Fabric calculators

Every calculator here answers the same underlying question — how much cloth does this project need — and every one of them answers it the same way: solve the width, solve the length, multiply, then round up to something a shop will cut. What differs between projects is which allowances apply and how the fabric has to be joined.

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What every calculator here does the same way

Rounding is always upward. Never to nearest, at any step. Buying 40 cm too much fabric costs a little and goes in the offcuts box; buying 20 cm too little means returning for more from a dye lot that may have sold out. The two errors are not equally bad, so they are not treated equally.

Whole widths, not fractions. Fabric is sold by the width, so a curtain needing 1.6 widths gets 2. The consequence is that you almost always end up with more fullness than you asked for — which looks like a bug until you see the arithmetic, so the figure you actually get is always shown next to the figure you requested.

Allowances are inputs, not constants. There is no single standard for how much fabric a heading or a hem consumes; a London workroom, a US drapery shop and a sewing textbook will give three defensible answers. The defaults below are the middle of the UK and European range, and every one of them is editable on the calculator that uses it.

Before you measure anything

Three mistakes account for most of the emails a fabric calculator receives, and all three happen before any arithmetic starts. If a word below is unfamiliar, the glossary defines every term these calculators use.

• Measuring the window instead of the track. A curtain track normally extends 15–20 cm past the glass on each side so the curtains can stack clear of it. Measure what the curtains hang from, not what they cover.
• Measuring the drop from the pole. The finished drop runs from the hook, ring or eyelet position — which sits below the pole — down to the hem. The gap is usually 2–4 cm, which is the difference between a floor-length curtain and one that hovers.
• Trusting the roll width on the label. A fabric sold as 140 cm is often 137–140 cm once the selvedge is trimmed, and "54 inch" is 137.16 cm rather than 140 whatever the bolt end says. If the width matters, measure it.

What is coming

Roman blinds, upholstery, cushion covers and bias binding were on this list and are now built. What is still open, in no particular order: quilt backing and binding, elastic length, half-drop pattern repeats, and the fold depth and rod spacing on a Roman blind. The last of those is unbuilt for a reason worth stating — published workroom methods disagree about how the bottom section relates to the folds above it, and this site would rather ship nothing than pick one and present it as settled. Adding a tool is otherwise a small job, because the page structure and the testing already exist; the work is in getting the maths and the assumptions right. Requests genuinely move things up the list.

Working from a pattern written in other units, or for a different roll width? The conversion tools handle both. The formulas behind every calculator are set out in full on the methodology page.