Raised Bed Soil Calculator

How to use this raised bed soil calculator

1. Enter the inside length and width of each bed in feet. For a round bed or a stock tank, choose “Round” and enter the diameter.
2. Enter the soil depth. That’s usually the height of the bed, measured inside from the ground to just below the top edge.
3. If you’re building several identical beds, set “Beds this size” instead of entering each one. Use “Add a bed of another size” for different dimensions.
4. Choose the soil mix. The compost and topsoil mixes come from University of Maryland Extension. “Bagged raised bed soil only” is for filling straight from the bag.
5. Tick the filler option if you plan to put logs or branches at the bottom of a tall bed, and enter how deep that layer will be.
6. Open “Settling, bag sizes, and prices” to match the bags at your store and your local prices. The prices there are examples.

How the calculator works

bed area     = length × width, or π × diameter² ÷ 4 for a round bed
soil depth   = bed depth − filler layer
soil         = bed area × soil depth ÷ 12 × number of beds
to buy       = soil × 1.10                     settling allowance
each part    = to buy × its share of the mix   1:2 means 1/3 compost, 2/3 topsoil
bags         = each part ÷ bag size, rounded up

The mixes are the ones UMD recommends for filling raised beds: compost and purchased topsoil in a 1:2 or 1:1 ratio, or compost and a soilless growing mix 1:1. The extension’s target is a bed that is 25 to 50% organic matter by volume, which is why compost makes up a third to half of every mix.

For bagged raised bed soil we use a 1.5 cubic foot bag, the size Miracle-Gro sells its raised bed soil in. Compost and topsoil come in more sizes, so the defaults of 1 and 0.75 cubic feet are only a starting point. Change them to match the bags you’re buying.

The 10% settling allowance is our estimate. A freshly filled bed is loose, and it sinks once it’s watered and the compost starts to break down. On a 12 inch bed, 10% is a little over an inch.

The bulk line prices the same amount as a topsoil and compost blend from a landscape supplier, which UMD notes some vendors sell. We round bulk up to the next half yard. When the blend costs less than the bags, the result leads with it. The compost and soilless mix is meant for beds on pavement, where topsoil doesn’t belong, so for that mix we compare bags only.

A worked example

Say you’re building two 4 × 8 foot beds, 18 inches tall, on an old lawn. You’ll put 6 inches of logs and branches in the bottom and fill the rest with UMD’s 1:2 mix.

• Each bed: 4 × 8 = 32 square feet. 18 inches minus 6 inches of filler leaves 12 inches of soil, or 32 cubic feet.
• Two beds: 64 cubic feet of soil
• With 10% for settling: 70.4 cubic feet
• Compost, a third: 23.5 cubic feet, or 24 bags of 1 cubic foot, $168 at $7
• Topsoil, two thirds: 46.9 cubic feet, or 63 bags of 0.75 cubic feet, $252 at $4

That’s $420 in bags. A delivered blend would be 3 cubic yards at $55 plus $75 delivery, or $240. The filler saves a lot too: without it, the same beds would need 105.6 cubic feet, almost 4 cubic yards. Filling them with bagged raised bed soil alone would take 47 bags, $517 at $11 each.

Soil for common raised bed sizes

Bagged raised bed soil in 1.5 cubic foot bags, before and after the 10% for settling:

The popular 4 × 8 bed at 12 inches deep takes 32 cubic feet, 1.19 cubic yards, or 22 bags before settling. At our example prices, a delivered blend always costs less than bags once you need more than about 16.5 cubic feet, roughly half of that one bed.

Tips for filling raised beds

• If you buy bulk topsoil, buy from a reputable dealer and ask for soil test results and where the soil came from. UMD says good topsoil is dark and crumbly with an earthy smell. Skip soil that smells foul, is very sandy or very clayey, or has grayish mottling.
• Read the bag. UMD points out that bags sold as “organic soil” usually contain no mineral topsoil at all, just mixes of wood waste, bark fines, compost, and peat moss.
• On a lawn, you don’t need to dig out the grass. UMD suggests covering it with plain cardboard or several layers of newspaper and 2 to 4 inches of compost. The turf dies in 6 to 8 weeks and breaks down in place.
• If you’ll grow food, have the soil tested for lead. UMD calls 10 to 50 ppm a typical background level and advises against planting vegetables in soil with more than 400 ppm.
• Fill a few inches at a time and water as you go. The soil settles before you plant instead of after.