Raised bed soil mix.

Worked example

A 4 ft by 8 ft bed filled 12 in deep holds 4 × 8 × 1 = 32 cubic feet. With 10 percent settling you buy 35.2 cubic feet, 1.30 cubic yards. In Mel’s Mix that is 11.7 cubic feet each of compost, peat moss or coco coir, and coarse vermiculite, which is 12 bags each at 1 cubic foot a bag (3 each if bags hold 4 cubic feet). As 2 parts topsoil to 1 part compost it is 23.5 cubic feet of topsoil and 11.7 of compost.

How it works

Bed volume = length × width × depth (depth in feet). Order volume = bed volume × (1 + settling%). Each ingredient is its share of the parts times the order volume; bags are rounded up. 27 cubic feet make a cubic yard, and 1 cubic foot is 28.317 litres.

What this is not. A mix recipe is a starting point, not a soil test. Mel’s Mix is the Square Foot Gardening blend of equal parts blended compost, peat moss (or coco coir) and coarse vermiculite, and the topsoil and compost blend is a common budget alternative. Peat moss is harvested from bogs, so many gardeners choose coir. Buy compost from a reputable supplier and check that topsoil is free of contaminants before growing food in it. Bulk soil sold by the yard is often cheaper than bags above a few cubic feet.

Sources and basis (checked 2026-10-06): Mel Bartholomew, Square Foot Gardening (the equal-parts compost, peat moss and vermiculite mix); the 2 to 1 topsoil and compost blend is a common rule of thumb, not a published standard. Volume conversions are exact unit definitions.

FAQ

What is Mel's Mix?

A raised bed soil recipe from Mel Bartholomew's Square Foot Gardening: equal parts by volume of blended compost, peat moss or coco coir, and coarse vermiculite. It is light and holds water, and costs more than topsoil blends.

How much soil do I need for a 4 by 8 raised bed?

For a 12 inch depth it is 32 cubic feet, about 1.2 cubic yards, plus 10 percent for settling, so 35 cubic feet.

How deep should a raised bed be?

Six to 12 inches is enough for most vegetables if the ground beneath is loose. Root crops and tomatoes like the deeper end. Use the depth you plan to build.

Should I buy bagged soil or bulk?

Bags are fine for a few cubic feet. Above a cubic yard or two, bulk delivered soil is usually cheaper. The calculator shows both bags and cubic yards.