Raised Bed Soil Calculator

The volume math behind raised beds

Filling a raised bed is a simple volume problem: length × width × depth. Because depth is usually measured in inches, convert it to feet first (divide by 12), then multiply. A classic 4×8 foot bed at 12 inches deep holds 4 × 8 × 1 = 32 cubic feet.

Soil is sold two ways, and the calculator reports both. Bagged soil is sold by the cubic foot (commonly 1, 1.5 or 2 cu ft bags). Bulk soil from a landscape supplier is sold by the cubic yard — one cubic yard equals 27 cubic feet. Our 4×8×12" example needs 32 cu ft ÷ 27 ≈ 1.19 cubic yards, or about 16 two-cubic-foot bags. Bulk delivery is almost always cheaper per cubic foot once you need more than about a cubic yard, but bags make sense for a single small bed.

Why soil settles (and why you should overbuy)

Freshly filled beds always sink. Watering, rain and gravity compress the loose mix, and beds typically settle 10–15% in the first season. That 12-inch-deep bed will sit closer to 10–10.5 inches after a few months. This is normal — but it means buying exactly the calculated volume leaves you short.

The calculator's bag count already rounds up to whole bags, and adding one spare bag for top-ups later is cheap insurance.

The Mel's Mix recipe, explained

Popularized by Mel Bartholomew's Square Foot Gardening, Mel's Mix is equal parts by volume of three ingredients:

• ⅓ blended compost — the nutrition. Use several compost sources (mushroom compost, worm castings, homemade) rather than one, for a broader nutrient profile.
• ⅓ peat moss or coco coir — the moisture holder. Coir is the more sustainable choice and works identically.
• ⅓ coarse vermiculite — the aerator. It keeps the mix fluffy and holds water and nutrients in its layered structure.

The result is a mix that drains freely yet holds moisture, resists compaction, and needs no added fertilizer in its first season. The calculator breaks your total volume into these thirds so you can shop with a list. Note that vermiculite is sold by volume too — a 4 cu ft bag covers a lot of bed.

What not to fill your bed with

The most common (and expensive) mistake is filling a bed with pure native garden soil shoveled from the yard. In a contained bed it compacts into a dense brick, drains poorly, and often brings weed seeds, pests and soil-borne disease with it. Similarly, pure topsoil — especially cheap bagged "topsoil" — is inconsistent in quality and usually too heavy on its own.

The exception: if your native soil is genuinely good loam, blending up to about a third of it into a raised-bed mix works well and cuts cost. Just don't make it the whole fill.

Filling tips that save money

• Hugelkultur the bottom. For deep beds (12"+), fill the bottom third with logs, branches and leaves. They decompose slowly, and you buy far less soil.
• Line the bottom with cardboard or landscape fabric to suppress weeds while letting water drain.
• Water in layers. Fill a few inches, water thoroughly, repeat. This settles the mix as you go so you're not surprised later.
• Top up yearly. Add 1–2 inches of compost each spring — use the compost needed calculator to figure out how much.