How much soil a raised bed needs
For a rectangular raised bed, the volume of soil you need comes down to three numbers: length, width, and depth. Multiply length in feet by width in feet, then by depth converted from inches to feet (depth in inches divided by 12), and the result is cubic feet. If you're filling several beds of the same size, multiply that per-bed figure by the number of beds to get your total.
Cubic feet = Length ft × Width ft × (Depth in ÷ 12) × Number of beds
Cubic yards = Cubic feet ÷ 27
Take the calculator's defaults as a worked example: an 8 ft by 4 ft bed filled 10 inches deep. That's 8 × 4 × (10 ÷ 12), or about 26.7 cubic feet for one bed.
26.7 cubic feet ÷ 27 ≈ 0.99 cubic yards
26.7 ÷ 1.5 ≈ 18 bags (1.5 cu ft each)
26.7 ÷ 2 ≈ 14 bags (2 cu ft each)
Two beds that size would simply double every figure, to about 53.3 cubic feet and 1.98 cubic yards. Most raised beds are filled 8 to 12 inches deep, which suits the root systems of nearly all vegetables and flowers — shallower beds work for herbs and lettuces but limit deeper-rooted crops like tomatoes and carrots.
Whichever bag size you buy, it's worth rounding the total up rather than down. Fresh soil settles as it waters in and the organic matter breaks down, often by an inch or more over the first season, so a little extra on hand means topping off the bed later instead of making a second trip for one more bag.
What to fill a raised bed with
A raised bed rarely does best filled with plain dirt straight from the ground or a single bag of topsoil. Because the bed is a contained space rather than open ground, blending in compost and a lighter, airy material generally gives roots better drainage and more nutrients than soil alone, while still holding together well enough not to wash out.
This calculator offers three common starting points:
- Simple mix (60/30/10): 60% topsoil, 30% compost, 10% aeration material such as perlite or coarse sand — a balanced, budget-friendly blend that suits most vegetable and flower beds.
- Mel's Mix: the square foot gardening blend — equal thirds compost, peat moss or coco coir, and vermiculite, with no native soil at all. It stays loose and well-drained, though the compost and vermiculite can cost more than topsoil.
- Just soil (100%): the cheapest, simplest option, workable for larger beds, shrubs, or trees, but it tends to compact and drain more slowly than a blended mix — which matters more for vegetables than for ornamentals.
Treat these ratios as reasonable starting points rather than fixed rules. Local soil, what your garden center stocks, and the results of a soil test are all good reasons to shift the proportions.
Save money by buying in bulk
Soil, compost, and aeration material are all sold both bagged and in bulk by the cubic yard, and which one costs less depends heavily on your total volume and local pricing. As a general rule of thumb, bagged material tends to work out more expensive per cubic foot, but it's convenient for a single small bed and doesn't require anywhere to dump a pile. Once a project needs more than roughly a cubic yard — a couple of standard beds, or one large one — bulk delivery is often the cheaper route per cubic foot, and many garden centers or landscape suppliers will deliver a custom soil-compost blend already mixed.
That said, prices vary a lot by region and supplier, and small bulk minimums or delivery fees can erase the savings on a modest project. It's worth pricing both bagged and bulk options for your specific total before ordering, especially if you're filling multiple beds at once.
A quick way to compare: take each option's total price and divide it by the cubic feet it delivers, then compare the two per-cubic-foot numbers directly. If you're also buying separate compost or aeration material for one of the mixes above, price those the same way — the cheaper option can flip once several materials are involved instead of just plain soil.