Why do some cushions look plump and others look tired?

Most of the time it is not the cover. It is the insert, the inner pad, and how its size compares with the cover around it. An insert that is exactly the cover's size leaves the corners a little empty. One that is slightly bigger pushes into the corners and gives the full, square look you see in shop photos. One that is much bigger strains the seams.

The idea is easy to feel once you can see it, so try it below. Pick a cover width, pick a filling, then slide the insert from smaller to bigger.

The rule of thumb, and why it works

A common rule is to buy an insert one size step bigger than the cover, which usually means about 2 to 5 cm wider. A 45 cm cover often takes a 50 cm insert. The extra width has to go somewhere, and it goes into the corners and edges, so the cover is filled out instead of draped.

It is a starting point, not a law. Tightly woven or stiff covers, such as heavy velvet or thick linen, show strain sooner. Stretchy or loose weaves forgive more.

Filling changes the answer

Feather inserts squash easily, slip through a zip opening and spring back when you plump them, so they cope well with a bigger step. They do need a shake now and then to redistribute. Polyester fibre ranges from soft to springy. The springy kind resists being squashed, so a very oversized one can push against the seams. A foam block does not compress at all. Match its size closely to the cover, and aim for just a little over.

Whatever you choose, the width is not the only number. Check the height too, because a square cover with an oblong insert will wrinkle across the short side.

Measure before you buy

1. Lay the cover flat, zipped up, and measure seam to seam along the top edge and down the side. That is the cover size.
2. If you already have an insert, measure it the same way, flat and unplumped.
3. Compare the two. Aim for the insert to be a few centimetres larger in both directions, and check the label for the size the pad is made for.
4. For a sofa, think about depth. A very deep cushion can push someone forward instead of supporting them.

Cheaper ways to fix a flat cushion

You do not have to buy anything new straight away. Take the insert out and give it a firm shake and a few pats on each side. Put it back in with the corners pushed all the way in first. If the pad is still too small, a bit of extra stuffing in the corners helps, and old but clean pillows or a spare insert can be reused. Second-hand shops often sell good feather pads for much less than new.

What to look for

• Small budget: a polyester fibre pad in the next size up from your cover. Shake it out before you put it in.
• Middle: a feather and fibre mix that squashes into corners and still holds some shape.
• Higher: a feather-rich insert with a closely woven casing, so the feathers stay inside and the shape lasts.

Check the size label first, and the filling second. If a pad is the wrong size, the nicest filling will not fix it.