How it works
volume = length × width × depth × number of beds
The calculator converts every dimension to feet, multiplies length by width by depth to get the volume of one bed in cubic feet, then multiplies by how many identical beds you are filling. Cubic yards are that figure divided by 27 (there are 27 cubic feet in a cubic yard), and litres come from the exact factor of 28.3169 litres per cubic foot. The number of bags is the total volume divided by your chosen bag size, rounded up to whole bags. When you pick a blended mix, the split shows how the volume divides between components — for example Mel’s Mix is equal thirds of compost, peat moss and coarse vermiculite by volume, while a 60·30·10 blend is mostly topsoil with compost and an aeration material such as perlite. Buy loose soil by the cubic yard once you pass roughly a cubic yard, since bulk is usually far cheaper than bagged.
Sources
- Clemson Cooperative Extension — Raised Beds Soil needed for a raised bed is its total volume (length × width × height); compost should make up 10–20% of the bed volume.
- Square Foot Gardening Foundation — Mel’s Mix Mel’s Mix is equal parts, by volume, of blended compost, peat moss (or coco coir) and coarse vermiculite.
- Volume conversions (exact by definition) 1 cubic yard = 27 cubic feet; 1 cubic foot = 28.316846592 litres, from 1 ft = 0.3048 m.
FAQ
How much soil do I need for a 4x8 raised bed?
A 4 ft by 8 ft bed filled 12 inches deep holds 32 cubic feet of soil, which is about 1.19 cubic yards or 32 standard 1-cubic-foot bags. Halve the depth to 6 inches and you need half as much, 16 cubic feet. Enter your own dimensions above for an exact figure.
How deep should a raised bed be?
Most vegetables root happily in 10 to 12 inches of good soil, and that is the depth this calculator defaults to. Deep-rooted crops such as tomatoes, carrots and potatoes appreciate 18 inches or more, while shallow greens manage in 6 to 8 inches. If the bed sits on open ground rather than a hard surface, roots can also reach into the native soil below.
Is it cheaper to buy soil in bags or in bulk?
Bags are convenient for one small bed, but once you need roughly a cubic yard (27 cubic feet, or about 27 one-cubic-foot bags) bulk soil delivered by the cubic yard is usually much cheaper per unit. The calculator shows both the cubic-yard figure and the bag count so you can compare against local prices.
What is the best soil mix for a raised bed?
A common, forgiving recipe is roughly 60% quality topsoil or bagged raised-bed soil, 30% compost and 10% of an aeration material such as perlite or coarse sand. Square Foot Gardening instead uses Mel’s Mix — equal thirds of blended compost, peat moss and coarse vermiculite. Both are options in the mix selector above.
Should I add extra soil for settling?
Yes. Fresh soil, and especially mixes heavy in compost or peat, settle and compact after the first few waterings, so a bed filled to the brim will drop an inch or two. Buying about 5 to 10% more than the calculated volume covers settling and lets you top the bed off later in the season.
Can I reuse the soil next year?
Raised-bed soil is reusable for years. Each season the level drops as organic matter breaks down, so refresh it by mixing in an inch or two of compost rather than replacing everything. Use the compost calculator linked above to work out how much you need to top the bed back up.
Volumes are estimates. Loose soil settles and compacts after watering, so order a little extra and top beds off later. Soil needs vary with plants, climate and drainage. General gardening guidance, not professional advice.