Fullness is just a ratio

Hold a curtain panel flat against a window and it looks like a sheet on a pole. Gather the same fabric into folds and it turns soft, warm and a little bit luxurious. The thing that decides which one you get is fullness: the width of the fabric divided by the width of the pole or track it hangs from.

A pole that is 200 cm wide with 400 cm of fabric on it has a fullness of 2×. Fabric that is only as wide as the pole (1×) has no spare material to fold, so it hangs flat. More fabric means deeper, rounder folds. Try it below: set your pole width, then slide the fullness and watch the folds appear from the front and from above.

Why 2× is the usual advice

Shops and curtain makers tend to agree on a few numbers. Pencil-pleat and eyelet headings are normally made at about 2×; wave headings a little more, around 2.2–2.3×; pinch pleats go up to roughly 2.5×. Below about 1.5× the folds start to flatten, and above 2.5× they get crowded, which is why you rarely see more than 3×.

The reason is simple geometry. A fold needs spare fabric to make a loop. The more spare you have per fold, the deeper the loop, and deep loops catch light and shadow, which is what makes a curtain look soft rather than stiff. That is also why curtains can feel like insulation and sound-dampening: the folds hold a little air. Treat that as a gentle bonus, not a promise, because the effect depends on the fabric and the lining.

Break the rule when the fabric asks you to

Two is a starting point, not a law. Thick, lined or velvety fabric is heavy: lots of extra width gets bulky, and 1.5–1.8× often looks neater. Thin, airy fabric such as a sheer looks flat and a bit sad at 1.5×, so give it more. Narrow windows and small rooms can take a lighter fullness; a big window wall can carry plenty.

And if the curtains you already own hang a little thin, you do not need to start again. Moving the rings closer together, or hanging an extra panel on each side, raises fullness without a purchase.

What to measure before you decide

• The pole or track, not the window. Fullness is counted against the pole width. Measure between the finials (the end caps), edge to edge, with a tape measure.
• How far the pole extends. A pole that sticks out roughly 15–30 cm past each side of the window lets curtains stack back off the glass, so the window gets all of its daylight. That makes a small room feel brighter.
• The drop. Measure from the pole (or the rings) down to where you want the hem: just above the floor, or just touching it. Check the floor at both ends, since floors are not always level.
• Your wall. Poles with heavy curtains pull on their fixings. Use plugs that suit your wall type, and ask your landlord first if you rent. Tension rods and adhesive hooks are options for light fabrics in a rented window.

Then multiply: pole width × fullness gives the fabric you need. Divide by the width of one panel, round up, and buy in pairs so both sides match. The planner above does this sum for you, including what a pair of ready-made panels gives you on your pole.

What to look for

Ready-made curtains are usually 135–150 cm wide per panel, so a pair gives about 270–300 cm of fabric. On a 140 cm pole that is a generous 2×; on a 250 cm pole it is only about 1.1×, which will look flat. Check the pole first, then pick the panels, not the other way round.

• Under a tight budget: aim for the right width and a fabric weight you like. A thin fabric at the right fullness looks better than a heavy one stretched flat.
• In the middle: a natural-look texture, such as a linen look, hides creases and folds softly. Check the heading type, since it decides how the fabric gathers (rings or eyelets suit most poles).
• When spending more: look at weight and drop. A fabric with some heft hangs straighter, and a longer drop gives the folds room to fall.
• Always: check the care label, and the lining if you need light control.

A reminder that cosy doesn't have to mean new. Second-hand panels, cut-down older curtains and an extra pair from a friend all count, as long as the width works for your pole.