Raised Garden Bed Soil Calculator
Raised garden beds create excellent growing conditions by providing deep, loose, well-draining soil above the native ground. Before you can plant, you need to fill the bed with a quality growing medium. This calculator determines the total soil volume required based on the internal dimensions of your bed, then breaks it down into the three components of the recommended blend: 60 percent topsoil, 30 percent compost, and 10 percent coarse sand or perlite. These ratios give a balanced mix with good drainage, moisture retention, and nutrient availability. For most vegetables, a bed depth of 12 inches is ideal. Shallow-rooted crops such as lettuce, herbs, and radishes can thrive in 6 inches, while deep-rooted vegetables like carrots, parsnips, and tomatoes prefer 18 inches or more. Results are shown in cubic feet and cubic yards. For small beds, bagged soil is convenient. For beds over 2 cubic yards, ordering bulk soil delivery is typically much more cost-effective. Confirm bag sizes with your supplier before buying.
Formula and recommended soil mix
Cubic feet = Length (ft) x Width (ft) x Depth (ft)
Cubic yards = Cubic feet / 27
Topsoil = Total cu ft x 0.60
Compost = Total cu ft x 0.30
Sand/Perlite = Total cu ft x 0.10
Bags = Ceiling(Total cu ft / Bag size)
Worked example
Bed: 8 ft x 4 ft x 12 in deep. Bag size: 1.5 cu ft.
- Cubic feet = 8 x 4 x 1 = 32 cu ft
- Cubic yards = 32 / 27 = 1.19 cu yd
- Topsoil = 32 x 0.60 = 19.20 cu ft
- Compost = 32 x 0.30 = 9.60 cu ft
- Sand = 32 x 0.10 = 3.20 cu ft
- Bags (1.5 cu ft each) = Ceiling(32 / 1.5) = 22 bags
Frequently asked questions
How much soil do I need for a raised garden bed?
Multiply the length (ft) by width (ft) by depth (ft) to get cubic feet of soil. Divide by 27 to convert to cubic yards. For example, a 4 ft x 8 ft bed that is 12 inches deep needs 4 x 8 x 1 = 32 cubic feet (about 1.19 cubic yards).
What is the best soil mix for a raised garden bed?
A widely recommended starting mix is 60 percent topsoil, 30 percent compost, and 10 percent coarse sand or perlite. The compost adds nutrients and improves drainage. Some gardeners use a variation with more compost (up to 50 percent) for intensive vegetable production.
How deep should a raised garden bed be?
A depth of 6 inches is the minimum for shallow-rooted crops like lettuce and herbs. Most vegetables need 12 inches. Root vegetables like carrots and parsnips benefit from 18 to 24 inches. A 12-inch depth is the most versatile choice for a general-purpose vegetable bed.
Can I fill a raised bed with just garden soil from a bag?
Bagged 'garden soil' alone is not ideal for raised beds. It can compact easily and lacks sufficient drainage for elevated planting. A blended mix (topsoil, compost, and aeration material like perlite or coarse sand) performs better for plant growth and water management.
How many bags of soil will I need?
Most bagged topsoil and compost come in 1 or 1.5 cubic foot bags. Divide the total cubic footage needed by the bag size to find the number of bags. For larger beds, ordering bulk soil by the cubic yard is usually more economical.
Sources
- USDA National Gardening Association: raised bed gardening guidance.
- University of Minnesota Extension: Build a Raised Bed Garden.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Soil mix ratios are general guidelines; adjust for your specific crops.