Raised Bed Soil Calculator

Quick answer: length × width × depth, all in feet, gives the cubic feet to fill — an 8 × 4 ft bed filled 12 inches deep takes 32 cubic feet, or about 1.2 cubic yards. What goes in it is where the sources part ways: Maryland Extension fills a bed on the ground with compost and topsoil (1 : 2 or 1 : 1) and a bed on concrete with compost and a soilless mix (1 : 1); the University of Vermont wants 70 percent soil and 30 percent compost; the Square Foot Gardening Foundation’s Mel’s Mix is a third each of compost, peat and vermiculite.

How to use it

Measure the inside of the bed — or the top of the pot — and pick how deep you will fill it. If you are building several beds the same size, say how many. Then choose a mix. The answer is the total volume, then each ingredient on its own line in cubic feet and bags of the size you are buying, so you can walk into the garden centre with a list. Metric works too: type 2.4 m or switch the toggle.

Three mixes, and why they disagree

They were written for different beds. Maryland Extension assumes a bed on ground you have loosened, where roots carry on down into the soil beneath, so it tops the bed with compost and purchased topsoil. For a bed on a patio or a drive, with nothing underneath, it switches to compost and a soilless growing mix, half and half, and only lets topsoil in — up to a fifth — once the bed is at least 16 inches deep.

The University of Vermont goes the other way: 70 percent mineral soil and 30 percent compost, because too much compost “will form a thick non-porous layer that inhibits drainage”. Rutgers makes the same point from the other side — mineral soil keeps its volume after it settles; organic matter breaks down and shrinks.

Mel’s Mix is the square-foot gardening recipe: a third blended compost, a third peat moss or coir, a third coarse vermiculite, in shallow beds kept topped up with compost at every harvest. The foundation’s own example is the one to check this calculator against: “a 4’x4’x6″ deep raised bed would need 8 cubic feet”.

How deep a bed needs to be

Maryland Extension: on a hard surface, at least 8 inches for leafy greens, beans and cucumbers, and 12 to 24 inches for peppers, tomatoes and squash. On loosened ground a bed can be shallower, because the roots do not stop at the bottom of the frame. Deeper beds take a lot more fill — every extra 6 inches on an 8 × 4 bed is another 16 cubic feet — which is why people ask what they can put in the bottom.

Logs in the bottom (hügelkultur)

We did not find an extension service that publishes a logs-in-the-bottom method for a vegetable bed, so this tool does not give one. What the sources we used do say applies directly: organic material breaks down and shrinks, so a bed filled with a lot of it sinks, and wood is the extreme case. If you fill the bottom with logs or branches anyway, measure the soil layer that goes on top and put that depth into the calculator — then expect to top the bed up as the wood settles.

Buying it

For one bed, bags are easier. Past a cubic yard or so, a delivery of a blended bed mix is usually cheaper and far less lifting — the topsoil calculator gives the tonnage range a supplier who sells by weight will quote. Ask what is in any ready-made mix: Rutgers notes the labels often list “arbor fines”, bark, peat, coir and composts, which give structure but little nutrition without fertilizer.