How Much Soil Do I Need for a Raised Garden Bed?

Filling a raised bed always seems to take more soil than people expect. Here’s how to get the number right the first time so you don’t make three trips to the garden center.

The basic formula

Soil volume is just length × width × depth, with every measurement in the same unit. Working in feet gives you cubic feet:

Volume (cu ft) = length (ft) × width (ft) × depth (ft)

For example, a classic 4 ft × 8 ft bed filled 6 inches deep (0.5 ft):

4 × 8 × 0.5 = 16 cubic feet

If you’d rather think in cubic yards (how bulk soil is often sold), divide by 27: 16 ÷ 27 ≈ 0.6 cubic yards.

Turning volume into bags

Bagged soil is usually sold in 1, 1.5, or 2 cubic-foot bags. Divide your volume by the bag size:

• 16 cu ft ÷ 1.5 cu ft per bag ≈ 11 bags
• 16 cu ft ÷ 2 cu ft per bag = 8 bags

Always round up, and add about 5-10% extra, because soil settles after watering and the first season.

What mix should you use?

A popular, well-balanced recipe is Mel’s Mix: one-third compost, one-third coconut coir (or peat), and one-third coarse vermiculite, by volume. It’s light, drains well, and holds moisture. Other common approaches are a classic blend (about 60% topsoil, 30% compost, 10% perlite) or, on a budget, mostly quality compost. Whatever you choose, compost should be a meaningful share of the mix - it’s what feeds the bed.

Don’t forget settling and shape

Soil compacts over the first few weeks, so a bed filled exactly to the brim will end up an inch or two low. And if your bed isn’t a simple rectangle - L-shaped, or a tapered planter - the math gets fiddlier. That’s the tedious part our Raised Bed Soil Calculator handles: enter your dimensions, pick a mix, and it gives you a bag-by-bag shopping list in the bag sizes your store actually stocks, with a settling buffer built in.

Quick reference

For a 6-inch-deep bed: a 4×4 needs ~8 cu ft, a 4×8 needs ~16 cu ft, and a 4×12 needs ~24 cu ft. Double those for a 12-inch-deep bed.