Raised Bed Soil Calculator

Filling raised beds takes far more soil than most people expect. This raised bed soil calculator works out exactly how much you need for one bed or a whole set of matching beds, adds an allowance for the settling every new bed goes through, and splits the total into the ingredients of the soil mix you choose. The results show cubic feet and cubic yards for ordering bulk, and bag counts for buying at the store, so you can price both before you commit.

How to use the raised bed soil calculator

1. Enter the number of beds. If all your beds are the same size, set the count and enter the size once. For beds of different sizes, run each size separately and add the totals.
2. Enter the bed height in inches. Use the actual height of the sides, not the nominal lumber size. A 2×12 board is about 11.25 inches wide, and two stacked 2×6 boards make about 11 inches.
3. Enter the inside length and width in feet. Measure inside the frame. Board thickness takes up space, especially on narrow beds.
4. Set the unfilled gap. Leaving about an inch below the rim stops soil and mulch from washing over the edge when you water.
5. Set the settling allowance. The default of 15 percent suits most fresh mixes. Use 20 percent for very fluffy, compost-heavy blends.
6. Choose your mix. The calculator divides the total into each ingredient so you know exactly what to order.

The raised bed soil formula

The volume of a rectangular raised bed is its length times its width times its fill height. Because length and width are in feet and height is in inches, the height is divided by 12:

Cubic feet per bed = length (ft) × width (ft) × (height − gap) (in) ÷ 12
Total = cubic feet per bed × number of beds × (1 + settling %)
Cubic yards = total cubic feet ÷ 27

Common raised bed sizes

This table shows the volume of popular bed sizes when filled to an 11-inch depth (or 17 inches for the tall metal bed), before any allowance for settling.

Worked example: three 4×8 beds

You are building three 4-by-8-foot beds from 2×12 lumber, about 11 inches tall, and want to leave a 1-inch gap at the top. Each bed holds 4 × 8 × 10 ÷ 12 ≈ 26.7 cubic feet. Three beds hold 80 cubic feet. Adding 15 percent for settling brings the order to 92 cubic feet, or about 3.4 cubic yards. Using the 60/30/10 blend, that is roughly 55 cubic feet of topsoil, 28 cubic feet of compost and 9 cubic feet of aeration material. In bags, it would be 46 bags of 2 cubic feet, which is a strong argument for bulk delivery.

What to fill a raised bed with

A good raised bed mix has three jobs. It must drain freely so roots do not sit in water, hold enough moisture that the bed does not dry out in a day, and supply nutrients and biology for healthy growth. No single material does all three well, which is why most recommended fills are blends. The calculator offers four common options.

60% topsoil, 30% compost, 10% aeration

This is a balanced, affordable blend for most vegetable and flower beds. Screened topsoil provides body and mineral content, compost adds organic matter and nutrients, and a coarse aerating ingredient such as perlite, pumice or fine bark keeps it from packing down. It is close to what many landscape suppliers sell as a premixed “garden blend,” so you can often order it in one load.

Equal parts compost, peat or coir, and vermiculite

This soilless blend, popularized by the square foot gardening method, is light, holds water well and is very easy to work. It is ideal for shallow beds and for gardeners whose native soil is poor or contaminated. The tradeoffs are cost, because all three ingredients are bought, and the need to use a mix of several different composts for a broad range of nutrients. Peat moss, coco coir and vermiculite are often sold compressed, so check the expanded volume on the bag. A bale labeled 3 cubic feet compressed can expand to considerably more.

50% topsoil, 50% compost

This is a rich, simple mix that suits heavy feeders such as tomatoes, squash and corn. It works well when the topsoil is a sandy or loamy type. With heavy clay topsoil, add an aerating ingredient or choose the 60/30/10 blend instead, because clay plus compost alone can still compact over time.

100% bagged raised bed mix

Bagged raised bed mixes are the easiest option for one or two small beds. They are consistent, weed-free and ready to plant. For larger projects, the cost adds up quickly, so compare the per-cubic-foot price with a bulk blend.

How much to allow for settling

Every new raised bed sinks. Loose soil compacts as it is watered, and compost slowly breaks down into a smaller volume. A bed filled to the brim in spring can easily be several inches lower by fall.

• 10 percent: mixes that are mostly screened topsoil.
• 15 percent: typical blends with 30 to 50 percent compost.
• 20 percent: very light, compost-heavy or soilless blends.

You do not need to build a mound to account for all of it. Many gardeners fill the bed to the top, water it thoroughly, let it settle for a few days, and then top it up with the extra before planting. After the first year, expect to add an inch or two of compost each season to replace what the plants and soil life use. The compost calculator works out that top-up.

Saving soil in tall beds

Tall beds, such as 24-inch or 30-inch beds built for easier access, take a lot of soil. Most vegetables do not need that full depth of premium mix, so it is common to fill the bottom with cheaper organic material and save the good soil for the top.

• Logs and thick branches can fill the lowest layer. As they rot they hold moisture and feed soil life, an approach often called hugelkultur. Avoid treated, painted or black walnut wood.
• Smaller branches, leaves and straw fill the gaps around larger wood and break down faster.
• Cardboard laid on the ground at the bottom smothers grass and weeds. Remove tape and glossy coatings first.

Keep at least 12 inches of good soil mix above any filler for most vegetables, and around 18 inches for deep-rooted crops. Woody fillers decompose and can sink noticeably over the first couple of years, so expect to top up more often. Decomposing wood can also tie up nitrogen near it for a while, which is another reason to keep a generous layer of finished soil on top. To find the soil volume above the filler, enter the depth of good soil as the bed height in the calculator.

Filling beds of other shapes

The calculator is built for rectangular beds, which are by far the most common, but other shapes are easy to handle with a little extra arithmetic.

Round beds and stock tanks. For a circular bed, multiply 3.1416 by the radius squared to get the area in square feet, then multiply by the fill depth in inches and divide by 12. A round metal bed 4 feet across and filled to 22 inches holds about 3.1416 × 2 × 2 × 22 ÷ 12 ≈ 23 cubic feet before settling. You can also enter a round bed in the general soil calculator, which has a circle option.

L-shaped and U-shaped beds. Split the bed into rectangles that do not overlap, calculate each one, and add the results. A U-shaped bed is usually easiest as one long back section and two shorter side sections.

Keyhole beds. A keyhole bed is a circle with a wedge-shaped path and central compost basket cut out. Calculate the full circle, then subtract the path and basket area. A rough 15 to 20 percent reduction from the full circle is a reasonable estimate for most designs.

Sloped sites. On a slope, one end of the bed is deeper than the other. Measure the soil depth at the shallow end and the deep end, average the two, and enter the average as the bed height.

How deep should a raised bed be?

Bed depth should match what you plan to grow, although roots will also grow into loosened ground below an open-bottomed bed. These are typical root depths for common crops.

An 11- or 12-inch bed placed on loosened ground works for nearly all vegetables, because deeper roots can grow down into the native soil. If the bed sits on concrete, a patio or very poor ground, choose a deeper bed, or keep deep-rooted crops in the deepest beds. To see what fits in your beds once they are filled, use the square foot gardening calculator or the plant spacing calculator.

Bags or bulk for raised beds

For one small bed, bags are usually the simplest choice. A 4-by-4-foot bed at 11 inches takes about 17 cubic feet with settling, which is 9 bags of 2 cubic feet. That fits in most cars over one or two trips.

Once you need more than about a cubic yard, compare bulk prices. Divide the bulk price per cubic yard by 27 to get a price per cubic foot, add the delivery charge, and compare it with the bag price divided by bag size. For a set of three or four full-size beds, bulk delivery is usually cheaper and saves opening and disposing of dozens of plastic bags. Plan where the pile will go and how you will move it. A wheelbarrow holds about 4 to 6 cubic feet, so three cubic yards means roughly fourteen to twenty trips. The raised bed cost calculator adds lumber and hardware to the soil cost for a complete budget.

Common mistakes to avoid

• Using nominal lumber sizes. A 2×12 is about 11.25 inches, not 12, and a 2×6 is about 5.5 inches. Using nominal sizes overestimates soil slightly, while using outside dimensions overestimates it further.
• Skipping the settling allowance. Beds filled to the exact calculated volume end up noticeably short within weeks.
• Making beds too wide. Beds wider than about 4 feet are hard to reach across without stepping in them, and stepping compacts the soil you paid for. Beds against a wall or fence should be no more than about 2 to 3 feet wide.
• Filling with a single material. All topsoil compacts and all compost shrinks. A blend performs better and holds its level longer.
• Forgetting drainage below the bed. Loosen compacted ground under open-bottomed beds so excess water can drain away.

Frequently asked questions