Curtain Size Calculatorv1.0.0

Curtain sizing depends far less on the glass than most people expect. Interior designers size panels against the rod, not the window, and the rod is deliberately wider and higher than the frame so the fabric stacks clear of the glass and the wall reads taller. This calculator applies that convention to every window you enter, converts the result into panel counts and fabric yardage, and totals the order across the whole room.

• Click Add Window for each window that needs curtains. Give each one a name so the results stay readable when a room has several.
• Enter the Window Width and Window Height measured across the outside of the frame or casing, not the glass. Decimals, fractions such as 3/4, and mixed numbers such as 36 1/2 are all accepted.
• Enter the Sill Height, the distance from the finished floor up to the bottom of the window frame. This is what lets the calculator work out where the rod lands and how far the fabric has to fall.
• Choose a Header Style. This sets the fullness ratio, which is how many times wider than the rod the fabric must be so the panels gather instead of hanging flat. Rod pocket uses 2.5, pinch pleat uses 2.25, grommet uses 1.75, and tailored flat panels use 1.2. Override the ratio for every window in Settings when a workroom specifies its own.
• Choose a Drop Style to set where the hem falls. Sill stops half an inch above the sill. Apron falls 4 inches below it. Floor stops half an inch above the floor. Break lets 1.5 inches rest on the floor. Puddle pools 6 inches of extra fabric.
• Open Settings to adjust the design constants. Rod extension per side defaults to 6 inches, which is the usual allowance for stack back. Mount above frame defaults to 6 inches. Single panel width defaults to 50 inches, the most common ready-made width. Hem and header allowance defaults to 12 inches, covering a 4 inch doubled header and a 4 inch doubled bottom hem.
• Set a Pattern repeat when the fabric carries a motif that has to align across panels. Each cut length is rounded up to a whole number of repeats, which is what drives the extra yardage on patterned goods.
• Switch the Unit system to metric to work in centimeters. Every default converts, and fabric is reported in meters instead of yards.
• Enter a Cost for any window to price it. The field is optional, so leaving it blank or at zero reports sizes only and no cost line appears. Set Cost basis in Settings to read the figure either as a rate per yard or meter of fabric, which suits cut goods, or as the price of one finished panel, which suits ready-made curtains. The currency symbol is configurable. Windows priced and windows left blank can be mixed, and the order total says how many of the windows carried a price.
• Press Calculate, or wait for the automatic update after you stop typing. Enable Show step-by-step formulas to see each substitution.

The formulas are: rod width equals window width plus twice the rod extension. Required fabric width equals rod width times the fullness ratio. Panels equals required fabric width divided by single panel width, rounded up, with a minimum of 2 so the window closes as a pair. Rod height above floor equals sill height plus window height plus mount above frame. Finished length equals rod height above floor adjusted by the drop style. Cut length equals finished length plus hem and header allowance, rounded up to the next whole pattern repeat when a repeat is set. Yardage per panel equals cut length divided by 36 inches, or by 100 centimeters when working in metric. Window cost equals the entered rate times total fabric when the basis is per length, or the rate times panel count when the basis is per panel.

Every option in this calculator changes either how much fabric the window needs or how the finished panels read in the room. Start from the look you want and the list below points to the settings that produce it. The sections after that work the other way, taking each option in turn and explaining the reasoning behind it, including when it is the wrong choice.

Each entry below names a common goal and the settings that achieve it. The reasoning behind every individual option follows in the sections after this one.

• Make the ceiling look taller: Raise Mount above frame to 6 inches or more, or place the rod halfway between the frame and the ceiling, and choose the Floor drop. Carrying the eye from the rod down to the floor in one unbroken line is what creates the height, so a short drop undoes the effect no matter how high the rod goes.
• Get the most daylight from the window: Raise Rod extension per side to 10 or 12 inches. The open panels then stack onto the wall instead of the glass, which is the single change that matters most for light.
• Curtains you open and close every day: Choose the Grommet header and the Floor drop. Grommets glide on the rod with almost no friction, and a hem that clears the floor does not drag or soil with daily use.
• A formal room where panels stay in place: Choose the Pinch Pleat header with the Break or Puddle drop. Both assume the fabric is decorative rather than working, so they suit panels that are rarely drawn.
• Block light and gain privacy: Choose a high-fullness header, either Rod Pocket at 2.5x or Pinch Pleat at 2.25x, with the Floor drop and a generous Rod extension. Fullness closes the gaps between folds, and the extra rod width lets the panels overlap the frame at both edges.
• A window above a counter, radiator, or window seat: Choose the Sill drop, or the Apron drop when a baseboard heater or furniture sits just below the sill. Both keep the hem clear of the obstruction.
• Show off a large printed or woven fabric: Choose the Tailored Flat header at 1.2x. Folds hide a pattern, so the flatter the panel hangs, the more of the design stays visible.
• Keep the fabric budget down: Choose the Grommet header at 1.75x and leave Pattern repeat at 0 by selecting a solid or a non-directional texture. Fullness and repeat are the two settings that drive yardage hardest, so lowering both cuts the order the most.
• A house with pets or small children: Choose the Floor drop and avoid Break and Puddle. Fabric resting on the floor collects hair and dust and has to be rearranged by hand after anyone walks past.
• Sewing the panels yourself: Set Single panel width to the usable width of the bolt rather than the 50 inch ready-made default, and raise Hem and header allowance above 12 inches for a deeper hem in heavy fabric. Read the cut length rather than the finished length when marking the fabric.
• Comparing making against buying: Set Cost basis to per yard or meter and enter the fabric price to cost a sewn order, then switch it to per finished panel and enter a ready-made price to cost the same windows the other way.

The drop style sets where the hem lands. It is the single most visible decision, because a hem in the wrong place is obvious from across the room even when every other measurement is correct.

• Sill (half an inch above the sill): Choose it when something directly below the window blocks a longer panel, such as a kitchen counter, a radiator, a window seat, or a run of built-in cabinets. The hem clears the obstruction and the panel stays out of the way. Avoid it in living rooms and bedrooms with clear walls below the window, where a short hem cuts the wall visually and makes the ceiling read lower.
• Apron (4 inches below the sill): Choose it when the wall below the window is partly obstructed, by a baseboard heater or a piece of furniture pushed against the wall, so a floor-length panel is impractical but a sill hem stops too abruptly. The extra 4 inches gives the eye somewhere to land. Avoid it over an empty wall, where the hem floats with nothing to relate to and reads as an unfinished measurement.
• Floor (half an inch above the floor): Choose it as the default for most rooms. The panel reads full length, the gap is small enough to be invisible from standing height, and the hem stays clear of the floor so it does not collect dust or catch underfoot. Avoid it only when the floor is uneven enough that a consistent gap cannot be held across the width of the panel.
• Break (1.5 inches resting on the floor): Choose it for a softer, tailored look in a room where the panels stay mostly closed or are drawn rarely. The slight fold at the bottom reads as deliberate. Avoid it on panels opened and closed daily, because the resting hem drags across the floor every time and soils along one edge.
• Puddle (6 inches pooling on the floor): Choose it for formal, stationary panels in a low-traffic room, where the pooled fabric is part of the effect. Avoid it anywhere with pets, children, or regular traffic. Puddled fabric collects dust, blocks a vacuum, and has to be rearranged by hand after anyone brushes past it.

The header style determines the fullness ratio, which is how many times wider than the rod the flat fabric must be. Fullness is what separates a panel that gathers into soft folds from one that hangs flat and looks undersized. Each header carries its own ratio because each gathers the fabric differently.

• Rod Pocket at 2.5x: The fabric is gathered directly onto the rod through a sewn channel, so the gathers are the entire look and they need the most fabric to hold. Choose it for stationary decorative panels and casual rooms. Avoid it on panels that must be opened, because the pocket binds against the rod and has to be pushed along by hand.
• Pinch Pleat at 2.25x: Pleats are sewn into the header at fixed intervals, which controls the folds precisely and hangs the most evenly of any option. Choose it for formal rooms and for panels that traverse on a track or with rings. It requires either a workroom or pleat tape, so it costs more to make than the others.
• Grommet at 1.75x: Metal rings set into the header slide directly on the rod, and the ring spacing fixes the wave, so less fabric is needed to look correct. Choose it for panels drawn every day, since they glide with almost no friction. Avoid it in formal rooms, where the exposed hardware reads casual.
• Tailored Flat at 1.2x: Almost no gathering, so the panel hangs nearly flat. Choose it when the fabric carries a large print or a woven pattern that folds would hide, or for a deliberately minimal look. Avoid it when the curtains need to block light, because flat panels leave gaps at the edges and do not overlap when closed.

• Rod extension per side, default 6 inches: This is the allowance for stack back, meaning the bunched fabric that sits at each end when the panels are open. Without it the open panels cover the glass, which makes the window look narrower and cuts the daylight the window was installed to provide. Increase it to 10 or 12 inches for heavy fabric or wide windows, where the stack is bulkier. Reduce it to 3 or 4 inches only when a corner, a wall return, or an adjacent window physically limits the rod.
• Mount above frame, default 6 inches: Hanging the rod above the frame rather than on it draws the eye upward and makes the wall read taller, which is the reason designers describe the rule as hanging high and wide. A common alternative is to place the rod halfway between the frame and the ceiling, or within 4 to 6 inches of the ceiling when that gap is small. Reduce it when crown molding or a low ceiling leaves no room.

• Single panel width, default 50 inches: This is the most common width for a ready-made panel, so it works as a starting point for shop-bought curtains. Change it to the actual width of the panels being bought, or to the usable width of the bolt when sewing, since that figure is what determines how many panels the required fabric width divides into.
• Hem and header allowance, default 12 inches: This covers a 4 inch doubled bottom hem, which consumes 8 inches of fabric, plus a 4 inch header. Doubling the bottom hem matters because the extra weight pulls the panel straight and encloses the raw edge, so the hem hangs plumb instead of curling. Increase the allowance for a deeper hem on floor-length panels in heavy fabric.
• Pattern repeat, default 0: Set this only when the fabric carries a motif that has to align across adjacent panels. Every panel must begin at the same point in the pattern, so each cut length rounds up to a whole number of repeats and the remainder is waste. A large repeat of 24 inches or more can add a yard to every panel, which is why patterned goods cost more than the finished dimensions suggest. Leave it at 0 for solids, textures, and non-directional weaves.
• Fullness override, default 0: Leave it at 0 so each window uses the ratio for its header style. Set a value when a workroom specifies its own fullness, or to price the same window at two different ratios and see what the extra fabric costs.
• Cost basis: Choose per yard or meter when buying cut fabric by the length, which is the usual case when sewing. Choose per finished panel when pricing ready-made curtains, where the unit sold is the panel rather than the yardage. The two answers differ substantially for the same window, so the basis has to match what is actually being bought.

Getting panel width wrong is the most common and most visible curtain mistake, because a panel that is too narrow hangs flat and skimpy no matter how expensive the fabric. Sizing against the rod with a real fullness ratio prevents that, and calculating every window in a room at once keeps a whole order consistent.

• Standard Living Room Window: A 36 by 60 inch window with a 30 inch sill height, floor-length drop, and pinch pleat header produces a 48 inch rod, 108 inches of required fabric width, and 3 panels at roughly 3 yards each. Seeing the panel count before ordering avoids the classic error of buying one panel per side regardless of width.
• Whole Room at Once: Add every window in a living room, including a wide slider and two flanking casements, to get a single fabric total. Ordering all panels from one dye lot matters because the same fabric run can shift shade between batches.
• Patterned Fabric Ordering: Set a 24 inch pattern repeat on a large floral. Each cut length rounds up to the next whole repeat so motifs align across adjacent panels, and the total yardage reflects the waste that alignment costs. Skipping this step is how orders come up short.
• Short Ceilings and Rentals: Reduce the mount height when a window sits close to the ceiling and there is no room for the usual 6 inch rise. The calculator recomputes the drop from the actual rod position rather than assuming a standard.
• Cafe Curtains and Kitchen Sinks: Choose the sill drop for a window over a counter or sink where floor-length fabric would be impractical. The hem lands half an inch above the sill so the panels clear the surface.
• Formal Dining Rooms: Choose break or puddle for a formal room where fabric resting on the floor reads as intentional. Puddle adds 6 inches to the finished length, which changes yardage enough to matter on a multi-window order.
• Grommet Panels on a Budget: Grommet headers use a 1.75 fullness ratio rather than 2.5, so a given window needs meaningfully less fabric. Comparing header styles on the same window shows the cost difference before committing.
• Budgeting a Room: Enter a fabric price per yard on each window and read the order total to see what a room costs before committing to a bolt. Switching the basis to per panel prices the same windows against ready-made curtains instead, which is the quickest way to compare making against buying.
• Sewing Your Own Panels: The cut length output already includes header and hem allowance, so it is the number to measure and cut against, not the finished length. Adjust the allowance when using a deeper hem than the 4 inch doubled default.