Guide
How Much Soil for a Raised Bed?
Length times width times height in feet, divided by 27, gives cubic yards. A 4x8 bed at 10 inches needs about 1 cubic yard. Formula, mix ratios, bag list, and a calculator.
The short answer
A 4x8-foot raised bed filled 10 inches deep needs about 26.7 cubic feet of soil, or roughly 1 cubic yard. The formula is length x width x height in feet, divided by 27 for cubic yards. Add 10 percent for settling, so order about 29 cubic feet.
Try it — Raised Bed Soil Calculator
Full calculatorA balanced, beginner-friendly mix. Good default for most vegetable beds.
Total soil needed
0.99cu yd
26.7 cu ft · buy ~29.3 cu ft to allow for settling
| Component | Volume | Bags |
|---|---|---|
| Topsoil | 16 cu ft | 11 × 1.5 cu ft |
| Compost | 8 cu ft | 8 × 1 cu ft |
| Aeration (perlite or vermiculite) | 2.7 cu ft | 2 × 2 cu ft |
A 4x8 bed swallows almost a full cubic yard at just 10 inches deep. Order by guess and you usually come up short on the first try.
The math is simple. What trips people up is the depth, the settling, and the fact that "soil" for a raised bed is really three ingredients in three different bag sizes, mixed to a specific ratio. Get those right and you order once.
This guide sits under the bigger raised bed soil hub, which covers mix, depth, and cost end to end. For the plain volume formula that applies to any soil type, see how much soil do I need. For a lawn or grading job, see how much topsoil do I need instead.
The formula, and the one division that matters
A raised bed is a box. So its volume is plain geometry: length times width times height.
The only catch is units. Length and width come in feet, but fill depth comes in inches. Convert the depth to feet first by dividing by 12.
cubic feet = length_ft x width_ft x (height_in / 12)
cubic yards = cubic feet / 27
Why both numbers? Bulk soil is sold by the cubic yard, bagged soil by the cubic foot.
A cubic yard is a block 3 feet on every side, which holds 27 cubic feet. That 27 is the whole conversion.
Bag math comes last. Divide each ingredient's cubic feet by its bag size and round up, because you cannot buy two-thirds of a bag.
A worked example: the 4x8 bed at 10 inches
Run the standard bed through it. A 4x8 footprint is 32 square feet.
At 10 inches deep, that is 32 x (10 / 12), which lands at 26.67 cubic feet, or 0.99 cubic yards. Call it a cubic yard.
Then add for settling. Fresh mix drops after the first few waterings, so the calculator adds a 10 percent buffer. That bumps the order to about 29.33 cubic feet.
Some gardeners go up to 15 percent on tall beds. But 10 percent matches what the tool plans and what most beds actually drop.
Pro tip
Fill that same 4x8 bed a full 12 inches instead of 10 and the number jumps to 32 cubic feet, about 1.19 cubic yards. Two extra inches of depth adds a fifth of a yard. Decide your depth before you price the order, because depth drives the whole bill.
That gives you a total in cubic yards. It still does not tell you what to put in the cart, which is where most guides stop.
Common bed sizes at a glance
Before the mix ratio, here is the raw volume for the sizes people search most, all at the common 10 inch and 12 inch fill depths.
| Bed size | Area | 10 in deep | 12 in deep |
|---|---|---|---|
| 2x4 ft | 8 sq ft | 6.7 cu ft | 8.0 cu ft |
| 3x6 ft | 18 sq ft | 15.0 cu ft | 18.0 cu ft |
| 4x4 ft | 16 sq ft | 13.3 cu ft | 16.0 cu ft |
| 4x8 ft | 32 sq ft | 26.7 cu ft | 32.0 cu ft |
| 4x12 ft | 48 sq ft | 40.0 cu ft | 48.0 cu ft |
| 8x8 ft | 64 sq ft | 53.3 cu ft | 64.0 cu ft |
| 8x16 ft | 128 sq ft | 106.7 cu ft | 128.0 cu ft |
Your per-component shopping list: the 60/30/10 default blend
A raised bed is not one product. The default vegetable blend that the Raised-Bed Soil Calculator plans for is 60 percent topsoil, 30 percent compost, and 10 percent aeration (perlite or vermiculite). Each comes in a different bag size.
So the real shopping list breaks the cubic feet into three stacks of bags. For the 4x8 bed at 10 inches, the 26.67 cubic feet splits like this.
| Ingredient | Share | Cubic feet | Bag size | Bags |
|---|---|---|---|---|
| Topsoil | 60% | 16.0 cu ft | 1.5 cu ft | 11 |
| Compost | 30% | 8.0 cu ft | 1.0 cu ft | 8 |
| Aeration (perlite/vermiculite) | 10% | 2.67 cu ft | 2.0 cu ft | 2 |
So one 4x8 bed is 11 bags of topsoil, 8 bags of compost, and 2 bags of aeration. That is the list you carry to the store, not a lonely "1 cubic yard" you translate at the register.
The 60/30/10 split is a balanced starting mix. The Raised-Bed Soil Calculator rebuilds this list for any bed size and depth, including the ready-made 4x8 size page.
60/30/10 vs Mel's Mix: the other common recipe
The 60/30/10 blend is not the only recipe gardeners use. Square-foot gardening popularized Mel's Mix: equal thirds by volume of compost, peat moss or coco coir, and coarse vermiculite, per Penn State Extension's square foot gardening guidance. It drains fast and suits intensive planting, but it costs more because it uses no native topsoil.
| Recipe | Topsoil | Compost | Other | Cost | Drainage |
|---|---|---|---|---|---|
| 60/30/10 Default Blend | 60% | 30% | 10% aeration | Lower | Good |
| Mel's Mix | 0% | 33.3% | 33.3% peat/coir + 33.3% vermiculite | Higher | Very fast |
Both lean heavily on compost, for good reason. Extension mixes for raised vegetable beds often run as high as one part topsoil to one part compost by volume.
Want to size the compost on its own? The how much compost do I need guide walks that number.
Potting-soil bag sizes: quarts to cubic feet
Some bagged mixes are sold in dry quarts instead of cubic feet, which is where a lot of the confusion comes from. One dry quart is about 0.0333 cubic feet.
| Bag size (quarts) | Cubic feet | Bags per cubic yard |
|---|---|---|
| 8 qt | 0.27 cu ft | 101 |
| 16 qt | 0.53 cu ft | 51 |
| 25 qt | 0.83 cu ft | 33 |
| 40 qt | 1.33 cu ft | 20 |
| 50 qt | 1.67 cu ft | 16 |
Common mistake
Assuming a 25 quart bag equals 1 cubic foot. It does not. A true cubic foot is about 25.7 dry quarts, so a 25 quart bag is a little short of a full cubic foot. Convert by quarts, not by rounding to "close enough."
How deep does a raised bed need to be?
Depth sets your whole order, and it depends on what you grow.
Shallow-rooted crops like lettuce, beans, and other greens are happy in at least 8 inches of mix. Root crops and deeper feeders like carrots, tomatoes, and squash want 12 to 24 inches, per the University of Maryland Extension guide to growing vegetables in raised beds.
So a 10 to 12 inch bed is a sensible default for a mixed garden. It clears the 8-inch floor for greens and gives most root crops enough room. The University of Minnesota Extension raised-bed guide lands in the same range for general vegetable beds.
The cheapest honest way to fill a deep bed
Spend on the top foot and save on the bottom. A 24-inch bed holds more than twice the mix of a 10-inch one, and buying premium blend all the way down is where the cost runs away.
The fix is to fill only the top 10 to 12 inches with growing mix, since that is the root zone. Pack the bottom 6 to 12 inches with cheaper coarse fill.
Coarse woody material, branches, or clean fill dirt all work down low. Burying logs and branches is the hugelkultur approach, and it can genuinely cut the volume of purchased mix in a deep bed.
Extension views are mixed, so treat it as a budget option that settles over time and may need topping up, not a guaranteed shortcut. Down low you are buying volume, not nutrition.
The Topsoil Calculator sizes that bottom layer if you would rather pay for screened topsoil than scavenge wood.
Common mistake
Do not fill a raised bed with pressure-treated lumber scraps, railroad ties, painted or stained wood, construction debris, or fill dirt of unknown origin. Older treated wood can leach preservatives like CCA into the soil, and mystery fill can carry contaminants. Clemson Extension and others also warn off tires and fly-ash cinder blocks. Keep the growing zone to clean topsoil, compost, and aeration.
What goes into the mix, ingredient by ingredient
- Topsoil — mineral base, sold by the cubic yard bulk or in 0.75 to 1.5 cu ft bags
- Compost — organic matter, feeds the mix, sold in 1 to 1.5 cu ft bags
- Aeration (perlite or vermiculite) — keeps the mix from compacting, sold in 2 cu ft bags
- Peat moss or coco coir — used in Mel's Mix instead of topsoil, sold in 2 cu ft compressed bales that expand
- Coarse fill (bottom layer only) — logs, branches, or fill dirt, never the growing zone
It comes down to four numbers. A cubic yard for a 4x8 bed at 10 inches. Eleven bags of topsoil, eight of compost, two of aeration. Ten percent more for settling. Run those and you load the truck once.
Got your bed dimensions? Open the Raised-Bed Soil Calculator for your exact bag list.
Common questions
How many bags of soil do I need to fill a 4x8 raised bed?
For a 4x8 bed filled 10 inches deep with the default 60/30/10 blend, plan on about 11 bags of topsoil (1.5 cu ft each), 8 bags of compost (1 cu ft each), and 2 bags of aeration (2 cu ft each). That covers roughly 26.7 cubic feet before the 10 percent settling buffer.
How many cubic feet is a 40 lb bag of potting soil?
A 40 lb bag of potting or garden soil mix typically holds about 0.75 to 1.5 cubic feet, depending on the product's density and moisture. Bagged soil is lighter per cubic foot than screened topsoil, so a 40 lb bag of potting mix usually runs toward the higher end of that range. Check the cubic-foot number printed on the bag rather than relying on weight.
Can I fill a raised bed with just compost?
Not for the whole bed. Straight compost holds too much water, compacts over time, and lacks the mineral structure roots need. Extension guidance points to a blend, such as the 60 percent topsoil, 30 percent compost, 10 percent aeration mix, or a compost-and-soilless-mix blend for the growing zone.
How many cubic feet is a 25 quart bag of potting soil?
About 0.83 cubic feet. One dry quart equals roughly 0.0333 cubic feet, so 25 quarts works out to 25 × 0.0333 ≈ 0.83 cubic feet. It takes about 33 of these 25-quart bags to fill one cubic yard (27 cubic feet).
Is 25 dry quarts the same as 1 cubic foot?
No. One cubic foot equals about 25.7 dry quarts, so a 25-quart bag is slightly under a full cubic foot, about 0.97 cubic feet by that conversion, or roughly 0.83 cubic feet using the common bagged-soil approximation of 0.0333 cu ft per quart. Bag labels vary, so check the cubic-foot figure printed on the bag when precision matters.
Is it a better deal to buy topsoil as a cubic yard or by the bag?
Once a raised-bed project needs more than about 1 cubic yard total, bulk delivery almost always beats bags per cubic foot. A single 4x8 bed sits right at that line on its own, but most gardeners fill more than one bed, which tips the math toward a bulk order for the topsoil and compost components.
Sources
Agronomic claims in this guide are checked against these primary sources.
- Growing Vegetables in Raised Beds — University of Maryland Extension
- Raised bed gardens — University of Minnesota Extension
- Raised Beds — Clemson Cooperative Extension HGIC
- Square Foot Gardening — Penn State Extension
Keep reading
How Much Compost Do I Need?
Multiply square feet by depth in inches, divide by 324 for cubic yards of compost. Formula, worked examples, and a bagged-vs-bulk guide.
Read →How Much Mulch Do I Need?
Measure your bed, multiply by depth, divide by 324, and you have the cubic yards of mulch you need. Formula, worked examples, and a coverage table.
Read →Raised Bed Soil: The Complete Guide (Mix, Depth, Cost)
Everything that goes in a raised bed, in one place: a shopping list for 10 bed sizes, 9 extension-tested mix recipes, what to put in the bottom, and how to fill a deep bed for less.
Read →How Much Soil Do I Need? (And Which Soil to Buy)
Multiply length × width × depth in feet, divide by 27 for cubic yards. Plus a which-soil chooser so you buy the right material, not just the right amount.
Read →What Soil Mix for Raised Beds?
The best raised bed soil mix is 60 percent topsoil, 30 percent compost, and 10 percent aeration. Here is why each part matters, plus the per-bag amounts for a 4x8 bed.
Read →