Curtains are the one soft-furnishing project where the fabric costs far more than everything else combined, and where the quantity is not obvious. It is not the width of the window; it is the width of the track multiplied by a fullness ratio, rounded up to whole bolt widths, times a cut length that includes allowances you cannot see in the finished curtain. This curtain fabric calculator does all of that, including the pattern repeat, which is the step people forget until the fabric is already cut.
Arb Digital keeps a free tools library of estimating maths for projects where an error costs material rather than time. This page covers flat panel curtains made from full fabric widths. It does not draft a pleat plan, size a lining separately or design a pelmet.
What This Curtain Fabric Calculator Does
It works in two directions at once. Across the window, it multiplies the track width by the fullness ratio, adds the returns and overlap, and divides by the usable fabric width to find how many full widths of cloth the curtains need — always rounded up, because you cannot buy four and a bit widths.
Down the window, it takes the finished drop and adds the header and hem allowances to get a cut length. If the fabric has a pattern repeat, it rounds that cut length up to the next whole repeat, so every width starts at the same point in the design and the pattern matches across the seams.
Multiply the two and you have the fabric to buy. The supporting figures give the number of widths, the cut length per width, the flat width the curtains need before rounding, and exactly how much fabric the pattern repeat costs you.
The boundary with the other fabric tools on the site is the fullness ratio. The fabric yardage calculator fits rectangular cut pieces to a bolt and has no concept of gathering. The circle skirt calculator solves a radius, and the cross stitch calculator works from thread counts. Only curtains are sized by deliberately buying more cloth than the opening.
How to Use It
- Measure the track, not the window. The track normally extends well beyond the frame so the curtains can stack clear of the glass when open.
- Choose the fullness for the heading you want. A pencil-pleat tape usually wants two to two and a half times; a pinch-pleat heading wants closer to three; sheers and eyelets can work at one and a half.
- Measure the drop from the track. Floor-length curtains are usually finished a centimetre or so clear of the floor unless they are deliberately puddled.
- Set the allowances. A double hem takes twice its finished depth, which is why the hem allowance is bigger than the header.
- Enter the pattern repeat if there is one. Then look at the last grid figure to see what it costs before you commit to that fabric.
The Formula: Fullness, Widths and Cut Length
Three steps, in order:
flat width = track width × fullness + returns and overlap
number of widths = round up (flat width ÷ usable fabric width)
cut length = drop + header allowance + hem allowance, rounded up to a whole pattern repeat
total fabric = number of widths × cut length
Worked example. A 120-inch track at 2× fullness needs 240 inches of flat fabric, plus 6 inches for returns and overlap, giving 246 inches. On 54-inch usable fabric that is 246 ÷ 54 = 4.56 widths, which rounds up to 5 widths. The drop is 84 inches, plus a 6-inch header allowance and an 8-inch hem allowance, so each width is cut at 98 inches. Total fabric is 5 × 98 = 490 inches, which is 13.6 yards or 12.4 metres.
Now add a 24-inch pattern repeat. The cut length rounds up from 98 to 120 inches, the next whole multiple of 24. Total fabric becomes 5 × 120 = 600 inches, or 16.7 yards. The repeat has cost 110 inches — over three yards — for a curtain whose finished size has not changed at all. That is the single most expensive line in a curtain estimate and the one most often left out.
Two unit notes. Fabric is sold by length at a fixed width, and inch and centimetre figures are exactly convertible: NIST's SI units of length reference gives the inch as exactly 25.4 millimetres, so a 54-inch bolt is exactly 137.16 cm. The rules governing how goods sold by length are measured and labelled sit in NIST Handbook 130, the uniform laws and regulations on legal metrology that US states adopt.
Fullness: What the Ratio Actually Buys
Fullness is the whole reason curtains look like curtains. A panel cut exactly to the track width hangs flat and reads as a blind. The gathered folds that give a curtain its depth come from having more cloth than the opening, and the ratio between the two is the only control you have over how deep those folds are.
Different headings need different ratios because they consume the excess differently. A pencil-pleat tape draws the fabric into small even gathers and typically wants somewhere between two and two and a half times the track width. A pinch-pleat heading groups the excess into fixed pleats at intervals and generally wants closer to three, since each pleat swallows a fixed amount. Eyelet headings form their own regular waves and work at lower fullness. Sheers are the exception in the other direction: because the fabric is transparent, low fullness looks thin and high fullness reads as texture, so sheers are often made fuller than opaque fabric on the same track.
The heading tape you buy will state the fullness it is designed for, and that figure is more reliable than any general rule. Use it. A tape designed for 2.5× fullness fitted to 2× fabric will not gather correctly.
Pattern Repeat: The Line That Blows the Budget
A patterned fabric can only be cut at points where the design lines up, so every cut length has to be a whole number of repeats. The consequence is a step function: as the drop increases, the fabric needed stays flat and then jumps by a full repeat.
The waste is worst when the required cut length sits just past a repeat boundary. A 98-inch requirement with a 24-inch repeat wastes 22 inches per width, five times over. Move the drop to 92 inches and the requirement falls to 96, exactly four repeats, and the waste disappears entirely. That is why it is worth calculating before you fix the drop, not after.
Large repeats are common on formal furnishing fabrics and can exceed two feet. Check the repeat on the bolt label before falling in love with a design, and if two windows in the same room are different heights, they will not share offcuts — each drop rounds independently.
One related decision: whether to match the pattern across the seams between widths. Matching is standard on anything patterned and is already accounted for by rounding the cut length to whole repeats. Not matching saves fabric and looks like a mistake, because it is one.
The Measurements People Get Wrong
Measure the track, never the window. A track normally extends 15 to 20 cm beyond the frame at each side so that open curtains stack clear of the glass and let the light in, and a curtain sized to the window will always look mean and block a strip of daylight.
Measure the drop from the track, not from the top of the frame — for a pole, from the bottom of the rings, since that is where the curtain actually hangs. Take the drop in three places across a wide window: floors are rarely level and a long curtain shows a slope of a few millimetres clearly.
Returns and overlap are small numbers that are always forgotten. The return is the fabric that wraps back to the wall at each end of the track, and the overlap is where the two curtains cross in the middle. Both need cloth, both matter for blocking light, and together they justify the small extra allowance in the calculation.
Finally, allow for shrinkage on anything that will be washed rather than dry-cleaned. Natural fibres in particular can shrink noticeably on their first wash, and a curtain that shortens after cleaning cannot be fixed. Pre-shrink the fabric or add to the hem allowance so the hem can be let down.
Arb Digital's free tools library covers materials estimating for decorating, sewing and building projects, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Measuring the window instead of the track — the track is wider so the curtains can stack clear of the glass, and the fullness is applied to the track.
- Ignoring the pattern repeat — it can add several yards to a job without changing the finished curtain at all.
- Using the bolt width rather than the usable width — the selvedges are not usable and will pull the seams if they are included.
- Undersizing the hem allowance — a double hem needs twice its finished depth, and a generous hem can be let down later if the fabric shrinks.
- Matching the fullness to a heading tape that wants a different one — the tape's stated fullness is the figure that must be used, not a general rule.
Related Free Tools From Arb Digital
Use the fabric yardage calculator for lining and other rectangular pieces, the wallpaper calculator for the same repeat-rounding problem on walls, the paint calculator for the rest of the room, the rug size calculator for the floor, and the circle skirt calculator when the project is a garment rather than a window. Everything else is in the free online tools hub.
Frequently Asked Questions
Multiply the track width by the fullness ratio, add the returns and overlap, and divide by the usable fabric width to get the number of widths, rounded up. Multiply that by the drop plus the header and hem allowances.
Follow the heading tape you are using. Pencil pleat typically wants two to two and a half times the track width, pinch pleat closer to three, and eyelet or sheer headings can work at one and a half.
The track. It normally extends well beyond the frame so open curtains stack clear of the glass, and the fullness ratio is applied to the track width rather than the window width.
Every cut length has to be a whole number of repeats so the pattern matches across the seams. That rounds each width up, and with a large repeat it can add several yards to the total.
Enough for a double hem, which is twice the finished depth, plus a little spare if the fabric may shrink. Eight inches is a common allowance for a four-inch double hem.
The return is the fabric wrapping back to the wall at each end of the track, and the overlap is where the two curtains cross in the middle. Both need cloth and both matter for blocking light.
Not usefully. Fabric is sold by length at a fixed width, so a job needing 4.1 widths costs the same as one needing 5. Adjusting the fullness slightly sometimes brings you under the line.
This tool estimates fabric quantity from the measurements you enter. Check the figures against your actual fabric and heading tape before cutting, and allow extra for shrinkage, flaws and pattern matching.
