Raised Bed Soil Calculator: Bags and Mix

Why our numbers are more realistic

The raised-bed calculators we looked at work out the volume of the box and stop there. Here's what changes:

Typical calculator	This calculator	Why it matters
None of the eight we checked adds anything for settling	Adds 15% for settling	University guidance says new beds sink as the compost in them breaks down. A 4 × 8 ft bed 12 in deep holds 32 cu ft; with the extra, that's 25 bags of 1.5 cu ft instead of 22, so you can fill it and still have it full after the first watering.
Mix recipes from 20% to 40% compost, with no source given	70% topsoil and 30% compost by default, from university guidance, and a warning above 50% compost	University guidance puts compost at 25% to 50% and says never to fill a bed with compost alone: plants struggle in it, and manure compost can be too salty.
One total, or one bag size for everything	Each ingredient rounded up to its own bags, or cubic yards for bulk	You buy topsoil and compost separately, often in different bag sizes, so each one rounds up on its own.
Rectangles only, or a round bed sent to another tool	Round beds too	Treating a 6 ft round bed as a 6 × 6 ft square orders 27% too much.
No way to count a filler layer	An optional filler for beds 24 in or deeper	University guidance suggests logs and branches in the bottom of tall beds. A 4 × 8 ft bed 24 in deep with 12 in of filler needs half the soil.
Fill a whole bed only, with the same compost rules	A top-up option, with compost alone allowed	University guidance says to add compost or mix to a sunken bed every year or two. The rule against compost alone is for filling a whole bed, so a top-up can be compost alone, with nothing added for settling.

How this works

Soil mix = bed area × soil depth (in inches ÷ 12) × number of beds × 1.15 for settling. A round bed's area is 3.14 × (diameter ÷ 2)². Each ingredient is its share of the mix, rounded up to whole bags, or to 0.1 cubic yard in bulk (27 cu ft per cubic yard).

Topsoil and compost, 70/30: the blend one university recommends, inside the 25% to 50% compost that university guidance suggests. Topsoil and compost are counted in 1 cu ft bags, the common size; the result also gives 0.75 cu ft topsoil bags.
Equal parts: topsoil, compost, and coarse sand or perlite, a mix university guidance gives for better drainage. Sand and perlite bag sizes vary, so we give that part in cubic feet.
Bagged raised-bed soil: sold ready to use in 1.5 and 2 cu ft bags.
Your own mix: topsoil plus the compost share you enter. For a new bed, we warn above 50% and won't calculate 100%.
Topping up: bed area × how far the soil sits below the rim, with no settling allowance, since it's a thin layer and the rim shows where to stop. We start you on compost alone, which is fine for a few inches; deeper than about 6 in, mix in topsoil.
Settling: 15%. University guidance says new beds sink as the compost breaks down, often "a couple of inches" by the second year, but gives no percentage. Trade estimates run 10% to 25%, so we use 15%, a planning allowance and the same one our topsoil calculator uses.

Filling a tall bed

Beds over about 2 ft deep take a lot of soil. University guidance suggests filling the bottom with sticks, logs, leaves and compost, a method often called hugelkultur. Other university guidance is cautious: it hasn't been tested in peer-reviewed studies, and the bed sinks as the wood rots, so it may need rebuilding every 5 to 6 years (another source says a well-made one lasts 5 to 10).

Use it only in beds 24 in or deeper, and keep at least 12 in of real soil mix on top (our rule, from the 6 to 12 in that roots need).
Skip black walnut, black cherry and pressure-treated wood.
Rotting wood can tie up nitrogen for a while, so plants above it may need extra feeding.
We don't add settling for the filler, because no source gives a figure. Expect to top up the bed more often.

Common questions

What is the best soil mix for a raised bed?

Topsoil with compost mixed in: university guidance suggests 25% to 50% compost by volume, and one university recommends about 70% topsoil and 30% compost, our default. For heavy clay, the equal-parts mix with coarse sand or perlite drains better. Don't fill with compost alone.

How deep should a raised bed be?

At least 6 in, and 8 to 12 in suits most vegetables, per university guidance. In a bed on open ground, roots can also grow into the soil below. Beds over 18 to 24 in tall need sturdy walls.

Why did my raised bed soil sink?

Compost and other organic matter break down, and loose soil packs down once it's watered. It's normal. Fill to the top and mound the extra at first, then add compost or mix every year or two to bring it back up. Pick "Topping up a sunken bed" above to see how much.

Should I buy bags or bulk?

From about 2 cubic yards (54 cu ft), bulk from a landscape yard is usually cheaper, and your result will say so. A 4 × 8 ft bed 12 in deep takes about 1.4 cubic yards with our settling allowance.

What this doesn't cover

Soilless mixes, such as the "1/3 compost, 1/3 peat or coir, 1/3 vermiculite" mix from a gardening book. University guidance backs a soilless mix mainly for beds on a patio or other hard surface, half compost and half soilless mix.
Building the frame, and the weight of a full bed on a deck or roof. Moist topsoil weighs about a ton per cubic yard, so ask a builder first.
Fertilizer and pH. A soil test tells you what the mix needs.

Sources

Iowa State University Extension, a good soil mix for a raised bed: about 70% topsoil and 30% compost, or equal parts topsoil, organic matter and coarse sand.
University of Minnesota Extension, raised bed gardens: half to two-thirds topsoil; filler for beds over 2 ft; keep adding soil.
Oregon State University Extension EM 9308: up to 25% compost; "don't fill raised beds with compost alone"; new beds sink.
South Dakota State University Extension, compost in raised beds: 25% most common, up to 50%; plants struggle in 100% compost.
University of Maryland Extension, soil to fill raised beds: 25% to 50% organic matter; soilless mix on hard surfaces; buy topsoil with a soil test.
Utah State University Extension, raised bed gardening: equal-parts mix; beds at least 6 to 12 in.
University of Missouri Extension G6985: 6 to 12 in rooting zone; tall beds need retaining walls.
Oregon State University Extension FS 270: about 8 in of mix is enough for most vegetables.
University of Minnesota Extension, too much compost and manure: salts and nutrients build up.
Washington State University Extension, hugelkultur: no peer-reviewed studies; mounds collapse and need rebuilding.
UC Master Gardeners, San Mateo and San Francisco, hugelkultur: 5 to 10 years; woods to avoid.
UConn Home and Garden Education Center blog, why raised beds shrink: "a couple of inches" lower by the second year.
Espoma, Kellogg Garden and Miracle-Gro raised-bed soil pages: 1.5 and 2 cu ft bags and their bed charts.
Rogue Engineer, raised bed soil calculator: actual lumber sizes.
Also used: the raised-bed calculators we compared (Omni, Almanac, Lurveys, Garden Betty, Eartheasy, Frame It All, Rogue Engineer, Mulch Mound) and trade settling estimates of 10% to 25%.

Numbers last reviewed: October 9, 2026.