How to use the raised bed soil calculator
- Choose the unit for bed lengths and widths. Soil depth is always entered in inches.
- Add one row per bed. Pick Rectangle or Round, enter the inside size, the soil depth and how many identical beds you have.
- Choose the soil mix: equal thirds, topsoil and compost, a single bagged mix, or your own parts. Keep the 10% extra for settling or change it.
- Read the cubic feet, cubic yards and the bags of each ingredient. Set the bag size under More options so the bag counts match the product you will buy.
Formula
Rectangular bed volume = length x width x soil depth in inches / 12
Round bed volume = 3.14159 x (diameter / 2)^2 x soil depth in inches / 12
Total volume = sum of every bed x its quantity
Volume with extra = total x (1 + extra percent / 100)
Ingredient volume = volume with extra x its parts / total parts
Bags = ingredient volume / bag size in cubic feet, rounded up for each ingredient
Cubic yards are cubic feet divided by 27. Cubic meters are cubic feet times 0.028316846592. Bags are rounded up for each ingredient separately, so the total can be a bag or two higher than the volume alone suggests.
Worked example: two vegetable beds and a round herb bed
Two beds measuring 8 ft by 4 ft with 12 inches of soil hold 8 x 4 x 1 = 32 cu ft each, so 64 cu ft. A round herb bed 4 ft across with 10 inches of soil holds 3.14159 x 2^2 x 10 / 12 = 10.47 cu ft.
The total is 74.47 cu ft. With 10% extra for settling it becomes 74.47 x 1.10 = 81.92 cu ft, or 3.03 cubic yards.
An equal thirds blend splits that into 27.31 cu ft each of compost, peat moss or coir, and vermiculite. With 2 cu ft bags each ingredient needs 13.65 bags, which rounds up to 14 bags, or 42 bags in all.
Choosing a soil mix
Raised beds do best with a loose, well-draining, fertile blend, and many gardeners mix their own rather than filling a bed with plain topsoil. A common recipe uses equal volumes of compost, a moisture-holding material such as peat moss or coconut coir, and vermiculite for aeration. Another popular approach is half topsoil and half compost, which costs less and works well in deep beds.
This tool does not judge which recipe is best for your plants. It splits the volume by the parts you choose, and the custom option lets you set up to three ingredients with any ratio, such as two parts topsoil, one part compost and one part sand.
Bags or bulk
Bagged soil is convenient for one or two small beds, but the bag count climbs fast. A single 4 by 8 foot bed that is a foot deep holds 32 cubic feet, which is 16 bags of 2 cubic feet. Bulk soil, delivered by the cubic yard, is often a better value once you pass about a yard, and 1 cubic yard equals 27 cubic feet.
Ask the supplier how they measure a load and whether the material is screened. Bulk soil can arrive wet and heavy, and it may settle more than bagged mix.
Settling and topping up
Fresh soil and compost compact as they get wet and as the organic material breaks down. A bed filled to the brim in spring may sit a few inches lower by summer. The extra allowance helps you buy enough to fill the bed fully the first time, and you can add compost to top it up each season.
Assumptions and limits
- Each bed is a perfect rectangle or circle with straight sides and a flat soil surface. Sloped sides and mounded soil are not modeled.
- Bag size is an example of 2 cubic feet that you can edit. Compressed bags may expand after opening, and some brands list quarts instead of cubic feet.
- The mix ratio is by volume. Weights of wet and dry ingredients differ and are not used here.
- Cost uses a single price per bag for every ingredient. If your ingredients cost different amounts, price each one on its own.
- Drainage rock, landscape fabric, lumber for the frame and watering systems are not included.
Frequently asked questions
How much soil do I need for a raised garden bed?
Multiply the bed length by width in feet, then by the soil depth in feet. A 4 by 8 foot bed filled 12 inches deep needs 32 cubic feet, before any extra for settling.
How many cubic feet are in a bag of garden soil?
Common sizes are 1, 1.5 and 2 cubic feet, but sizes vary by brand. Read the volume on the bag and enter it as the bag size.
Can I fill the bottom of a deep raised bed with something cheaper?
Some gardeners fill the lower part of a very deep bed with logs, branches or coarse material to save on soil. Set the soil depth to the depth you plan to fill with soil mix, and the tool will size the order for that layer.
How do I calculate soil for a round raised bed?
Use the round option and enter the diameter. The area is pi times the radius squared, which is then multiplied by the soil depth.
Should I add extra soil to the order?
Soil settles after the first few waterings, so a margin of around 10 percent is a reasonable starting point. You can set any percentage, including zero.