Almost every curtain that disappoints its owner disappoints for the same reason: there isn’t enough fabric in it. The colour was right, the fabric was beautiful on the roll, and the finished curtain still hangs flat and thin against the wall like a bedsheet on a line. Fullness is not a luxury upgrade. It is the entire reason a curtain reads as drapery rather than as a panel.

So before you fall in love with a fabric, it is worth understanding the arithmetic that sits behind it — partly so you can budget honestly, and partly so that when a quote comes back with more metres than you expected, you know exactly what those metres are doing.

Measure the track, not the window

This is the single most common error, and it compounds through every other step. Your curtain hangs from the track or pole, not from the window frame, and a well-placed track is considerably wider than the window it serves.

The extra width on each side is called the stack-back: the space the curtain occupies when it is fully open. Without it, an open curtain sits permanently across the glass, stealing daylight from a room and hiding the window you paid to dress. Decide where the track goes first. Fit it, or at least mark it. Then measure it.

For the drop, measure from the top of the track for a tracked system, or from the bottom of the ring eye for a pole — that ring is where the fabric actually begins. Measure at several points across the window: older Cape Town homes in particular are rarely square, and a 15 mm difference between left and right is normal and worth knowing about before anything is cut.

Fullness is where the fabric goes

Fullness is the ratio between the flat fabric width and the finished track width. A 2x fullness curtain on a 3 metre track contains 6 metres of flat fabric, gathered into 3 metres of hanging curtain. The gathers are the point.

Different headings need different ratios, because each one folds the fabric differently:

These figures are the trade’s working conventions, published in guides such as Direct Fabrics’ fullness guide, and they describe raw flat fabric before making up — side hems and seams take a little back. They are ranges rather than laws. A heavy velvet at 2x already looks generous because the cloth carries its own weight; a fine linen at 2x can look sparse.

“Fabric is not the expensive part of a curtain. Regret is. The metre you leave out to save money is the metre you notice every morning for the next fifteen years.”

Turning flat width into a number of drops

Fabric arrives on a roll of a fixed width, printed on the label. Check yours rather than assuming — it changes the answer completely.

You cannot use the full roll width. Each drop loses fabric to two side hems, and where drops are joined together they lose a little more to the seam allowance. Subtract that from the roll width to get your usable width per drop, then divide the flat fabric width by it.

Then round up. Always up. Four and a bit drops is five drops, and the half drop that feels wasteful is what keeps the pair symmetrical.

Pattern repeat: the cost nobody budgets for

If your fabric carries a pattern, every drop has to begin at exactly the same point in that pattern, or the design will step up and down across the finished curtain and across the pair.

In practice this means each cut length is rounded up to a whole number of pattern repeats. A fabric with a 64 cm repeat can add the better part of half a metre to every single drop — and on a five-drop pair, that is two and a half metres of fabric you did not plan for. It is not waste, and it is not padding on the quote. It is the price of the pattern matching across the window.

Large-scale prints are the most affected. If the budget is tight and the repeat is enormous, a plain or a textured weave will go considerably further.

The cut length

The cut length is not the finished drop. To the finished drop you add:

• A bottom hem — generous, because weight at the bottom is what makes a curtain hang straight rather than kick out at the floor.
• A heading allowance at the top, which varies with the heading style and the tape or buckram used.
• The pattern repeat rounding described above, if the fabric is patterned.

A deep bottom hem also buys you something else: room to let the curtain down later if it settles, which natural fibres such as linen reliably do in the first season.

A worked example

A 2.4 metre track, a 2.6 metre finished drop, a pinch pleat heading, and a plain fabric with a usable width of 1.25 metres per drop after hems and seams:

• Flat width: 2.4 m × 2.5 = 6.0 m
• Drops: 6.0 ÷ 1.25 = 4.8, rounded up to 5 drops
• Cut length: 2.6 m drop + 0.20 m bottom hem + 0.10 m heading = 2.90 m
• Total fabric: 5 × 2.90 m = 14.5 m

Fourteen and a half metres, for one pair of curtains on a window under two and a half metres wide. That number surprises people, and it is the honest one. Change the heading to a pencil pleat and it drops to four drops and 11.6 metres. Add a patterned fabric with a substantial repeat and it climbs again.

Where the maths usually goes wrong

• Measuring the window instead of the track. Every downstream number is then too small.
• Forgetting stack-back. The curtains fit, but they never fully clear the glass.
• Rounding drops down. The pair ends up asymmetrical and under-full.
• Ignoring the repeat. The pattern marches out of line across the pair.
• Applying the same maths to sheers. Many sheer fabrics are woven extra-wide and are made up sideways, so the roll width becomes the drop. The calculation is different, and assuming otherwise will over-order badly.

If you would rather not carry any of this yourself, it is exactly what a measuring appointment is for. Our made-to-measure curtain service works the numbers from the fitted track, and if you are weighing up hardware at the same time, our note on curtain installation covers how track placement changes the result.

Fullness conventions referenced from Direct Fabrics’ published guide to curtain fullness. Figures in the worked example are illustrative and assume a plain fabric with 1.25 m usable width per drop.