader's Digest, Complete Guide to Sewing