🛋️ Cushion Insert Size Calculator
Enter your finished cover size, pick how plump you want it, and get the insert size to buy — sized larger than the cover so your cushions look full instead of saggy.
🧮 Size Your Insert
🛋️ Recommended insert size
| Finished cover | 18″ × 18″ |
| Oversize added | +1.5″ each dimension |
| Recommended insert | 19.5″ × 19.5″ |
| Nearest standard square | 20″ insert |
Common off-the-shelf insert sizes: 16″, 18″, 20″, 22″, and 24″ squares, plus 12×20″ lumbar. If your figure lands between two, size up for a fuller look.
Plump pillows start with the right insert
The single most common cushion mistake is buying an insert the same size as the cover. The result looks deflated within days — soft corners, loose fabric, and that sad, half-empty slump. Designers deliberately over-stuff, choosing a form an inch or two larger so the cover is filled to the seams.
This calculator does that maths for you: enter the finished cover, choose your plumpness, and it returns the insert dimensions plus the nearest standard square size, so you can shop with confidence.
❓ Frequently Asked Questions
Why should the insert be bigger than the cover?
An insert that exactly matches the cover looks limp, with saggy, empty corners. Sizing the form 1–2 inches larger on each dimension packs the cover full so the corners fill out, the fabric stays taut, and the pillow holds a crisp shape — including that tidy 'karate chop' dent if you want one. This calculator adds +1″ for a light fill, +1.5″ for medium, and +2″ for a firm, plump look.
What are the common standard insert sizes?
Off-the-shelf square inserts come in 16, 18, 20, 22, and 24 inches, and the usual lumbar form is 12×20 inches. If your recommended size lands between two standards, size up rather than down — a slightly oversized insert always looks fuller and more expensive than an underfilled one.
Does this work for lumbar and rectangular cushions?
Yes. Pick the lumbar or rectangular shape and enter both the finished width and height; the calculator adds your chosen plumpness allowance to each dimension. For those shapes it won't suggest a 'nearest standard square', since lumbar and rectangular forms are usually bought to size or made to order.
How firm should I go?
For decorative throw pillows you actually lean on, medium (+1.5″) is the sweet spot — full but still soft. Choose firm (+2″) for a designer, showroom look where the pillows stand up on their own, and light (+1″) if you like a softer, more relaxed, lived-in feel.