A 45x45 cm cushion is 17.7 x 17.7 inches, and it's sold as an 18 x 18 inch cushion. One inch is exactly 2.54 cm, so you divide centimetres by 2.54 to get inches, then round to the nearest whole inch to get the size on the label. This guide gives the full conversion chart from 30x30 cm to 75x75 cm, the answers for 45x45 cm, 30x45 cm and 18 x 18 inches, why the exact and labelled figures don't match, and how the label affects the pad you buy for a cover.
What are cushion sizes in cm and inches?
Cushion pad sizes on our listings run in 5 cm steps from 30 to 75 cm, and each step is close to 2 inches, so 40 cm is sold as 16 inches, 45 cm as 18 inches and 50 cm as 20 inches.
| Size in cm | Exact size in inches | Inch label on our listings |
|---|---|---|
| 30 x 30 cm | 11.8 x 11.8 in | 12" x 12" |
| 30 x 45 cm | 11.8 x 17.7 in | 12" x 18" |
| 35 x 35 cm | 13.8 x 13.8 in | 14" x 14" |
| 40 x 40 cm | 15.7 x 15.7 in | 16" x 16" |
| 45 x 45 cm | 17.7 x 17.7 in | 18" x 18" |
| 50 x 50 cm | 19.7 x 19.7 in | 20" x 20" |
| 55 x 55 cm | 21.7 x 21.7 in | 22" x 22" |
| 60 x 60 cm | 23.6 x 23.6 in | 24" x 24" |
| 65 x 65 cm | 25.6 x 25.6 in | 26" x 26" |
| 70 x 70 cm | 27.6 x 27.6 in | 28" x 28" |
| 75 x 75 cm | 29.5 x 29.5 in | 30" x 30" |
The exact inches are the centimetres divided by 2.54, rounded to one decimal place. The labels are the ones on our hollowfibre cushion inserts, which list every size as, for example, 45cm x 45cm (18" x 18" inches). Each label is the exact figure rounded to the nearest whole inch.
What is 45x45 cm in inches?
45x45 cm is 17.72 x 17.72 inches, which rounds to 18 x 18 inches. That's why a 45 cm cushion and an 18 inch cushion are sold as the same size.
The working is short: 45 divided by 2.54 is 17.716, so 17.7 to one decimal place. The rounding adds about a quarter of an inch to each side. In practice you won't see that on a sofa, but it matters when you're matching a pad to a cover measured in the other unit, which we come back to below.
45x45 cm comes in three of our four pad ranges, all of them hollowfibre. The 2 pack is £9.49 for 2 pads, the quilted inserts are £16.99 for a pack of 4, and the 18 x 18 inch hollowfibre pad is £9.49 for 2 or £12.99 for 4. The duck feather pads don't come in 45 cm.
What is 30x45 cm in inches?
30x45 cm is 11.81 x 17.72 inches, which is sold as 12 x 18 inches. It's the lumbar size on our listings, the long, shallow cushion that sits in the small of your back.
Both sides round up by a fraction: 30 cm becomes 12 inches and 45 cm becomes 18 inches. Every one of our pad types comes in this size. The hollowfibre 2 pack is £9.99, the quilted inserts are £15.99 for 4, and the duck feather pads are £10.99 for 2, listed as 12 X 18. If you're torn between fillings, our guide to feather vs hollowfibre cushion pads sets out how each one feels and how much plumping it needs.
What is 18x18 inches in cm?
18 x 18 inches is 45.72 x 45.72 cm. Multiply inches by 2.54 to get centimetres, so a true 18 inch square is about 7 mm bigger on each side than a 45 cm square.
This is the conversion that catches people out. A cover sized in true inches, or one you've measured with a tape at home, can be a genuine 18 inches. A pad labelled 18" but made at 45 cm is the smaller of the two. Going the other way, here's what each whole-inch size works out to in centimetres:
| Inches | Exact cm | Nearest UK cm size |
|---|---|---|
| 12 in | 30.48 cm | 30 cm |
| 14 in | 35.56 cm | 35 cm |
| 16 in | 40.64 cm | 40 cm |
| 18 in | 45.72 cm | 45 cm |
| 20 in | 50.80 cm | 50 cm |
| 22 in | 55.88 cm | 55 cm |
| 24 in | 60.96 cm | 60 cm |
| 26 in | 66.04 cm | 65 cm |
| 28 in | 71.12 cm | 70 cm |
| 30 in | 76.20 cm | 75 cm |
Why don't exact and labelled inches match?
Exact and labelled inches don't match because cushions are made in round centimetre sizes and then given a round inch label. 5 cm is 1.97 inches, so each 5 cm step is labelled as 2 inches.
The inch itself isn't approximate. NIST, the US measurement institute, says the inch is exactly equivalent to 25.4 mm. UK law agrees: the Weights and Measures Act 1985 defines the yard as 0.9144 metre and the inch as 1/36 of a yard, which also comes to 2.54 cm. The fudge is only in the label.
That small shortfall adds up. Because each 2 inch label stands for 1.97 inches of real size, the difference grows with the cushion. At 12 inches the true size is 0.48 cm bigger than the 30 cm pad. At 30 inches it's 1.2 cm bigger than the 75 cm pad. For most cushions that's a few millimetres. But if you're filling a big floor cushion cover measured in true inches, go by the centimetres.
Should a cushion pad be bigger than the cover?
Yes, a cushion pad should be a little bigger than its cover so the cushion looks full. Our hollowfibre pad listing says to buy one size up, and our 2 pack listing says 2 to 3 cm larger.
Our hollowfibre pad listing gives an example in both units: an 18 x 18 inch pad fills a 17 x 17 inch or 43 x 43 cm cover. So read the cover's size in centimetres where you can, then pick the pad label above it. Our cushion pad size guide has the cover-to-pad chart for every size from 30 to 70 cm, and the guide to cushion inserts that keep their shape covers why quilted inserts stay squarer than plain ones.
How do you convert a cushion size yourself?
To convert a cushion size yourself, divide centimetres by 2.54 for inches, or multiply inches by 2.54 for centimetres. Measure the cover flat, seam to seam, before you convert.
Lay the cover flat with the zip closed and measure across the middle, edge to edge. Then round the result the way shops do, to the nearest whole inch, and you'll land on the label you need to look for.
A quick check you can do in your head: 2.5 cm is about an inch, so 10 cm is about 4 inches and 50 cm is about 20 inches. It gets you within half an inch for anything under 50 cm. For a pad, that's close enough to shop by. You'll find every size in this chart in our cushion pads collection, from a 12" x 12" square at £8.99 for 2 to a 30" x 30" at £18.99 for 2.
Written by the Adam Home team in the UK.
