How Much Soil Do I Need for a Raised Bed? (Charts & Calculator)

“How much soil do I need?” is the first question almost everyone asks when they build a raised bed, and it is easy to get wrong. Order too little and you are making a second trip, or paying a second delivery fee. Order too much and you have a pile of soil sitting on the driveway. And raised beds hold far more soil than most people expect: a single 4-by-8-foot bed a foot deep needs more than a cubic yard.

This guide shows you how to work out the exact amount for any bed, with ready-made charts for common sizes, how much extra to allow for settling, what to fill the bed with, and how to cut the cost. If you just want the number, our raised bed soil calculator does it in seconds.

The quick answer

For the most popular bed size, a 4-by-8-foot bed:

• 11 inches deep (two 2×6 boards): about 29 cubic feet, or 34 cubic feet with settling, roughly 1.25 cubic yards.
• 12 inches deep: 32 cubic feet, or about 37 cubic feet with settling, roughly 1.4 cubic yards.
• 18 inches deep: 48 cubic feet, or about 55 cubic feet with settling, roughly 2 cubic yards.

How to calculate soil for a raised bed

Raised bed soil is a simple volume calculation. Measure the inside of the bed, not the outside of the frame, because the boards themselves take up space.

Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12
Cubic yards = cubic feet ÷ 27

For example, a bed 8 feet long, 4 feet wide and 11 inches deep holds 8 × 4 × 11 ÷ 12 = 29.3 cubic feet. Divide by 27 and that is about 1.1 cubic yards.

Add 10 to 20 percent for settling

Fresh soil is fluffy. As you water it and as the compost in it breaks down, it sinks, often by an inch or two in the first season. Adding 10 to 20 percent to your order fills the bed to the top after settling. The calculators on this site add 15 percent by default. Our guide on why raised bed soil sinks explains what is happening and how to top up.

Soil chart for common raised bed sizes

These figures are before the settling allowance. Add about 15 percent when you order.

Remember that 27 cubic feet make one cubic yard. A 4-by-12 bed 18 inches deep, at 72 cubic feet plus settling, needs about 3 cubic yards: a full pickup truck load or more.

Round and oval beds

For round beds, including the popular metal ring kits, use the area of a circle:

Cubic feet = 3.14 × radius (ft) × radius (ft) × depth (in) ÷ 12

A round bed 6 feet across, 12 inches deep, has a radius of 3 feet and holds about 28 cubic feet. Oval and modular metal kits vary by design, so check the manufacturer’s stated volume, or measure the bed and use the soil calculator, which handles circles and other shapes.

How deep should a raised bed be?

Depth has the biggest effect on how much soil you buy, so it is worth getting right rather than defaulting to the tallest bed you can build.

• On decent ground, 11 to 12 inches is plenty for most vegetables, especially if you loosen the soil underneath so roots can grow down into it. Oregon State University Extension’s raised bed gardening fact sheet notes that even a mounded bed with a soil and organic matter mix about 8 inches deep is enough for most vegetable roots, and recommends loosening the native soil about 6 inches deep beneath a framed bed and mixing it in as you fill.
• Over poor, compacted or contaminated ground, or on paving, go deeper, 18 to 24 inches, because roots cannot use what is underneath. Over suspected contamination, a barrier between the bed and the ground is sensible, and a soil test is worth doing first.
• For accessibility, beds 24 to 30 inches tall reduce bending, and can be worked from a seat on the edge.

Halving the height halves the soil. Going from 18 inches to 11 inches on a 4-by-8 bed saves about 19 cubic feet of soil, roughly two-thirds of a cubic yard.

What to fill a raised bed with

Plain topsoil on its own packs down in a raised bed, and bagged potting mix on its own is light, expensive and dries out fast. Most gardeners fill beds with a blend.

A practical mix

A widely used, affordable recipe is roughly:

• 60 percent topsoil or garden soil for structure and minerals.
• 30 percent compost for nutrients and moisture.
• 10 percent aeration such as perlite, pumice or fine bark, to keep it loose.

Many landscape suppliers sell a premixed “raised bed” or “garden mix” that is close to this. Ask what is in it, and choose a mix with plenty of organic matter and good water-holding capacity rather than a coarse lawn soil.

The square foot gardening mix

Square foot gardening popularized a soilless blend of equal parts compost, peat moss or coconut coir, and vermiculite. It is light and drains well, but it costs considerably more per cubic foot than a soil-based mix, so it suits small beds better than large ones.

The raised bed soil calculator splits your total into topsoil, compost and aeration, so you can buy each ingredient separately and mix your own.

How to fill a tall bed for less

Tall beds can swallow several cubic yards of soil. A popular way to cut the cost is to fill the bottom part with organic material that slowly breaks down:

1. Lay cardboard on the ground inside the bed to smother grass and weeds.
2. Fill the bottom third with untreated logs, branches and twigs.
3. Add a layer of leaves, grass clippings or straw to fill the gaps.
4. Top with at least 10 to 12 inches of your soil mix for the root zone.

Expect the filler to settle over the first few years as it decomposes, and plan to top up the bed with compost. Avoid treated or painted wood, and anything that has been sprayed with herbicide.

Bags or bulk delivery?

Once you know the volume, decide how to buy it. Bags are convenient for small jobs, but bulk soil is usually much cheaper per cubic yard once you need about a cubic yard or more.

• A 4-by-8 bed, 12 inches deep, with settling needs about 37 cubic feet: roughly 25 bags of 1.5 cubic feet, or 19 bags of 2 cubic feet.
• The same soil in bulk is about 1.4 cubic yards, often a fraction of the bagged price, plus a delivery fee.

The soil bag calculator works out the exact bag count and shows the break-even point between bags and bulk for your local prices. If you are filling several beds, order everything in one delivery.

Topping up every year

Raised bed soil gradually sinks as organic matter breaks down. Each spring or fall, top up with an inch or two of compost. For a 4-by-8 bed, one inch of compost is about 2.7 cubic feet, roughly two 1.5 cubic foot bags. The compost calculator works this out for all your beds at once, and our beginner’s guide to composting shows how to make your own.

Budgeting the whole bed

Soil is often the single biggest cost of a raised bed, frequently as much as the frame. The raised bed cost calculator adds lumber or kit prices to your soil cost, so you can see the full picture before you build.

Common mistakes

• Measuring the outside of the frame instead of the inside.
• Forgetting the settling allowance and ending up with half-empty beds.
• Using nominal board sizes. Two stacked 2×6 boards make an 11-inch bed, not 12.
• Filling with topsoil alone, which compacts and drains poorly.
• Building taller than you need, which multiplies the soil bill.
• Buying dozens of bags when bulk delivery would cost less.

Checklist before you order soil

1. Measure the inside of every bed in feet, and the depth you plan to fill in inches.
2. Add the volumes together if you are filling several beds, so you can order in one delivery.
3. Add 10 to 20 percent for settling.
4. Decide on your mix: a premixed raised bed soil, or separate topsoil, compost and an aerating ingredient.
5. Ask the supplier what is in their mix, whether it has been screened, and whether they can deliver soil and compost together.
6. Plan where the load will go. A cubic yard of soil makes a pile about the size of a small car’s trunk, and moving it takes roughly five to seven wheelbarrow loads.
7. Lay a tarp where the pile will be dumped, and prepare cardboard to line the beds.

Improving soil in an existing bed

If you already have raised beds, you rarely need to replace the soil. Each year, mix in or spread an inch or two of compost to replace what has broken down. If plants struggle despite good watering and sun, a soil test is the best next step: your state Cooperative Extension service usually offers inexpensive tests that show pH and nutrient levels, so you can add exactly what is missing rather than guessing. Signs that a bed needs attention include water pooling on the surface, a hard crust after watering, or soil that has sunk well below the top of the frame.

Sources and further reading

These university Extension and government resources informed this guide and are useful for going deeper. Recommendations can vary by region, so your local Cooperative Extension office is always a good next step.

• Vegetable Garden Planning, University of Maryland Extension
• Composting at Home, U.S. Environmental Protection Agency
• Planning a Garden (PDF), Texas A&M AgriLife Extension

Frequently asked questions