Soil Volume Calculator

How to measure for soil volume

Soil is sold by volume, not weight, so the only numbers that matter are the inside dimensions of the space you are filling. Measure the inside length and width of a bed or container — the outside dimensions include the timber or pot wall and will overstate the volume. For a round pot, measure the diameter across the top opening and the depth from the rim to the base. If the pot tapers (most do), the top diameter is a reasonable approximation; the result will be slightly generous, which is better than running short. Depth is the depth of soil you actually want, not the full height of the container — a pot with a drainage layer or a bed you only fill to an inch below the rim needs less than its total height suggests.

How deep should the soil be?

Depth matters more than most gardeners expect. Shallow-rooted crops such as lettuce, radishes and most herbs do well in 6 to 8 inches of soil. Most vegetables — tomatoes, peppers, beans, squash — need 10 to 12 inches at minimum, and 12 to 18 inches is better for strong root growth. Root crops such as carrots and parsnips need the full depth of their mature root plus a few inches to spare, so a 12-inch bed suits short varieties and deeper beds suit long ones. For houseplants, the depth is set by the pot, and the rule is simpler: the plant should sit with its root ball about an inch below the rim, with fresh mix packed around it. If you are filling a new raised bed, remember that the soil level will drop by roughly 10 to 20 percent in the first season as it settles and as organic matter breaks down, so topping up in autumn is normal, not a sign that you bought too little.

Bags or bulk — which is cheaper?

Bagged soil makes sense for pots, small beds and top-ups: it is easy to carry, consistent in quality and available in mixes blended for specific jobs. Once a project passes roughly one cubic yard — about 27 cubic feet, or the contents of a standard 4 by 8 foot bed filled 10 inches deep — bulk delivery usually costs far less per cubic foot. Bulk soil is sold by the cubic yard, so this calculator’s cubic-yard figure is the number to quote to the supplier. Order a little extra, because a yard delivered and dumped is hard to top up in small amounts, and leftover soil is easy to store under a tarp. Check what is actually in the mix before you buy: a good raised-bed blend is mostly topsoil and compost, while cheap “fill dirt” can be heavy clay with almost no organic matter. If you are unsure, ask the supplier for the ingredients and, for food beds, whether the compost feedstock is suitable for vegetables.

Common mistakes to avoid

The most common mistake is measuring in mixed units — feet for length and inches for depth — and forgetting to convert. This calculator handles the conversion if you keep every measurement in the same unit you selected. Another is forgetting that a bed already part-filled with branches, leaves or old compost (the hugelkultur or lasagne approach) needs less purchased soil; measure only the depth you still need to fill. Finally, do not fill a container with garden soil straight from the ground: it compacts, drains poorly in pots and can carry weed seeds and soil-borne problems. Use a potting mix for containers and a raised-bed blend for beds. If you want to work out the mix itself rather than just the volume, try the raised-bed soil calculator or the potting-mix calculator, and use the mulch calculator to cover the finished bed. For planting dates once the bed is filled, see the planting calendar and the full list of garden tools.