Soil & Compost

Raised Bed Soil Calculator

Enter your raised bed dimensions to see exactly how much soil to buy — in cubic feet, cubic yards, and number of bags — plus an optional Mel’s Mix breakdown.

How to use the raised bed soil calculator

Enter your bed’s length and width in feet and its depth in inches, then choose how many identical beds you are filling and the bag size you plan to buy. Press Calculate Soil Needed and you will instantly see the total volume in cubic feet and cubic yards, plus the exact number of bags to purchase. Tick the Mel’s Mix box to split that total into equal parts of peat, compost, and vermiculite.

The formula behind the numbers

Soil volume is a simple box calculation. Multiply length by width to get the surface area in square feet, then multiply by the depth converted to feet (inches divided by 12):

  • Cubic feet per bed = length × width × (depth ÷ 12)
  • Total cubic feet = cubic feet per bed × number of beds
  • Cubic yards = total cubic feet ÷ 27
  • Bags needed = total cubic feet ÷ bag size, rounded up

For example, two beds that are each 8 ft long, 4 ft wide, and 12 inches deep hold 32 cubic feet each, for a total of 64 cubic feet. That is about 2.37 cubic yards, or 43 bags of 1.5-cubic-foot soil once rounded up to whole bags.

What is Mel’s Mix?

Mel’s Mix is the soil blend recommended for square foot gardening. It combines three ingredients in equal parts by volume: peat moss (or coconut coir as a sustainable alternative), blended compost from several sources, and coarse vermiculite. The result is a lightweight, fertile medium that holds moisture without becoming waterlogged and rarely needs replacing. When you enable Mel’s Mix in the calculator, your total volume is divided into three equal parts and each part is rounded up to whole bags at your chosen bag size.

Bags vs. bulk delivery

Bagged soil is the easiest option for one or two beds: it is clean to handle, simple to carry, and available at any garden center. But the price per cubic foot adds up quickly. Once your project needs roughly a cubic yard or more — 27 cubic feet — ordering bulk soil from a landscape supplier is usually cheaper and far less work than opening dozens of bags. Because this calculator reports both the bag count and the cubic-yard figure, you can quickly compare the two options before you buy.

Tips for filling a raised bed

  • Measure the interior dimensions of the frame, not the outside, for an accurate volume.
  • Soil settles after watering, so fill to the top — it will drop an inch or two in the first few weeks.
  • Mixing in compost each season tops up nutrients and replaces the small amount of volume lost to settling and decomposition.
  • If the bed sits on open soil rather than a patio, roots can reach below the frame, so you do not always need to fill the full theoretical depth.
  • Buy a little extra; it is better to have a spare bag than to stop mid-project one bag short.

Frequently Asked Questions

A 4 ft by 8 ft bed that is 12 inches deep needs 32 cubic feet of soil (4 x 8 x 1). That is about 1.19 cubic yards, or roughly 22 bags if you buy 1.5-cubic-foot bags. Change the depth and the volume changes proportionally — the same bed at 6 inches deep needs half as much, 16 cubic feet.
Volume in cubic feet equals length times width times depth, with depth converted from inches to feet by dividing by 12. So a bed that is 8 ft long, 4 ft wide, and 12 inches deep is 8 x 4 x (12 / 12) = 32 cubic feet. Multiply by the number of identical beds for your total, and divide by 27 to convert cubic feet to cubic yards.
One cubic yard equals 27 cubic feet. So a cubic yard is about 36 bags of 0.75 cu ft, 27 bags of 1 cu ft, 18 bags of 1.5 cu ft, or 9 bags of 3 cu ft. This calculator rounds up to whole bags, because you cannot buy a partial bag.
Mel's Mix is the growing medium recommended in the book All New Square Foot Gardening. It is equal parts (by volume) of peat moss or coconut coir, blended compost, and coarse vermiculite. The blend is light, drains well, holds moisture, and is rich in nutrients. Tick the Mel's Mix box and the calculator splits your total volume into three equal parts and tells you how many bags of each to buy.
For one or two small beds, bagged soil is convenient and easy to handle. Once you need more than about a cubic yard (27 cubic feet), buying in bulk from a landscape supplier is usually cheaper per cubic foot and saves a lot of bag opening. The calculator shows both the bag count and the cubic-yard total so you can compare.
Six inches is the minimum for leafy greens and herbs, but 10 to 12 inches is better for most vegetables and gives roots room to grow, especially for carrots, tomatoes, and other deep-rooted crops. If your bed sits on solid ground rather than a hard surface, roots can grow deeper still, so you can fill a little less.

More Tools to Explore

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GardenCalc Editorial Team

Horticulture Writers & Master Gardeners

Our calculators and guides are written and fact-checked by gardeners with hands-on experience in vegetable production, soil management, and home landscaping.