Raised Bed Soil Calculator

Measure your beds, choose a cited mix, and get the volume of each ingredient to buy — not just a total. You cannot buy "raised bed soil"; you buy two or three things and blend them, and which blend is right depends on what your bed is standing on.

How this is calculated

Each bed footprint is multiplied by the fill depth and summed: volume = area × depth (at depth d inches, cubic yards = square feet × d ÷ 324).
That volume is split by the parts in your chosen blend — ingredient = total × its parts ÷ total parts. Parts, not percentages, so an equal-thirds mix stays exact.
No settling allowance and no weight are applied. Neither is published for a blend, and both are explained below rather than guessed at.

Data used, with sources

Value	Used as	Source
1 : 1 compost to topsoil	blend option — building the bed up over compacted or clay-heavy ground — the richer end of the published ratio	University of Maryland Extension, Home & Garden Information Center
1 : 2 compost to topsoil	blend option — building the bed up over compacted or clay-heavy ground — the leaner end, and the cheaper one	University of Maryland Extension, Home & Garden Information Center
1 : 1 compost to soilless growing mix	blend option — a bed on a hard surface, where nothing roots into the ground below; topsoil may be added up to 20 % by volume once the bed is at least 16 in deep	University of Maryland Extension, Home & Garden Information Center
three-way mix — equal parts topsoil, sand, compost	blend option — a common bulk soil mix sold by landscape suppliers, for filling larger beds	Oregon State University Extension Service
four-way mix — equal parts topsoil, sand, compost, lava rock or vermiculite	blend option — a common bulk soil mix sold by landscape suppliers, where more aeration is wanted	Oregon State University Extension Service
excavated native soil with up to 25 % compost	blend option — reusing the soil you dug out while installing the bed — Oregon State caps the compost at 25 % by volume, which is what the 3 : 1 parts here express	Oregon State University Extension Service
8 in	bed on a hard surface (driveway, patio) — leafy greens, beans, cucumbers: 8 in minimum	University of Maryland Extension, Home & Garden Information Center
12–24 in	bed on a hard surface (driveway, patio) — pepper, tomato, squash	University of Maryland Extension, Home & Garden Information Center
2–4 in	cited for context, not a fill preset — topping up an existing garden bed — spread compost and mix it into the top 4 in of the soil already there (NOT a fill depth)	University of Maryland Extension, Home & Garden Information Center

Choosing the blend is the actual decision

The ratio matters less than matching it to what is underneath the bed. If your bed sits on open ground, roots will eventually work into the native soil, and the job of the fill is to be good soil that connects to it — a compost-and-topsoil blend at 1:2 or 1:1 — the topsoil calculator sizes the purchased half — or the soil you dug out with up to a quarter compost mixed back in, which the compost calculator puts in yards or bags. That last option is the cheapest available and often the best, because excavated soil brings the mineral fraction that bagged products charge for.

If the bed sits on a hard surface, nothing roots anywhere: the bed is a large container, and it needs a container's mix. That is why Maryland's guidance switches to compost and a soilless growing mix at 1:1, admitting topsoil only up to 20 % by volume and only once the bed is 16 inches deep or more. Filling a driveway bed with topsoil-heavy mix gives you a compacted, badly drained block.

The bulk three-way and four-way mixes are what a landscape supplier will deliver by the yard, and they are the sane choice once you are filling more than a couple of beds — at which point the sand and the aeration ingredient are doing the drainage job that a container mix does in a smaller bed.

What we deliberately do not tell you

How much it will settle. It will. Oregon State says to expect newly filled beds to sink as the compost in the mix decomposes, and to refill them as needed. No source we could find puts a number on it, and the honest consequence is that a settlement percentage on this page would be a number we made up and you paid for. Buy what fills the bed today; top it up next season.

What it weighs. A raised bed's weight depends entirely on which of these blends you chose and how wet it is when it arrives, and no source publishes a bulk density for a blend. If you need the weight — a rooftop, a deck, a balcony — that is a structural question for the person who designed the structure, and it deserves a real number rather than an estimate from a web page.

How much organic matter your bed ends up with. Maryland gives a target: 25–50 % organic matter by volume, or 5–15 % by weight as a lab measures it. We do not compute your mix against that, because compost is not 100 % organic matter, and pretending that 50 % compost means 50 % organic matter would be arithmetic dressed up as a fact. A soil test tells you where you actually are.

Common mistakes

Filling with pure compost. The single most common one, and cited guidance is explicit against it. Compost has no mineral fraction and slumps hardest.
Using a ground-bed mix in a bed on pavement. A bed on a hard surface is a container and needs a container mix.
Buying "topsoil" without asking what it is. Maryland notes topsoil sales are not regulated there, and that bagged "organic soil" often contains no mineral soil at all — ask for test results and look at it before it is tipped.
Over-ordering to cover settling. Nobody publishes a settling figure. Top up later for the price of a bag.
Skipping the soil test near an old house or road. Lead above 400 ppm rules out food crops, and you cannot see it.

Frequently asked

How much soil do I need for a 4 × 8 raised bed?

At 12 inches deep, 1.19 cubic yards — 32 cubic feet, which is 32 one-cubic-foot bags. At 6 inches it halves to 0.59. The arithmetic is length × width × depth, and you can check ours against Oregon State's published example: they say it takes 1.7 cubic yards of soil mix to fill four 3 × 8 ft beds to 6 inches, and the exact figure is 1.78.

What is the best soil mix for a raised bed?

There is no single best mix — there is a mix that matches your situation, which is why this page makes you pick one. Over compacted or clay-heavy ground, University of Maryland Extension gives compost and purchased topsoil "in a 1:2 or 1:1 ratio". For a bed on a hard surface where roots cannot reach the ground below, it gives compost and a soilless growing mix at 1:1, with topsoil added up to 20 % by volume once the bed is at least 16 inches deep. Oregon State lists the bulk mixes landscape suppliers sell — a three-way of equal parts topsoil, sand and compost, or a four-way that adds lava rock or vermiculite. Cheapest and often best: reuse the soil you excavated, with up to 25 % compost by volume.

How deep should a raised bed be?

If the bed sits on open ground, the honest answer is that roots reach past the frame into the soil below, so the frame depth is a convenience decision rather than a horticultural one. The cited depths apply to the case where that is not true. For beds on a hard surface — a driveway, a patio — University of Maryland Extension calls for a minimum of 8 inches for leafy greens, beans and cucumbers, and 12 to 24 inches for pepper, tomato and squash, noting those beds dry out faster and stress plants more. For a bed you need to reach without bending, Oregon State puts wheelchair-accessible beds at 28 to 34 inches tall.

Can I just fill it with compost?

No. Oregon State puts it as their top tip: "Don't fill raised beds with compost alone. Fill beds with a mix of native soil, compost or a soil mix. Compost lacks the mineral component of earthen soil and planting mixes. Compost must be mixed with these materials to support plant growth." Every blend on this page keeps a mineral fraction for that reason. Pure compost also slumps hardest as it decomposes, so the bed you filled in April is noticeably lower by autumn.

Why does the calculator not include settling?

Because no source quantifies it. Oregon State says to "expect your newly filled beds to sink over time as the compost in the mix further decomposes" and to refill as needed, which is real and worth planning for — but nobody publishes a percentage, and inventing one would inflate every order on this page. Buy what fills the bed, and expect to top it up next season with a bag or two rather than paying for guesswork now.

Is it safe to grow food in my soil?

Test it before you decide, particularly near an older house or a road. University of Maryland Extension advises having the lab test for lead if you plan to grow food, notes that "a background soil lead level of 10-50 ppm is typical", and sets a clear line: "Don't plant vegetable crops in soils with a total lead level over 400 ppm." That test also tells you the pH — 6.2 to 6.8 suits most vegetables — and it is the one input on this page that no calculator can supply.

Sources

Soil to Fill Raised Beds (J. Traunfeld, updated 6 November 2024) — University of Maryland Extension, Home & Garden Information Center, accessed Aug 11, 2026.
How to use compost in gardens and landscapes (EM 9308) — Oregon State University Extension Service, accessed Aug 11, 2026.

Quantities on this page are planning estimates, not engineering advice or a purchase guarantee — product sizes vary by producer, so confirm coverage with your supplier before ordering. Beds on a roof, deck, or balcony are a structural load question, and soil contamination is a laboratory question. Neither is answered by this estimate.