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Raised Bed Soil Calculator
Use this free raised bed soil calculator to work out how much soil a garden bed needs. Pick a rectangular or circular bed, enter the size and the soil depth, and it gives the volume in cubic feet and cubic yards, how many 1.5-cubic-foot bags to buy, and the topsoil, compost, and perlite split.
How it works
Soil volume is area times depth. For a rectangular bed, multiply the length by the width in feet, then multiply by the depth in inches divided by 12 to convert inches to feet. A circular bed uses pi times the radius squared for the area instead. The calculator shows the result in cubic feet, converts it to cubic yards by dividing by 27, and works out how many standard 1.5-cubic-foot bags that volume needs, rounding up so you never come up short.
The soil mix is 60 percent topsoil, 30 percent compost, and 10 percent perlite. Topsoil gives the bed its body and holds nutrients, compost feeds the soil life and holds moisture, and perlite keeps the mix loose so roots get air and water drains instead of pooling. For a 32-cubic-foot bed that means 19.2 cubic feet of topsoil, 9.6 of compost, and 3.2 of perlite — easy to scale to any bed size.
Depth depends on what you grow. Most vegetables do well in 12 inches of soil, while root crops such as carrots and potatoes prefer 18 inches, and shallow herbs and lettuce manage in 8 inches. If the bed sits on open ground, roots can reach into the soil below, so 12 inches is usually enough; a bed on concrete or a rooftop needs the full depth inside the frame.
Bags suit small beds, but bulk soil by the cubic yard is much cheaper once you pass about a yard — roughly 27 bags' worth. A cubic yard of delivered soil typically costs less than half the bagged price, though delivery fees apply. Soil settles after the first few waterings, so fill the bed to about an inch below the rim and top it up with compost after a week or two.
Worked example
A gardener in Oregon builds a rectangular raised bed 8 feet long, 4 feet wide, and fills it 12 inches deep.
Step 1: Area — 8 × 4 = 32 sq ft.
Step 2: Volume — 32 × (12 ÷ 12) = 32 × 1 = 32 cu ft.
Step 3: Cubic yards — 32 ÷ 27 = 1.19 cu yd.
Step 4: Bags — 32 ÷ 1.5 = 21.33, rounded up to 22 bags.
Step 5: Mix — topsoil 32 × 0.60 = 19.2 cu ft, compost 32 × 0.30 = 9.6 cu ft, perlite 32 × 0.10 = 3.2 cu ft.
The bed needs 32 cubic feet of soil (1.19 cubic yards) — 22 bags of 1.5 cubic feet, or just over a yard of bulk soil — made of 19.2 cubic feet of topsoil, 9.6 of compost, and 3.2 of perlite.
FAQ
How deep should soil be in a raised bed?
Twelve inches suits most vegetables, 18 inches is better for root crops such as carrots and potatoes, and 8 inches is enough for herbs and lettuce. Beds on open ground can be shallower because roots reach into the soil below.
What is the best soil mix for a raised bed?
A common mix is 60 percent topsoil, 30 percent compost, and 10 percent perlite. The topsoil gives body, the compost feeds plants and holds moisture, and the perlite keeps the mix loose and draining well.
How many bags of soil do I need?
Divide the cubic feet by the bag size and round up. A 32-cubic-foot bed needs 22 bags of 1.5 cubic feet. For larger beds, bulk soil by the cubic yard is much cheaper than bags.
How many cubic feet are in a cubic yard?
One cubic yard is 27 cubic feet — a cube 3 feet on each side. Divide cubic feet by 27 to get cubic yards when ordering bulk soil.
Why does my raised bed soil settle?
Fresh soil settles as air pockets collapse and organic matter breaks down, usually losing an inch or two after the first waterings. Fill the bed to about an inch below the rim and top it up with compost after a week or two.