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Raised Bed Soil Calculator

Find the cubic feet and bag count of potting mix needed to fill a raised garden bed to a given fill depth.

Raised bed soil volume
Enter the interior dimensions of the bed and the fill depth -- not the wall height, but how deep you want the soil.
Bed dimensions
ft

Interior length of the raised bed. Measure inside the boards.

ft

Interior width of the raised bed.

in

How deep you want to fill the bed with soil mix. This is the fill depth, not the wall height. A standard 4x8 raised bed is often filled 10 to 12 inches deep.

pcs

Number of identical beds to fill.

Bag size

Volume per bag. Check the label on the potting mix. Common sizes are 1.5, 2, and 3 cubic feet.

Volume

1.19 cu yd

0.91 m³

Bags needed: 22 pcsTotal volume: 32 cu ft

What does the raised bed soil calculator estimate?

It finds the cubic feet of potting mix needed to fill a raised bed to a chosen depth and converts that to bag count.

A raised bed calculator converts the interior dimensions of the bed frame and the intended fill depth into a volume of growing medium. That volume, divided by the bag size and rounded up, gives the number of bags to purchase.

Fill depth is the key variable: it is how deep you want the soil, not how tall the frame is. A 12-inch-tall frame filled 10 inches deep needs only 10-inch worth of volume. Many gardeners leave the top 2 inches unfilled to allow watering without overflow.

The standard potting mix for raised beds is a blend of compost, peat or coir, and perlite or vermiculite. Native soil compacts in a raised frame and drains poorly; use a formulated mix designed for containers or raised beds.

How to calculate raised bed soil volume

The length-times-width-times-depth formula, unit handling, and how bag count rounds up.

Formula

volume (cu ft) = length x width x (fill depth in / 12) x bed count; bags = ceil(volume / bag size)

How the formula works

Volume (cu ft) = bed length (ft) x bed width (ft) x (fill depth (in) / 12) x bed count. Fill depth is divided by 12 to convert from inches to feet so the product of three linear feet gives cubic feet. Bags = ceil(volume cu ft / bag size cu ft).

Bed count multiplies the single-bed volume for users filling multiple identical beds at once.

Measure interior dimensions

Length and width are the interior measurements of the frame, not the outside. Lumber has thickness; a 2x12 board adds about 1.5 inches to each exterior wall. Measure the inner face of the boards.

Fill depth vs wall height

The calculation uses fill depth only. If you are refilling a partially full bed, enter only the depth of new material needed.

Metric

Bed length and width may be entered in meters; fill depth in centimeters. The calculator converts to feet and inches before applying the formula.

Example

One 4 x 8 ft bed filled 12 in deep, 1.5 cu ft bags:
  1. 1
    Volume in cubic feet4 x 8 x (12/12) x 1 = 32 cu ft
  2. 2
    Volume in cubic yards32 / 27 = 1.19 cu yd
  3. 3
    Bags (1.5 cu ft each)ceil(32 / 1.5) = ceil(21.33) = 22 bags
Result32 cu ft / 22 bags

FAQ

What is the difference between fill depth and wall height?

Wall height is how tall the raised bed frame is. Fill depth is how much soil you actually put in. You may not fill the bed all the way to the top, or you may have started with some existing soil in the bed. Enter only the depth you need to add.

What kind of soil should I use?

A typical raised-bed mix is one part compost, one part peat or coconut coir, and one part coarse perlite or vermiculite. Many gardeners use the Mel's Mix ratio popularized by Square Foot Gardening (1/3 each of compost, peat, and coarse vermiculite). Avoid using only native soil, which compacts and drains poorly in a raised frame.

How much will the mix settle?

New soil mix settles 10 to 20 percent in the first growing season as organic matter decomposes and particles compact. If you need a 12-inch final depth, fill to 13 or 14 inches. Add a top-dress of compost each year to replenish what settles.

Sources

High-trust standards and documented references.

  1. 1. Square Foot Gardening, 2nd Edition

    Square Foot Gardening Foundation

    Introduced the Mel's Mix formula (equal parts compost, peat, and coarse vermiculite) for raised-bed growing; basis for the volume calculation approach used here.

    https://www.squarefootgardening.org/

  2. 2. Raised-Bed Gardening

    Iowa State University Extension and Outreach

    Iowa State Extension covers soil selection, bed dimensions, and fill depth recommendations for raised garden beds.

    https://store.extension.iastate.edu/product/Raised-Bed-Gardening

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