How to use the raised garden bed soil calculator
- Measure the interior length and width of your raised bed in feet.
- Decide how deep you want the soil layer and note it in inches.
- Enter the three numbers into the fields above.
- Read the results in cubic feet, cubic yards, and liters to plan your soil purchase.
How the soil volume is calculated
The formula used by this raised garden bed soil calculator is straightforward: multiply the interior length by the interior width, then multiply by the depth converted from inches to feet.
Volume (cubic feet) = Length (ft) x Width (ft) x Depth (in) / 12
For example, a bed that is 8 feet long, 4 feet wide, and filled to a depth of 12 inches holds 8 x 4 x 12 / 12 = 32 cubic feet of soil. To convert that to cubic yards, divide by 27 because one cubic yard equals 27 cubic feet. So 32 cubic feet is about 1.185 cubic yards. One cubic foot also equals approximately 28.3168 liters.
Common raised bed sizes and soil needs
Gardeners tend to build beds in a few standard sizes. This table shows the soil volume for popular dimensions at two common depths, which helps when ordering bulk soil or counting bags.
| Bed size (feet) | Depth (inches) | Cubic feet | Cubic yards |
|---|---|---|---|
| 4 x 4 | 6 | 8 | 0.296 |
| 4 x 4 | 12 | 16 | 0.593 |
| 4 x 8 | 6 | 16 | 0.593 |
| 4 x 8 | 12 | 32 | 1.185 |
| 3 x 6 | 10 | 15 | 0.556 |
| 2 x 8 | 12 | 16 | 0.593 |
Why measure the interior dimensions
Lumber has thickness. A bed built from 2-inch thick boards that measures 4 feet on the outside is only about 3 feet 8 inches on the inside. If you use exterior measurements in any garden bed fill calculator, you will overestimate the soil needed and spend more than necessary.
Always measure from the inside face of one board to the inside face of the opposite board. For pre-built kits, check the manufacturer specifications for interior dimensions rather than relying on the advertised nominal size.
Choosing the right soil depth
Different plants need different amounts of root space. Shallow-rooted crops like lettuce, spinach, and most herbs grow well in 6 to 8 inches of soil. Tomatoes, peppers, carrots, and potatoes perform best with 12 to 18 inches of depth.
If your bed sits directly on native ground, you can sometimes get away with less imported soil because roots will penetrate the earth below. However, if the bed sits on a patio, driveway, or has a weed barrier at the bottom, all root growth must happen within the fill you provide. Plan your depth accordingly before running the numbers through this raised garden bed soil calculator.
Buying soil: bags versus bulk delivery
Retail potting soil typically comes in bags sized by quarts or cubic feet. One cubic foot equals roughly 25.7 dry quarts. A standard 1.5 cubic foot bag covers a small area, but filling a large bed with bags becomes expensive quickly.
As a rule of thumb, if you need more than one cubic yard (27 cubic feet), ordering bulk topsoil or compost by the truckload is significantly cheaper per unit. Bulk suppliers sell by the cubic yard. Use the cubic yard output from this tool to tell the supplier exactly what you need. Always order about 5 to 10 percent extra to account for settling after the first few waterings.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| Standard 4x8 bed, 12 inches deep | Interior length: 8; Interior width: 4; Soil depth: 12 | Cubic feet: 32; Cubic yards: 1.185; Liters: 906.139; Bags needed (1.5 cu ft): 22 |
| Small square bed, 6 inches deep | Interior length: 3; Interior width: 3; Soil depth: 6 | Cubic feet: 4.5; Cubic yards: 0.167; Liters: 127.426; Bags needed (1.5 cu ft): 3 |
| Deep bed for root vegetables | Interior length: 6; Interior width: 3; Soil depth: 18 | Cubic feet: 27; Cubic yards: 1; Liters: 764.555; Bags needed (1.5 cu ft): 18 |
| Long narrow herb bed | Interior length: 10; Interior width: 2; Soil depth: 8 | Cubic feet: 13.333; Cubic yards: 0.494; Liters: 377.558; Bags needed (1.5 cu ft): 9 |
FAQ
How many bags of soil do I need for a 4x8 raised bed?
A 4x8 bed filled to 12 inches deep requires 32 cubic feet of soil. If your bags are 1.5 cubic feet each, you will need about 22 bags. If they are 2 cubic feet each, you need 16 bags.
Should I fill the entire raised bed with expensive potting mix?
Not necessarily. Many gardeners use the hugelkultur method, placing logs, branches, and leaves in the bottom third of the bed to save on soil costs. Only the top 8 to 12 inches need high quality growing medium.
Does soil settle after I fill the bed?
Yes. Freshly added soil settles by 10 to 20 percent after watering and rain. It is wise to slightly overfill the bed or keep a few extra bags on hand to top it off after the first season.
What is the difference between cubic feet and cubic yards?
One cubic yard equals exactly 27 cubic feet. Bulk soil suppliers usually quote prices per cubic yard, while bagged soil is sold by the cubic foot or quart.
Can I use this raised garden bed soil calculator for irregular shapes?
This tool calculates volume for rectangular beds. For L-shaped or circular beds, break the shape into rectangles, calculate each section separately, and add the totals together.
Is my data saved or sent anywhere?
No. All calculations run locally in your browser. Your dimensions are never uploaded to a server.