Curtain Size Calculator

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. The curtain size calculator applies that convention to every window entered, converts the result into panel counts and fabric yardage, and totals the order across the whole room.

• Window width and height. Measured across the outside of the frame or casing, not the glass, since the rod width and mount height are both set from the frame rather than the opening. Decimals, fractions such as 3/4, and mixed numbers such as 36 1/2 are all accepted. Each window can be named and added separately, so a room with several windows keeps its results legible.
• Sill height. The distance from the finished floor up to the bottom of the window frame, which fixes where the rod lands and how far the fabric has to fall.
• Header style. Sets the fullness ratio, how many times wider than the rod the flat fabric must be so the panel gathers instead of hanging flat. Rod pocket uses 2.5, pinch pleat 2.25, grommet 1.75, and tailored flat 1.2; a fullness override applies a workroom's own ratio to every window instead of the header default.
• Drop style. Sets 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, and puddle pools 6 inches of extra fabric.
• Design constants. Rod extension per side defaults to 6 inches, 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. Any of these can be overridden for a specific project.
• Pattern repeat. Set when the fabric carries a motif that has to align across panels; each cut length rounds up to a whole number of repeats, which is what drives the extra yardage on patterned goods.
• Unit system. Metric mode converts every default and reports fabric in meters instead of yards.
• Cost. Optional per window; left blank or at zero, a window reports sizes with no cost line. Cost basis reads the entered 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, with a configurable currency symbol. Priced and unpriced windows can sit in the same order, and the total reports how many carried a price.

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 wanted 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: A mount height of 6 inches or more above the frame, or a rod placed halfway between the frame and the ceiling, paired with a floor length drop, carries the eye from rod to floor in one unbroken line, which is what creates the height. A short drop undoes the effect no matter how high the rod goes.
• Get the most daylight from the window: A rod extension of 10 to 12 inches per side stacks the open panels onto the wall instead of the glass, the single change that matters most for light.
• Curtains opened and closed every day: A grommet header glides on the rod with almost no friction, and a floor length hem that clears the ground does not drag or soil with daily use.
• A formal room where panels stay in place: A pinch pleat header with a break or puddle drop assumes the fabric is decorative rather than working, which suits panels that are rarely drawn.
• Block light and gain privacy: A high fullness header, rod pocket at 2.5x or pinch pleat at 2.25x, with a floor length drop and a generous rod extension, closes the gaps between folds while the extra rod width lets the panels overlap the frame at both edges.
• A window above a counter, radiator, or window seat: A sill drop, or an apron drop when a baseboard heater or furniture sits just below the sill, keeps the hem clear of the obstruction.
• Show off a large printed or woven fabric: A tailored flat header at 1.2x hangs nearly flat; folds hide a pattern, so the flatter the panel, the more of the design stays visible.
• Keep the fabric budget down: A grommet header at 1.75x fullness, paired with a solid or non-directional texture that needs no pattern repeat, cuts the order the most, since fullness and repeat are the two factors that drive yardage hardest.
• A house with pets or small children: A floor length drop, rather than break or puddle, keeps fabric off the floor; fabric resting on the floor collects hair and dust and has to be rearranged by hand after anyone walks past.
• Sewing the panels yourself: The single panel width becomes the usable width of the bolt rather than the 50 inch ready-made default, and a hem and header allowance above 12 inches gives a deeper hem in heavy fabric. The cut length, not the finished length, is the number to mark and cut against.
• Comparing making against buying: Pricing by the yard or meter costs a sewn order, while pricing by the finished panel costs the same windows against ready-made curtains, so the two bases put the same measurements against two different purchases.

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): Suits a window with something directly below it that 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. It is a poor fit 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): Suits a wall below the window that 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. Over an empty wall the hem floats with nothing to relate to and reads as an unfinished measurement.
• Floor (half an inch above the floor): 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. It works poorly 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): Suits 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. On panels opened and closed daily the resting hem drags across the floor every time and soils along one edge.
• Puddle (6 inches pooling on the floor): Suits formal, stationary panels in a low-traffic room, where the pooled fabric is part of the effect. It is a poor fit anywhere with pets, children, or regular traffic, since 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. It suits stationary decorative panels and casual rooms, and works poorly 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. It suits formal rooms and panels that traverse on a track or with rings, and 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. It suits panels drawn every day, since they glide with almost no friction, and reads casual in formal rooms because of the exposed hardware.
• Tailored Flat at 1.2x: Almost no gathering, so the panel hangs nearly flat. It suits fabric carrying a large print or a woven pattern that folds would hide, or a deliberately minimal look, and works poorly 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, 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. It runs to 10 or 12 inches for heavy fabric or wide windows, where the stack is bulkier, and drops 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 places the rod halfway between the frame and the ceiling, or within 4 to 6 inches of the ceiling when that gap is small. It comes down when crown molding or a low ceiling leaves no room.

• Single panel width, default 50 inches: The most common width for a ready-made panel, so it works as a starting point for shop-bought curtains. It changes to the actual width of the panels being bought, or to the usable width of the bolt when sewing, since that figure determines how many panels the required fabric width divides into.
• Hem and header allowance, default 12 inches: 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. The allowance increases for a deeper hem on floor-length panels in heavy fabric.
• Pattern repeat, default 0: Set 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. It stays at 0 for solids, textures, and non-directional weaves.
• Fullness override, default 0: Left at 0, each window uses the ratio for its header style. A value here applies when a workroom specifies its own fullness, or to price the same window at two different ratios and compare what the extra fabric costs.
• Cost basis: Per yard or meter suits buying cut fabric by the length, the usual case when sewing. Per finished panel suits 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: Every window in a living room, including a wide slider and two flanking casements, rolls up into a single fabric total, so all panels can come from one dye lot; the same fabric run can otherwise shift shade between batches.
• Patterned Fabric Ordering: A 24 inch pattern repeat on a large floral rounds each cut length 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: A window that sits close to the ceiling, with no room for the usual 6 inch rise, calls for a reduced mount height; the drop then follows from the actual rod position rather than a standard assumption.
• Cafe Curtains and Kitchen Sinks: A sill drop suits 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: Break or puddle suits a formal room where fabric resting on the floor reads as intentional. Puddle adds 6 inches to the finished length, enough to change yardage 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: A fabric price per yard on each window totals what a room costs before committing to a bolt; switching the basis to per panel prices the same windows against ready-made curtains instead, 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. The allowance increases when using a deeper hem than the 4 inch doubled default.