What this calculator does
Working out what size curtain you need comes down to two separate measurements: how wide the fabric should be, and how long it should hang. Width is driven by fullness, the amount of extra fabric bunched into folds rather than stretched flat across the rod, and this calculator lets you set your own fullness ratio since it varies with fabric weight and the look you want, from a fairly flat 1.5× ratio to a heavily gathered 3× ratio.
Drop is simpler in principle but easy to get wrong in practice: it is measured from the rod, not the top of the window frame, down to wherever you want the hem to finish, whether that is the windowsill, just above the floor, or pooling on the floor. Add a header allowance for any fabric folded over above the rod line, and this calculator gives you the finished fabric dimensions to buy or sew to.
The formula
Enter the width of your curtain rod or track in cm, and a fullness ratio: 1.5 to 2 for a flatter, more tailored look, 2 to 2.5 for a standard gathered look, and up to 3 for a very full, heavy-fabric look. Enter the drop length measured from the rod down to your chosen finish point, plus any header allowance for fabric that folds over above the rod line. The calculator multiplies rod width by fullness ratio to get total fabric width, divides that across however many panels you are hanging, and adds header allowance to drop length for the finished panel length.
| Term | Meaning |
|---|---|
| Fullness ratio | How much wider the fabric is than the rod, expressed as a multiplier. Higher means fuller, more gathered curtains. |
| Drop | The length of a curtain panel from the rod down to its hem, measured before any header allowance is added. |
| Header allowance | Extra fabric folded over above the rod line to form the top heading, such as a rod pocket or pleat tape. |
The inputs explained
| Field | What to enter |
|---|---|
| Rod or track width (cm) | The width of the rod or track the curtains will hang from, not the window opening itself. |
| Fullness ratio (fabric width per rod width) | A multiplier for how gathered the curtains should look. Try 2 as a standard starting point and adjust for your fabric. |
| Length from rod to chosen finish point (sill, floor, puddle) (cm) | Measured from the rod down to wherever you want the curtain to end: sill, just above the floor, or pooling on the floor. |
| Header/heading allowance above the rod line (cm) | Extra length folded over above the rod line. Leave at 0 if your heading style needs none. |
| Number of panels | How many separate curtain panels will share the total fabric width, usually 2 for a pair. |
When to use it
Ordering fabric for a standard living room window
A 150cm rod with a 2× fullness ratio needs 300cm of total fabric width, split across two panels of 150cm fabric width each, before accounting for any seam allowances the fabric itself needs.
Choosing between sill-length and floor-length curtains
Running the same rod width and fullness with different drop lengths shows how much more fabric a floor-length or puddling curtain needs compared with a shorter sill-length one, useful when comparing fabric costs before committing.
Adjusting fullness for a heavier fabric
Heavy fabrics like velvet often look better with less fullness than sheer or lightweight cotton. Lowering the fullness ratio from 2.5 towards 1.8 reduces the total fabric width needed while keeping the same rod and drop.
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 width does a 150cm rod need at different fullness ratios?
Holding rod width and panel count fixed and varying the fullness ratio shows how much extra fabric each style adds.
| Fullness ratio | Total fabric width | Fabric width per panel |
|---|---|---|
| 1.5 | 225 cm | 113 cm |
| 1.8 | 270 cm | 135 cm |
| 2 | 300 cm | 150 cm |
| 2.5 | 375 cm | 188 cm |
| 3 | 450 cm | 225 cm |
Questions
What size curtain do I need for a standard window?
There is no single standard size because it depends on your rod width, chosen fullness and desired drop. Measure your own rod and window, pick a fullness ratio for the look you want, and use the calculator above rather than relying on a generic size.
Should curtains touch the floor?
That is a style choice. Common options are just above the floor (around 1 to 2cm clearance), just touching the floor, or pooling with extra fabric on the floor. Set your drop measurement to whichever finish point you prefer.
How much wider should curtains be than the window?
Curtains are usually sized against the rod width, not the window opening, and the rod itself is typically mounted wider than the window so the curtains can stack fully open beside it without covering the glass.
What fullness ratio should I use?
1.5 to 2× suits flat, tailored looks and heavier fabric; 2 to 2.5× is a common general-purpose default; up to 3× suits lightweight or sheer fabric where a heavily gathered look is wanted. There is no single correct ratio, so adjust it to taste.
For other home measuring jobs, see the concrete block fill calculator and the flow rate calculator.