Raised Bed Soil Calculator - Cubic Yards and Litres for Any Bed

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How much soil does a raised bed need

Raised beds fail most often before a single seed goes in, when the gardener buys too little soil. A bed that looks full in the shop’s delivery pile settles to half depth after the first watering, and plants in shallow soil dry out fast and grow weak roots. Counting soil volume before ordering takes two minutes and saves a second delivery fee. This calculator does the counting.

Enter the bed length, width, and the soil depth you want. The calculator gives the volume in cubic feet, cubic yards, and litres, plus the number of 40-litre bags to buy. Use the unit switch to work in feet or metres. Measure the depth to the soil surface you want, not to the top of the boards, because soil sits a little below the rim.

A worked example

Take a common bed eight feet long, four feet wide, and one foot deep. Multiply 8 by 4 by 1 and you get 32 cubic feet. Divide by 27 and you get about 1.19 cubic yards, which is the unit bulk soil is usually sold in. Multiply cubic feet by 28.3 and you get about 906 litres, or 23 bags of 40 litres. Order a little extra, since soil settles.

Deeper beds need much more soil than people expect. The same eight by four bed at eighteen inches deep needs 48 cubic feet, or about 1.78 cubic yards. At two feet deep it needs 64 cubic feet, or 2.37 cubic yards. Depth is the number that surprises buyers, so measure it honestly instead of guessing.

Soil settles, so buy extra

Fresh soil is fluffy, full of air pockets. Watering collapses those pockets, and the surface drops by an inch or more in the first week. Organic matter keeps breaking down all season, dropping the level further. Buy about ten percent extra and keep a bag aside for topping up after the first heavy rain.

Filling the bottom of a deep bed with logs, branches, and leaves, a method called hugelkultur, cuts the soil bill. The woody layers rot slowly and feed the soil for years. They also settle a lot, so top up yearly. Never fill the bottom with rubble or plastic, which block drainage and roots.

What kind of soil to buy

Raised beds want a mix, not pure topsoil and not pure compost. A common blend is about 60 percent topsoil, 30 percent compost, and 10 percent perlite or coarse sand for drainage. Pure compost holds water but shrinks fast and lacks structure. Pure topsoil compacts into a brick in a raised bed. Ask the supplier what the mix contains before ordering.

Bagged soil suits small beds, while bulk delivery suits large ones. Bulk soil is far cheaper per litre, but a cubic yard weighs over a tonne when wet, so check that the delivery truck can reach your garden. For a single small bed, bags are easier to carry and store, even at a higher price per litre.

Common mistakes to avoid

The classic mistake is measuring the boards instead of the soil depth. A bed with 12-inch boards often holds only 10 inches of soil, because the soil surface sits below the rim for watering room. Another mistake is forgetting that beds on slopes need more soil at the low end. Level the bed first, or measure depth at several points and average them.

People also forget paths and access. A bed filled to the brim spills soil every time you water. Leave two to three centimetres below the rim so water soaks in instead of running off with your soil.

Frequently asked questions

How deep should raised bed soil be? Most vegetables need at least 12 inches of soil, and root crops like carrots need 18 inches or more. Herbs and lettuce manage with 8 to 10 inches. Deeper is better for drought, since deep soil holds water longer.

Can I reuse old soil? Yes, with refreshment. Remove old roots, mix in fresh compost at about one part in three, and check drainage. Soil that smells sour or stays soggy needs more perlite or a fresh start.

Should I line the bed? A weed barrier under the bed stops grass but also blocks worms and drainage. Cardboard is a good compromise: it smothers weeds and rots away within a season, letting roots and worms through.

Use the calculator above before you order, buy ten percent extra, and top the bed up after the first rain. Garden results are general guidance estimates only. Your local climate, your soil, and the instructions on your seed packets govern what actually grows, and results vary with weather and conditions.

Raised Bed Soil Calculator – Cubic Yards and Litres for Any Bed