How to measure a raised bed
- Measure the inside length and width of the frame, or the inside diameter of a round bed.
- Decide how deep the soil will be. Leaving an inch or two below the rim keeps soil and mulch from washing over the edge.
- Group identical beds in one row and set the count, then add rows for beds of other sizes.
How the soil is calculated
Bed volume (cu ft) = inside area (sq ft) × soil depth (in) ÷ 12 × number of beds
Total soil = sum of all beds × (1 + settling allowance)
Each ingredient = total soil × its share
Bags = ingredient volume ÷ bag size, rounded up
Worked example
Two 4 by 8 foot beds and one round bed 3 feet across, all filled 12 inches deep, hold 32 + 32 + 7.1 = 71.1 cubic feet of soil, or 2.63 cubic yards. With Mel's Mix, each of the three ingredients is a third of that: 23.7 cubic feet, or 16 bags of 1.5 cubic feet each.
Choosing a raised bed soil mix
Published recipes differ. The presets in the calculator come from these sources:
- Mel's Mix from Square Foot Gardening: equal volumes (not weights) of blended compost, sphagnum peat moss or coconut coir, and coarse vermiculite.
- Topsoil and compost, about 70/30, as Penn State Extension and Iowa State University Extension suggest.
- Topsoil and compost, 1:1, as University of Illinois Extension suggests. The University of Maryland Extension gives 1:2 or 1:1 compost to topsoil.
Whichever you choose, plan to top the bed up with compost each season, because organic matter breaks down and the soil level drops.
Frequently asked questions
How much soil do I need for a 4 by 8 raised bed?
A 4 by 8 foot bed filled 12 inches deep holds 32 cubic feet of soil, which is 1.19 cubic yards. In 1.5 cubic foot bags that is 22 bags. At 6 inches deep it takes half as much.
What soil mix should I use in a raised bed?
There is no single right answer. Square Foot Gardening's Mel's Mix is equal volumes of blended compost, peat moss or coconut coir, and coarse vermiculite. Penn State Extension suggests about 70% soil to 30% compost, and University of Illinois Extension a 1:1 mix of compost and topsoil. Pick a preset or enter your own shares.
How many bags of soil make a cubic yard?
A cubic yard is 27 cubic feet: 27 bags of 1 cubic foot, 18 bags of 1.5 cubic feet or 13.5 bags of 2 cubic feet. Once a project needs more than a yard or two, compare the price of bulk delivery.
Why does the soil in my raised bed sink?
Compost keeps breaking down after you fill the bed, so the level drops over time. University of Illinois Extension notes that more soil and compost may need to be added each year. Use the settling allowance if you want extra on hand.
Should I measure the inside or outside of the bed?
Measure the inside. Board thickness takes up real space: a 4 by 8 foot frame built from 1 1/2 inch lumber is only 3 3/4 by 7 3/4 feet inside, about 9% less soil.
Do compressed bales match the volume on the bag?
No. Peat moss and coconut coir are often sold compressed and expand when loosened. Use the loose or expanded volume printed on the label as the bag size for that ingredient.