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.

Step by step procedure before you buy soil

Measure the empty frame after it is assembled and sitting square, not while the boards are loose on the ground. Use a tape measure across the inside face of one long wall to the inside face of the opposite wall, then repeat across the short direction. Measure the planned soil height from the bottom of the bed to the fill line, not to the absolute top of the board if you plan to leave a lip.

Check the ground under the bed before you calculate the final order. A bed placed on a flat patio has a clear bottom plane, so the soil height is easy to define. A bed placed on uneven soil may have one corner 1 or 2 inches lower than another corner. In that case, measure the average usable height at the four corners, then use that average for the order.

Use a rigid tape measure long enough to span the bed in one pull, because bending the tape around a corner can add 0.25 to 0.5 inch.
Use a pencil, paper, and calculator, then round the final order upward by 5 to 10 percent for settling and small measurement errors.
Use a wheelbarrow or large tub to mix partial bags before filling, because separate layers of compost and soil can settle unevenly.
Keep one small bag or a covered bucket of the same mix aside for the first top up after watering.

If the bed depth is still undecided, choose that before ordering soil. A shallow herb bed, a standard vegetable bed, and a deep root bed do not use the same volume. See how deep should a raised bed be if the crop depth is not fixed yet.

Working dimensions and fill allowances

The most useful order is rarely the exact calculated number. Loose soil compacts when it is watered, compost can shrink as it continues to break down, and fine material drops into gaps around the base. For most raised beds, add a small allowance rather than trying to buy the exact cubic footage to the last decimal.

Situation	Practical allowance to add	Reason
New bed filled with dry bagged mix	5 percent	Dry material fluffs up in the bag, then drops after the first deep watering.
Bulk soil tipped on a driveway	5 to 10 percent	Some material is lost to spillage, shoveling, and uneven moisture.
Bed placed over lawn	10 percent	Soil settles into the cut grass layer and low spots under the frame.
Bed placed over coarse gravel	10 to 15 percent	Fine material works down into the voids between stones unless a barrier is used.
Top up after one season	1 to 3 inches of depth	Organic matter breaks down and the surface drops with watering and cultivation.

For carrying work, use weight only as a planning concern, not as the volume calculation. Moist soil can be several times harder to move than dry mix. A standard wheelbarrow load is often around 3 to 5 cubic feet, depending on how high it is heaped and how wet the material is. A full cubic yard is 27 cubic feet, which can mean 6 to 9 wheelbarrow trips for careful loads, or more if the path is narrow.

Frequent mistakes and how to recover

If you ordered too little, do not stretch the mix thin over the whole bed and plant into a shallow layer. Fill the bed to a consistent depth first, water it, then measure the missing height. A missing 2 inches over a medium bed is often several cubic feet, so calculate the top up as its own shallow layer instead of guessing by bag count.

If you ordered too much, store the surplus dry and covered if possible. Keep it off bare ground on a tarp or in lidded tubs so it does not become contaminated with weed seed or wash away in rain. Surplus mix is useful for topping up after the first month, filling pots, or correcting low spots around transplants.

If the bed was filled with very light material and drops more than 2 inches after watering, compact it gently by soaking it again rather than pressing it hard with feet. Foot compaction can create dense zones that drain poorly. Add more mix in 1 inch layers, wet each layer, then rake level.

If the bed was filled too high and soil washes over the side, remove the extra while it is moist, not muddy. Scrape back to about 1 inch below the rim, then move the excess to a bucket for later top up. Repeated overflow wastes compost and can stain paving or bury the base of wooden boards.

Special cases that change the soil order

A bottomless bed over good garden soil does not always need to be filled to the full board height with imported mix. If the native soil is loose and weed free, the lower 2 to 4 inches can be forked open and blended with the new fill. This reduces the amount to buy, but it only works when the underlying soil drains well and is not compacted clay.

A bed on a hard surface needs the full calculated depth because roots cannot continue downward into native ground. It also needs a clear drainage path, since water has nowhere to disappear except through the bottom edge, drain holes, or a designed outlet. In this case, do not reduce the order on the assumption that roots will use the soil below.

Tall beds often are not filled entirely with finished planting mix. Clean woody material, coarse compost, or mineral soil may be used in lower layers, depending on the design, but the upper root zone still needs a suitable growing mix. For vegetables, keep at least the top 8 to 12 inches as planting soil, and deeper for long carrots, parsnips, or similar roots.

Irregular beds should be divided into rectangles before calculating. An L shaped bed can be treated as two rectangles, and a keyhole or angled bed can be estimated by splitting it into simple sections. Calculate each section separately, add the results, then apply the settling allowance. For general raised bed questions, see the FAQ.