Raised bed soil calculator
Filling a raised bed takes more soil than most people expect, and the bags at the garden center are small. Enter the bed dimensions and how many beds you have, pick a mix, and the calculator returns the total volume, bag count, and how much of each ingredient to buy.
How the raised bed soil calculator works
Volume is length × width × depth, multiplied by the number of beds. The calculator adds 10 percent because fresh soil settles as it is watered in. For the mix recipes it splits that total into the ingredients by volume, so you can buy each one separately and blend them in the bed. For a hugelkultur bed, the bottom half is filled with logs and branches that you have on hand, so only the top half needs purchased soil.
Soil depth is measured from the bottom of the bed to a couple of inches below the frame top, not the full frame height. If the bed sits on open ground, roots will go deeper than the frame, so 10 to 12 in of soil is plenty for most vegetables.
Formula
cubic feet = length × width × (depth ÷ 12) × beds × 1.1
bags = cubic feet ÷ bag size, rounded up
Example: one 8 × 4 ft bed filled 12 in deep is 32 cubic feet, or 35 cubic feet with the settling allowance. That is 24 bags of 1.5 cu ft, or 1.3 cubic yards in bulk. With the classic mix you would buy 21 cubic feet of topsoil, 11 of compost, and 4 of perlite or bark fines.
Choosing a mix
- Classic 60/30/10 is the economical choice for beds over about 20 cubic feet. Bulk screened topsoil, finished compost, and a little aeration material.
- Equal thirds is the Square Foot Gardening recipe. Light, fluffy, drains fast, and expensive at scale because of the vermiculite. Good for small beds and containers.
- Bagged raised bed mix is the no-mixing option. Fine for one or two small beds; costly beyond that.
- Hugelkultur fills the bottom with wood that slowly decays, holding water and cutting the soil bill in half. Expect the bed to sink a few inches the first year.
Topping beds with mulch after planting? The mulch calculator covers it. Buying bulk topsoil by the yard? See the topsoil calculator.
Common sizes, already worked out
Common questions
How much soil do I need for a 4x8 raised bed?
About 32 cubic feet for a 12 in depth, or 35 with a settling allowance. That is 24 bags of 1.5 cubic feet, or about 1.3 cubic yards in bulk. At 6 in deep it is half that.
How many bags of soil for a raised bed?
Divide the cubic feet by the bag size. A 4 × 4 ft bed 10 in deep holds 13.3 cubic feet, so 9 bags of 1.5 cu ft or 14 bags of 1 cu ft. Round up and add one for settling.
What is the best soil mix for raised beds?
A blend of about 60% quality topsoil, 30% compost, and 10% something airy like perlite, vermiculite, or pine bark fines works for nearly everything. Avoid straight potting mix (too light and dries fast) and straight topsoil (compacts).
How deep should a raised bed be?
10 to 12 in of soil suits most vegetables, herbs, and flowers. Root crops like carrots and parsnips like 12 to 18 in. Beds on concrete or with weed barrier underneath should be at least 18 in deep.