A raised bed warms up early in spring, drains well and saves your back, but filling it takes more soil than most people expect. A standard 4 × 8 ft bed one foot deep holds almost 30 cubic feet, more than a cubic yard once settled. This calculator works out the soil, how to split it between topsoil and compost, and the lumber for the frame.
Soil volume
Volume = length × width × depth. In feet and inches:
cubic feet = length (ft) × width (ft) × depth (in) / 12
Divide by 27 for cubic yards, the unit used for bulk delivery. Bagged soil is usually sold in 1, 1.5 or 2 cubic foot bags. Add 10 to 20% if you fill to the brim, because the mix settles over the first season.
What to fill it with
| Mix | Proportions | Notes |
|---|---|---|
| Classic | 60% topsoil, 30% compost, 10% other | cheap, holds water, good for deep beds |
| Lighter | 50% topsoil, 50% compost | rich, ideal for vegetables |
| Soilless | 1/3 compost, 1/3 peat-free coir, 1/3 vermiculite | light, for shallow beds and tabletops |
For beds deeper than 18 in, you can fill the bottom third with logs, branches and leaves (the hugelkultur method) to save soil; they rot and feed the bed over the years, but the surface will sink.
Lumber
Boards are stood on edge in rows until they reach the soil depth. A 2 × 6 is actually 5.5 in wide, so two rows give an 11 in bed. The linear length is the perimeter × rows; this calculator assumes off-cuts are reused, which works when the bed sides divide evenly into the board length (4 and 8 ft sides with 8 ft boards).
Example
A 4 × 8 ft bed 11 in deep holds 29.3 cubic feet, about 1.1 cubic yards, or 20 bags of 1.5 cubic feet. It takes 6 boards of 8 ft for two rows of 2 × 6 and 4 corner posts.
Which wood?
Cedar and redwood resist rot naturally and last 10 years or more. Modern pressure-treated lumber (ACQ or copper azole) is considered safe for vegetable beds by most extension services; old CCA-treated wood is not. Avoid railroad ties. Galvanized steel and stone are long-lasting alternatives.
Buying bags or bulk
Bags are convenient for one or two beds. Beyond about a cubic yard, bulk delivery is usually much cheaper, and you can ask the supplier for a ready-made raised bed blend.