What this calculator does
Curtain fabric is driven by fullness, the ratio of fabric width to rod width that gives curtains their gather. A 150 cm rod at double fullness needs 300 cm of fabric width, which on a 137 cm bolt comes to three panels.
Because panels round up to whole numbers, fullness often comes free. On that same rod, both double and two and a half times fullness need three panels, so the extra gather costs nothing but a little more sewing. Checking whether you are just under a panel boundary is worth doing before settling on a ratio.
The formula
The rod width is multiplied by the fullness ratio to give the total fabric width needed, which is divided by the bolt width and rounded up to whole panels. The cut length per panel is the finished drop plus the hem allowance for the top and bottom together. Total fabric is the cut length multiplied by the number of panels.
| Term | Meaning |
|---|---|
| Fullness ratio | Fabric width divided by rod width. 1.5 is modest, 2 is standard, 2.5 to 3 is full. |
| Panel | One bolt width of fabric. Panels are joined to make up the total width. |
| Bolt width | The width the fabric comes in, commonly 137 cm for curtain fabric. |
| Cut length | Finished drop plus hem allowances, which is what you actually cut. |
The inputs explained
| Field | What to enter |
|---|---|
| Rod / window width (cm) | The rod or track width, not the window width, since curtains usually extend past the glass. |
| Fullness ratio | How gathered the curtains should be. 2 is the usual default; sheers often use 2.5 or 3. |
| Fabric width per panel (bolt width) (cm) | The bolt width of your fabric. 137 cm is common, but check before buying. |
| Finished drop (rod to floor/sill) (cm) | Finished drop from rod to where the curtain should end. |
| Hem allowance (top + bottom) (cm) | Top and bottom hem allowances added together. 20 cm is a reasonable default. |
When to use it
Buying fabric for curtains
Fabric is sold by length off a fixed-width bolt, so the panel count and the cut length are the two numbers the shop needs.
Choosing a fullness
Because panels round up, the difference between two ratios is often nothing at all, and checking first can get you more gather for free.
Costing a room
Fabric is usually the dominant cost in making curtains, and the total length figure turns a price per metre into a budget.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much fabric does each fullness need?
The same window at a range of fullness ratios.
| Fullness ratio | Panels needed | Total fabric width needed | Total fabric length to buy |
|---|---|---|---|
| 1.5× | 2 | 225 cm | 440 cm (4.40 m) |
| 2× | 3 | 300 cm | 660 cm (6.60 m) |
| 2.5× | 3 | 375 cm | 660 cm (6.60 m) |
| 3× | 4 | 450 cm | 880 cm (8.80 m) |
Questions
What fullness ratio should curtains have?
Two times the rod width is the standard for most lined curtains. One and a half looks sparse on anything but heavy fabric. Sheers and voiles are usually 2.5 or 3 times, since light fabric needs more gather to read as full.
Do I measure the window or the rod?
The rod or track. Curtains normally extend 15 to 20 cm past the window on each side so they clear the glass when open, and the rod reflects that. Measuring the window alone gives curtains that are too narrow.
How much hem allowance do I need?
Around 20 cm total is a reasonable default, split between a heading allowance at the top and a hem at the bottom. Formal curtains often use a deeper bottom hem of 10 to 15 cm on its own, which weights the fabric and helps it hang.
What about pattern repeat?
A patterned fabric needs extra so the pattern matches across panels, and this calculation does not include it. Add up to one full repeat per panel beyond the first. On a large repeat that can add a metre or more to the order.
For cross-stitch fabric instead, see the cross-stitch fabric calculator. For table linen, see the tablecloth size calculator.