Raised Bed Soil Calculator: Cubic Feet, Yards, Bags
How much soil do I need to fill a raised bed? Enter the inside length, width and depth of your bed, the number of beds and a little extra for settling. You get the volume in several units, the bags to buy and a mix split such as 60 percent topsoil, 30 percent compost and 10 percent other. After switching units, re-enter your measurements.
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Formula
volume (ft3) = length (ft) x width (ft) x depth (in) / 12 x number of beds x (1 + extra %)- Metric:
m3 = length (m) x width (m) x depth (cm) / 100 x beds x (1 + extra %) cubic yards = ft3 / 27;litres = ft3 x 28.316846592bags = round up (ft3 / bag size)for 1.5 and 2 cu ft bags- Blend:
litres x sharefor topsoil and compost; the rest is other amendments
Worked example
One bed 8 ft by 4 ft, filled 12 in deep, 0% extra, 60/30/10 mix:
- 8 x 4 x 12 / 12 = 32 ft3
- 32 / 27 = 1.19 yd3; 32 x 28.3168 = 906.14 L
- 32 / 1.5 = 21.33, so 22 bags; 32 / 2 = 16 bags
- Mix: 543.68 L topsoil, 271.84 L compost, 90.61 L other
With the default 10% extra the same bed needs 35.2 ft3, which is 24 bags of 1.5 ft3.
When to use it
Use it when you are building a new raised bed or topping one up and need the whole fill, not just a surface layer. A typical starting mix is about 60 percent topsoil, 30 percent compost and 10 percent something for drainage, but you can change the shares. Fresh soil settles, which is why a little extra is built in.
Common mistakes
- Using the outside dimensions of the bed instead of the inside.
- Filling the whole depth with soil when the bottom is lined with logs or branches. Enter only the soil depth you will buy.
How it differs from nearby calculators
The topsoil calculator and mulch calculator size a layer spread over a lawn or border, and the compost calculator sizes a thin top dressing. The cubic yard calculator handles any box-shaped volume. This page is for a bounded bed with a set depth and a soil blend.
Frequently asked questions
How much soil does an 8 by 4 foot raised bed need?
At 12 inches deep, 8 x 4 x 1 = 32 cubic feet, which is 1.19 cubic yards or about 906 litres. In bags of 1.5 cubic feet that is 22 bags, and in bags of 2 cubic feet it is 16 bags.
What is a good soil mix for raised beds?
A common starting point is about 60 percent topsoil, 30 percent compost and 10 percent something that improves drainage or structure, such as coarse sand, perlite or aged bark. It is a typical recipe, not a rule; change the shares to suit your plants and local soil.
Why add extra for settling?
Fresh soil and compost settle by roughly 10 to 20 percent over the first season as air pockets close, so most gardeners order a little extra. With 10 percent extra, the 32 cubic foot bed above needs 35.2 cubic feet.
Can I fill the bottom of a deep bed with other material?
Many people fill the lower part of a tall bed with logs, branches or leaves and put soil only in the top layer. If you do, enter only the depth of soil you will actually buy. Roots of most vegetables go down at least 8 to 12 inches, so keep that much good soil on top.
How many bags of soil for a 4x8 raised bed?
With the default 12 inch depth and 10 percent extra for settling, a 4x8 bed needs 35.2 cubic feet. That is 24 bags of 1.5 cubic feet or 18 bags of 2 cubic feet. Without the extra it is 32 cubic feet, or 22 and 16 bags.
How much soil for a 4x4 raised bed 6 inches deep?
4 x 4 x 6 / 12 = 8 cubic feet. With 10 percent extra it is 8.8 cubic feet, or 0.33 cubic yards, which is 6 bags of 1.5 cubic feet or 5 bags of 2 cubic feet.
How is this different from the topsoil and mulch calculators?
The topsoil and mulch calculators cover a shallow layer spread over an area, while this one fills a box to a set depth and splits the volume into a topsoil, compost and other blend. It also gives the bag counts for 1.5 and 2 cubic foot bags.