The right cushion insert size is two inches larger than the cover. A 45 cm (18 inch) cover takes a 50 cm (20 inch) insert, a 50 cm cover takes a 55 cm insert, and a 30 by 50 cm lumbar takes a 35 by 55. Size up, every time, and the flat, sad corners that make cushions look cheap simply disappear.
This is the one rule that separates cushions in photographs from cushions on most sofas. The cover is rarely the problem. The insert inside it is either too small, too light or the wrong fill for the job, and each of those has a fix.
Why you size a cushion insert up
A cover is sewn flat, so its stated size is the size of the fabric edge to edge. Once it wraps around an insert, some of that width is used up going around the sides, and a same-size insert ends up rattling around inside with nothing pushing into the corners. The corners collapse, the cover creases across the middle, and the cushion looks like it has been sat on even when it has not.
An insert that is two inches larger fills the corners, pulls the fabric taut and gives the cover the slightly overstuffed shape that reads as plump rather than deflated. It also helps the cushion hold its shape after use. When someone leans on it and gets up, a well-filled insert springs back; an under-filled one stays crushed.
Two inches is the standard because it works for most fills and most fabrics. Go up by three or four with very soft feather inserts, which compress more, and stay at two with firmer synthetic fills, which do not.
Cushion insert size chart
Covers are still sold in inches in much of the world and centimetres elsewhere, so both are here. The pairings below assume a standard fill; nudge the insert up one more size if you like a very full look or are using feather.
- 40 cm / 16 inch cover: 45 cm / 18 inch insert. A small square for armchairs and shallow seats.
- 45 cm / 18 inch cover: 50 cm / 20 inch insert. The most common size and the one most people own.
- 50 cm / 20 inch cover: 55 cm / 22 inch insert. Generous, suits deep low sofas and beds.
- 60 cm / 24 inch cover: 65 cm / 26 inch insert. A floor or bed cushion size, rarely right on a sofa.
- 30 by 50 cm lumbar: 35 by 55 cm insert. Lumbars need the extra length more than the extra width, so do not skimp.
If a cover has a very generous seam allowance or a thick fabric such as a quilted or heavy woven panel, the finished internal size is smaller than the label. In that case treat the cover as one size down and choose the insert accordingly.
Fill types, honestly compared
The fill decides how the cushion feels, how it ages and how it behaves after washing. There is no single best fill; there is a best fill for the job.
Feather and down
Feather inserts are soft, heavy and shape easily. They drape into a corner, hold a fold and feel expensive in the hand. They also compress over time, need plumping every few days, and the quills can work through thinner covers. Choose feather for a sofa you sit on every evening and want to feel deep and yielding, and size the insert up by three inches rather than two.
Polyester and hollowfibre
Synthetic fills are light, springy and resilient. They hold a square shape without plumping, survive the washing machine and cost less. Cheaper versions go lumpy within a year; better ones use siliconised clusters that stay loose and even. Choose a good synthetic for children's rooms, guest beds, outdoor seating and any cushion that will be washed often.
Feather-wrapped and blended
Some inserts wrap a springy synthetic core in a layer of feather, which gives most of the softness with less of the sag. They are a sensible middle ground and the one we tend to recommend for a living room sofa in a home where the cushions get used rather than admired.
How plump is plump enough
A well-filled cushion should look slightly overfull when you put it down and stay that way after you lean on it. The corners of the cover are pushed out fully with no loose fabric in the tips. When you press the middle with a flat hand, it gives, then returns within a few seconds.
If the cover creases in a diagonal line from corner to corner when you stand it up, the insert is too small or too soft. If you struggle to close the zip, or the seams look strained, the insert is too big for the fabric and will stress the stitching. Somewhere between those two is right, and two inches up usually lands there.
Do not chop the top of the cushion to make a dent. That habit belongs to staged photographs; in a real room it collapses the moment anyone sits down, and a properly filled insert looks better without it.
Caring for inserts and covers
Most covers can be washed on a cool cycle and dried flat; the insert usually cannot, or not often. Separate the two before washing and let the cover dry fully before you refill it. A damp cover on a feather insert is how mould starts.
Feather inserts benefit from a monthly airing in fresh air and a good shake to redistribute the fill. Synthetic inserts can go through a gentle machine cycle every few months and come back to shape in the dryer on low, with a couple of clean tennis balls in the drum to break up any clumps. Our note on caring for linen and linen-look fabrics covers the cover side in more detail.
Once the insert has lost its spring, replace the insert and keep the cover. A new insert costs a fraction of a new cover and gives the cushion a second life. That is also the moment to check the size and, if you bought same-size the first time, to go up.
Insert first, cover second
Choosing a cushion insert size well is unglamorous work, but it is what makes every other choice look right. A beautiful cover on an under-filled insert looks cheap; a plain cover on the right insert looks considered. Size up by two inches, pick a fill that suits how the cushion will be used, and keep the middle plump rather than dented.
With the inserts sorted, the rest of the sofa is about restraint. How many you need, and which sizes to mix, is covered in our guide to how many cushions belong on a sofa, and the way cushions work with throws and rugs is in layering textiles. For the room around them, start with the Japandi living room guide.
Frequently asked questions
What size insert for a 45 cm cushion cover?
Use a 50 cm (20 inch) insert in a 45 cm (18 inch) cover. The extra size fills the corners and keeps the cover taut. If the insert is a very soft feather one, a 55 cm will give an even fuller shape.
Should a cushion insert be bigger than the cover?
Yes, by about two inches (5 cm) in each direction. A same-size insert leaves empty corners and creases across the middle, which is why so many cushions look flat. Sizing up is the standard practice in upholstery and interior styling.
Are feather or polyester cushion inserts better?
Feather feels softer and heavier and drapes well, but it compresses and needs plumping. Polyester holds its shape, washes easily and costs less, but can feel springy. For an everyday sofa, a feather-wrapped synthetic core gives most of the softness with less of the sag.
