How much soil do I need for a raised bed

Soil volume for a raised bed comes down to a single formula: interior length times interior width times height, divided by 1728 to convert cubic inches to cubic feet. The word interior matters, since the boards and corner posts take up a small amount of space inside the outside dimensions most people measure and think in.

Why interior dimensions, not outside dimensions

A bed built from standard 2x6 boards, actual thickness 1.5 inches, loses 3 inches of interior space in both length and width once the walls are accounted for, since there is a board thickness on each side. A 48 by 24 inch outside footprint is actually 45 by 21 inches on the inside. Using the outside dimensions instead of the interior ones overstates the soil needed by a noticeable margin, especially on smaller beds where the wall thickness is a bigger share of the total size.

Worked example: a 4x2 bed

A 48 by 24 by 11 inch outside bed has an interior floor of 45 by 21 inches. Multiplying 45 by 21 by 11 gives 10,395 cubic inches, which divided by 1728 comes to about 6.02 cubic feet, or roughly 0.22 cubic yards. At a common 1.5 cubic foot bag size, that is about 5 bags.

Worked example: a 4x8 bed

A 96 by 48 by 11 inch bed has an interior floor of 93 by 45 inches. That works out to about 26.6 cubic feet, roughly 0.99 cubic yards, or about 18 bags at 1.5 cubic feet each, close to a full cubic yard of bulk soil.

Bagged versus bulk

For a single small bed, bagged soil is usually more convenient despite costing more per cubic foot, since it avoids a bulk delivery minimum and is easy to carry and store any leftover. Once a project approaches a full cubic yard, roughly the amount a 4x8 bed at 11 inches needs, bulk delivery from a landscaping supplier often becomes the cheaper option once the bag total is compared against a delivery fee, though the exact break-even point depends on local bagged and bulk prices, which this site does not track.

Filling above or below the interior volume

Plan to fill to about an inch below the top edge of the frame rather than exactly level, since watering will wash loose soil over the top otherwise. Soil also settles after the first watering, typically dropping slightly as it wets down, so filling almost to the top and topping up once after settling gives a more accurate final level than trying to hit the exact calculated volume on the first fill.

A recipe for the fill itself

Cubic footage tells you how much material to buy in total, not what to buy. A common starting recipe is roughly sixty percent topsoil or a bagged garden soil blend, thirty percent compost, and ten percent a coarse amendment such as perlite or coarse sand for drainage; see the soil mix recipe guide for more on proportioning a fill across those categories.

Let the configurator do the math

Rather than working the formula by hand for a custom size, the free configurator on the homepage calculates the interior volume automatically from whatever length, width, and height you enter, and returns the result in cubic feet, cubic yards, and bag count at the same time it builds the cut list, so the soil order and the lumber order come from the same set of numbers.

Quick Soil Volume and Bag Counts

Use these reference numbers for the page examples, with 2x walls already subtracted from the inside length and width.

Reference	Volume	1.5 cu ft bags
4x2, 11 in	6.02 cu ft	5 bags
4x2, 10 in	5.47 cu ft	4 bags
4x8, 11 in	26.6 cu ft	18 bags
4x8, 10 in	24.2 cu ft	17 bags
1 cu yd bulk	27 cu ft	18 bags

Use the 10 inch rows for filling an 11 inch frame 1 inch low. For custom beds, divide inside cubic inches by 1,728, then round bags up.