How this calculator works
The cover of a bean bag is close to a box, so the calculator takes length times width times height to get the volume of that box. A filled bean bag does not reach the corners, so a fullness factor reduces the volume to what the filler actually occupies; the default of 80 percent gives a firm bag and can be lowered for a slouchier one. Multiplying the filled volume by the bulk density of the beads gives the weight, because bulk density already accounts for the air gaps between beads. Dividing by the weight in one bag of filler shows how many bags to buy.
Formula
filled volume = length x width x height x fullness. mass = filled volume x bulk density. bags = mass / weight per bag, rounded up.
Worked example
A cover measuring 100 cm by 80 cm by 70 cm holds 0.56 m3 of space. At 80 percent fullness the filler occupies 0.448 m3, which at 15 kg/m3 weighs 6.72 kg. Bags of 1.5 kg give 4.48 bags, so buy 5 bags.
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Frequently asked questions
Why does 1.5 kg of beads look like such a large bag?
Expanded polystyrene is mostly air. Its bulk density is only about 10 to 20 kg/m3, roughly one fiftieth the density of water, so a small weight fills a large volume.
How full should a bean bag be?
Adult bean bags are usually filled to roughly 70 to 90 percent of the cover volume. Less filling gives a softer bag that sinks, more gives a firm seat that holds its shape.
Can I top up an existing bean bag?
Yes. Measure the cover, choose a fullness, and subtract the filler already inside if you know its weight. Otherwise buy about half the calculated amount and add until the bag sits as you like it.
Does the shape of the cover matter?
A box estimate is close for a filled cube or a squarish lounger, and it overestimates a rounded or pear-shaped cover. For those shapes reduce the fullness factor until the volume matches how the bag sits.