Curtain Calculator

Choosing the right curtain size is one of the most impactful decisions you can make for a room's appearance. Curtains that are too narrow look skimpy and fail to frame the window properly, while curtains that are too short can make a room feel smaller and less finished. Getting the sizing right transforms ordinary windows into a dramatic architectural feature that anchors the entire space.

The most common mistake homeowners make is buying curtains based only on the window opening itself. In reality, curtain sizing depends on a combination of factors: the rod position, the desired fullness level, the fabric panel width, and the preferred hem style. Each of these variables interacts with the others to determine how many panels you need and how much fabric to purchase.

A properly sized curtain treatment accomplishes several things at once. When panels are wide enough, they stack neatly to the sides when open, revealing the full glass area and flooding the room with natural light. When closed, they create a unified, luxurious look that makes even an average window appear grander. Understanding the relationship between rod width, fabric fullness, and panel count is the foundation of every successful curtain project.

This curtain calculator takes the guesswork out of the process. Enter your window dimensions, choose your preferred mount style and length, select a fullness level, and the calculator instantly tells you how many panels to buy, what rod length to get, and how many yards of fabric you need for custom-made drapes. Every result is based on the actual measurements you provide, not generic estimates.

Accurate measurements are the starting point for any curtain project. Use a metal tape measure rather than a fabric tape, and always measure twice to confirm. The calculator asks for the window width and window height as inside-frame measurements — that is, the width and height of the actual glass opening within the frame.

Measuring window width: Place your tape measure at the left inner edge of the window frame and extend it to the right inner edge. Record this number in inches. For most standard single windows, this falls between 24 and 72 inches. Double or triple windows, sliding glass doors, and bay windows can be much wider.

Measuring window height: Measure from the top of the window frame to the bottom sill. This inside-frame height is your baseline. The calculator adds extra length depending on your chosen length style (sill, below-sill, floor, or puddle) and mount position (inside, above-frame, or ceiling), so you only need to supply the raw window measurement.

Choosing a rod position: The mount style dropdown controls where the rod sits. "Inside mount" places the rod at the window frame level with no extra height added. "Above window (4 inches)" raises the rod four inches above the top of the frame, which visually elongates the window — a popular designer trick. "Ceiling mount" positions the rod near the ceiling line, adding twelve inches, and creates the most dramatic, loft-like appearance.

Once you have your two measurements and have selected your preferences, the calculator handles all the arithmetic. You do not need to manually account for rod extensions, hem allowances, or fullness multipliers — these are all built into the formulas automatically.

The curtain calculator uses a straightforward sequence of formulas to move from your raw window measurements to finished panel counts and fabric yardage. Understanding these formulas helps you make informed decisions when adjusting inputs like panel width or fullness level.

Step 1 — Rod Width: The rod must extend beyond the window frame on both sides so curtains can stack clear of the glass when open. The calculator adds 8 inches on each side (16 inches total) to your window width to arrive at the rod width.

Step 2 — Total Fabric Width: The rod width is multiplied by a fullness factor that depends on how gathered you want the fabric. Minimal fullness uses 1.5× for flat, contemporary panels; standard fullness uses 2× for the classic gathered look; sheer or luxurious fullness uses 3× for maximum drape and volume.

Step 3 — Panels Needed: The total fabric width is divided by the width of each individual panel (commonly 52 to 54 inches for off-the-shelf curtains), and the result is rounded up to the nearest whole number using ceiling math. You cannot buy half a panel.

Step 4 — Curtain Length: The base curtain length starts from your window height and is adjusted based on your chosen length style. Sill length equals the window height. Below-sill adds 4 inches. Floor length adds 35.5 inches (assuming the window sill sits 36 inches from the floor, with a 0.5-inch clearance). Puddle style adds 39 inches. On top of that, the mount height is added: 0 for inside mount, 4 inches for above-frame, and 12 inches for ceiling mount.

Step 5 — Fabric Yardage: Eight inches of hem allowance (for top and bottom hems) is added to the finished panel length to get the cut length per panel. That cut length is multiplied by the number of panels, then divided by 36 to convert from inches to yards.

Fullness is one of the most important — and most misunderstood — aspects of curtain design. It refers to how much fabric is used relative to the rod width. More fabric means more gathers, more volume, and a richer, more formal appearance. Less fabric creates a cleaner, more modern silhouette.

Minimal fullness (1.5×): At 1.5 times the rod width, panels have a relatively flat appearance when closed. This works well for contemporary or Scandinavian interiors, for structured fabrics like linen or canvas, and for situations where you want a clean, uncluttered look. Flat-panel and ripple-fold curtains typically use 1.5× fullness. Be aware that minimal fullness panels show any unevenness in the fabric more readily than gathered panels.

Standard fullness (2×): The most widely used fullness level, 2× provides soft gathers without excessive fabric. It works with almost every fabric type and every decorating style from traditional to transitional. Most ready-made curtain panels are designed for 2× fullness, which is why buying two panels for a standard window usually gives good results. This is the calculator's default setting.

Full/Sheer fullness (3×): Three times the rod width creates luxurious, heavily gathered fabric that pools dramatically and fills a window with volume. This level is commonly used for sheers, voiles, and silk taffeta — fabrics that are lightweight enough that extra fullness adds beauty rather than bulk. Heavyweight fabrics like velvet or thick blackout lining can feel stiff and awkward at 3× fullness. Custom drapery in formal living rooms and master bedrooms often uses 2.5× to 3× fullness.

When in doubt, go with 2× fullness. It is the standard for a reason: it flatters nearly every fabric and window shape, and most commercially available panels are already sized to work with it.

The length of a curtain dramatically changes how a room feels. Short curtains can look dated or cramped; floor-length curtains elongate walls and add elegance. The right choice depends on the room's function, the window height, and your personal aesthetic preference.

Sill length: The curtain hem falls at the bottom of the window sill. This is practical and easy to clean, making it popular in kitchens and bathrooms where fabric needs to stay clear of splashes. Sill-length curtains on tall windows can look undersized, so this style works best on shorter windows in more casual spaces.

Below-sill (4 inches below): Dropping the hem four inches below the sill bridges the gap between the casual sill look and the more formal apron look. It is a good choice when the window does not reach close to the floor and you want something more finished than sill-length but not floor-length.

Floor length (1/2-inch clearance): The most popular choice for living rooms, dining rooms, and bedrooms. The calculator positions the hem half an inch above the floor, assuming the window sill is 36 inches from the floor. This slight clearance prevents the fabric from dragging, collecting dust, or becoming a tripping hazard. It gives a clean, tailored appearance and works with nearly any window size.

Puddle (3 inches on the floor): A romantic, formal style where extra fabric intentionally pools on the floor. Three inches of fabric on the floor creates a gentle puddle effect popular in formal dining rooms and master suites. It requires more fabric but creates an undeniably luxurious result. Puddle-style curtains generally should not be used in high-traffic areas or in homes with pets or young children.

When you are sewing curtains from fabric rather than buying ready-made panels, accurate yardage estimation prevents costly shortfalls mid-project. The calculator gives you a total yardage figure that already includes an 8-inch hem allowance (4 inches at the top for rod pocket or heading tape, and 4 inches at the bottom hem).

However, there are additional factors to consider when shopping for fabric. If your chosen fabric has a repeating pattern — stripes, florals, geometric prints — you will need extra fabric to match the pattern at seam lines and across panels. A general rule of thumb is to add one full pattern repeat per panel cut. Pattern repeats typically range from 6 to 27 inches; the repeat length is usually printed on the fabric bolt's label.

Fabric shrinkage is another consideration. Natural fibers such as cotton, linen, and silk can shrink 3 to 5 percent in the wash or when steamed during pressing. If you plan to pre-wash your fabric before cutting (which is recommended for washable curtains), add about 5 percent to your total yardage to account for shrinkage.

Seam allowances add very little to total yardage when panels are cut at full fabric width, but if you plan to sew multiple fabric widths together per panel, add an extra half inch per seam per panel.

Finally, consider that most fabric is sold in 54-inch-wide bolts (though some decorator fabrics come in 60-inch widths). If your panel width exceeds the fabric width, you will need to seam two lengths of fabric side-by-side per panel, which roughly doubles the fabric needed for that panel width. The calculator assumes you are buying panels that fit within a standard bolt width — adjust accordingly if your design requires wider panels.