🧵 Cushion Fill Calculator
Enter your cushion size, loft, and firmness to estimate how much polyester fiberfill to buy — in ounces and pounds — so you stuff it plump without running short.
🧮 Estimate Your Fiberfill
🧵 Fiberfill needed
| Cushion volume | 1620 in³ |
| Fill density used | 0.38 oz per 100 in³ |
| Density estimate (volume × density) | 6.16 oz (0.38 lb) |
| Bag-label check (20 oz per 18″ pillow, scaled by face area) | 20 oz |
| To buy (the larger) | 20 oz (1.25 lb) |
Buy enough fill the first time
Running out of fiberfill halfway through a batch of pillows means a second trip and a mismatched bag. This calculator turns your cushion's finished dimensions into a defensible weight estimate using a stated density, so you know roughly how many bags to grab.
Because fiberfill compresses, treat the number as a starting point — a firmer pillow simply packs more of the same fill into the same space. Keep any leftover for topping up corners as pillows settle over time.
❓ Frequently Asked Questions
How does the calculator work out the fill weight?
It multiplies length × width × thickness to get the cushion volume in cubic inches, then multiplies by a fiberfill density in ounces per 100 cubic inches, and divides by 16 for pounds. It then checks that against a fiberfill bag label (20 oz for one 18-inch pillow, scaled by face area) and recommends the larger. The volume is the finished, stuffed size you want — not the flat, empty cover.
What fiberfill density do you use?
Polyester fiberfill is compressible, so the density depends on how firmly you pack it. This tool uses 0.32 oz per 100 in³ for a soft fill, 0.38 oz per 100 in³ for medium, and 0.45 oz per 100 in³ for firm. These sit inside the range craft suppliers commonly quote (about 0.32–0.45 oz per 100 in³). Buy a little extra so you can top up the corners.
Ounces or pounds — which should I buy by?
Bagged fiberfill is sold by weight in ounces or pounds (common bags hold 12, 16, 20 or 32 oz). The calculator gives both, so match whichever the label uses. Our density estimate runs low against makers' packaging: one widely sold 20 oz bag is described as enough for a single 18-inch pillow, while the density formula says about 6 oz for an 18 x 18 x 5 inch pillow. So the tool also scales that label figure by your pillow's face area and tells you to buy the larger. Round up to whole bags.
Why did my finished pillow come out softer or firmer than expected?
Firmness is really about how much fill you force into the same space. If you want it plumper, add more fill than the medium estimate; for a squishy, relaxed pillow, use less. The soft/medium/firm setting shifts the density to reflect exactly that, but hand-feel while stuffing is the final judge.