Curtain Size Calculator
Enter your window measurements to get the exact curtain panel width, finished drop length, rod width, and total fabric yardage or meterage you need. Choose your fullness ratio, heading style, drop style, and pattern repeat, and the results update instantly in metric or imperial units.
How to measure your window for curtains
Start by measuring the window width from the outer left edge to the outer right edge of the frame. For the height, measure from where the rod will sit (typically 4 to 6 inches above the frame) down to your chosen finishing point: the sill, below the sill, or the floor. Use a steel tape measure for accuracy and measure twice. Write down all measurements before buying fabric or curtain panels, as small errors multiply across multiple panels. If your window is wider than standard (over 72 inches / 180 cm), consider using 4 panels instead of 2 for better fullness and easier operation.
Fullness ratio: why it matters and how to choose
Fullness ratio is the key variable in curtain sizing. A 2x fullness means you use twice as much fabric as the rod is wide, which creates the soft, even folds most people expect from a curtain. At 1.5x the curtain is slimmer and more tailored, which suits heavy fabrics like velvet or structured linen. At 2.5x to 3x the fabric gathers into rich, voluminous folds that suit sheers, voiles, and lightweight cottons. The Window Covering Association of America (WCAA) recommends 2x as the standard for a balanced, professional look that works across most fabric weights and heading styles. If in doubt, choose 2x for medium-weight fabrics and increase to 2.5x for anything lightweight or sheer.
Understanding cut length, heading allowance and hem allowance
The finished drop is the visible length of the curtain from the top of the heading to the bottom hem. The cut length is longer: it adds a heading allowance at the top (the fabric folded or taped to form the heading) and a hem allowance at the bottom. A standard double-fold bottom hem uses about 4 inches (10 cm) of fabric. Heading allowances vary by style: an eyelet heading needs only 1.5 inches (4 cm), while a pinch pleat heading needs 6 inches (15 cm) for the structured fold. If your fabric has a vertical pattern repeat, the cut length must be rounded up to the next full repeat so the pattern lines up across panels. This calculator handles all of those adjustments automatically.
Choosing the right rod width and placement
The rod should be wider than the window opening so that the curtain panels can stack clear of the glass when open, letting in maximum light. A standard extension of 3 to 6 inches (8 to 15 cm) on each side is typical. Mounting the rod 4 to 6 inches (10 to 15 cm) above the top of the window frame makes the ceiling appear higher and the window taller. For rooms with low ceilings, position the rod close to the ceiling to maximise the perceived height. The rod width is the window width plus twice the side extension, and the calculator fills that in for you as soon as you enter your measurements.
Standard fullness ratios by fabric weight and heading
| Fullness ratio | Look | Best for |
|---|---|---|
| 1.5x | Tailored, minimal gathers | Structured fabrics, Roman blinds hybrid, minimalist rooms |
| 2x | Standard full (WCAA recommended) | Most fabrics, eyelet and pencil pleat headings, everyday use |
| 2.5x | Full, generous drape | Mid-weight linens, rod pocket and tab top headings, bedrooms |
| 3x | Ultra full, luxurious | Sheers, voiles, lightweight cottons, grand rooms and hotels |
Recommended fabric-to-rod width ratios for a balanced drape. Lighter fabrics need more fullness.
Frequently asked questions
How wide should curtain panels be?
Each panel should be wide enough so that together all panels equal 1.5 to 3 times the rod width. The most common standard is 2x (twice the rod width) shared across 2 panels, so each panel is the same width as the rod. Wider panels create fuller, more gathered curtains. Use this calculator to find the exact finished width for any fullness ratio and panel count.
How do I calculate the drop length for floor-to-ceiling curtains?
Measure from the rod location (typically 4 to 6 inches above the window frame) down to the floor. Add 0.5 inch (1 cm) clearance for curtains that just kiss the floor, or add 1 inch (2.5 cm) for a slight break, or 6 inches (15 cm) for a puddle effect. Then add your heading and hem allowances to get the cut length. This calculator does all of that from the inputs you provide.
What is a curtain pattern repeat and how does it affect fabric quantity?
A pattern repeat is the vertical distance before the pattern design starts over. When you join multiple fabric drops side by side to make a wide panel, the pattern must align across them. To match the pattern, you have to cut each drop to the next full repeat above your required cut length, which wastes some fabric. The larger the repeat, the more fabric you use. Plain or textured fabrics with no repeat waste nothing. Enter the repeat length in this calculator and it adjusts the cut length automatically.
How many curtain panels do I need?
Most windows use 2 panels (one on each side). Very wide windows - typically over 72 inches (180 cm) - benefit from 4 panels, which stack better when open and give a layered look. Single-panel designs suit small windows or a contemporary asymmetric style. Enter your panel count here and the calculator splits the total fabric width evenly across them.
How much extra fabric should I buy?
Always buy 10 to 15 percent more than the calculated total. Fabric can shrink when washed, cutting lines are rarely perfect, and having spare fabric lets you repair or re-hem later. If your fabric has a large pattern repeat, you may need even more - up to 20 percent extra - to ensure you can match the pattern across all drops.
What is the difference between the finished width and the cut width?
The finished width (or panel width) is the visible width of the curtain after hemming the sides. The cut width adds a side seam allowance on each edge - typically 1 to 1.5 inches (2.5 to 4 cm) per side - to the finished width. This calculator outputs the finished panel width. If you are sewing your own curtains, add 2 to 3 inches (5 to 8 cm) to the panel width for side hems before cutting.