Raised Bed Soil Calculator
How many bags or cubic yards of soil a raised garden bed needs, and the mix to fill it with.
Quick estimating rule of thumb
For a raised bed: inside length (ft) × inside width (ft) × depth (in) ÷ 12 = cubic feet. A 4 × 8 ft bed at 10 inches is 26.7 cubic feet, almost exactly one cubic yard, or 27 bags of 1 cubic foot. Mix roughly 60% topsoil, 30% compost, 10% perlite or bark fines. Add 10% for first-season settling.
This raised bed soil calculator turns bed dimensions into the bags of soil to buy, or the cubic yards to have delivered. Enter the inside measurements of the bed, not the outside frame: a 4 by 8 foot bed built from 2-inch lumber has an inside area closer to 3.7 by 7.7 feet.
The reference figure worth remembering: a 4 by 8 foot bed filled 10 inches deep takes 26.7 cubic feet, which is almost exactly one cubic yard, or 27 bags of the standard 1 cubic foot garden soil. That is the point where bagged soil stops making sense on price.
How deep does a raised bed need to be?
Six inches is enough for lettuce, herbs and shallow greens if the bed sits on decent soil the roots can grow into. Ten to twelve inches suits most vegetables and is the usual depth for a bed on grass or bare ground. Eighteen inches or more is needed for carrots, parsnips and potatoes, or for any bed sitting on a hard surface such as a patio or a driveway where roots cannot go deeper.
You do not have to fill the full frame with bought soil. A tall bed can take coarse woody material, branches, or upturned turf in the bottom third, topped with the soil mix; the woody layer breaks down over a few years and the level drops, which is why beds built that way need topping up annually. Calculate only the depth you are actually filling with purchased mix.
The mix, not just the volume
A workable raised-bed mix is roughly 60% screened topsoil, 30% compost, and 10% something for aeration such as coarse bark fines, perlite or leaf mould. For the one cubic yard a 4 by 8 foot bed needs at 10 inches, that is about 16 cubic feet of topsoil, 8 of compost and 3 of bark or perlite.
Do not fill a bed with pure compost or pure bagged potting mix. Compost alone holds too much water, shrinks dramatically as it finishes breaking down, and runs out of structure by mid-season. Potting mix is designed for containers and is far too expensive per cubic foot at this volume: a cubic yard of it costs several times what the same volume of blended garden soil does.
Bags or bulk
Garden soil comes in 1, 1.5 and 2 cubic foot bags, so a cubic yard is 27, 18 or 13.5 bags respectively. Under about ten bags, bagging is the easier choice: no delivery fee, no pile on the driveway, and you can buy it as you go. Past that, a bulk yard delivered is commonly half the price per cubic yard even after the delivery charge, and one bed is often already at that threshold.
Bagged soil is also lighter than the same volume delivered loose, because retail bags are screened, dried and usually blended with peat or bark. Do not assume a 40 lb bag and 40 lb off a bulk pile fill the same space: bagged product runs about 0.75 cubic feet for a 40 lb bag, while bulk topsoil is 75 to 85 lb per cubic foot.
Filling several beds, and the crossover to bulk
One 4 by 8 foot bed at 10 inches is about a cubic yard, or 24 to 27 bags depending on whether you measure inside the frame. Two beds is two yards and roughly 50 bags, around 1,300 lb of soil to move from a car to a garden, in 50 individual trips. Three or more beds is unambiguously a bulk delivery, and most landscape yards will blend a raised-bed mix to order at that volume.
The price gap is large. Bagged garden soil commonly works out to $40 to $70 per cubic yard once you total the bags, against $30 to $45 per yard for a blended bulk mix plus a flat delivery fee of $50 to $100. Two yards delivered is usually cheaper than two yards bagged, and it arrives in one pile instead of fifty bags to split open and flatten.
Common raised bed filling mistakes
Measuring the outside of the frame. A nominal 4 by 8 foot bed in 2-inch lumber holds about 11% less than 4 × 8 suggests: three bags on a 27-bag order.
Filling with pure compost or potting mix. Compost shrinks hard as it finishes decomposing and holds too much water; potting mix is formulated for containers and costs several times more per cubic foot at bed volumes.
Filling to the very top. Leave 1 to 2 inches of freeboard below the frame, or every watering washes soil over the edge.
Forgetting settling. A mix with real organic content drops 10 to 20% in the first season. Order 10% extra and expect to top up with an inch of compost each spring.
Landscape fabric under the soil. On soil, leave the bottom open so roots and worms can move through: cardboard to smother grass is enough. Fabric only makes sense over a hard surface, and then only for drainage separation.
| Material | lb / ft³ | tons / yd³ | Sold by | Typical use |
|---|---|---|---|---|
| Screened topsoil (dry) | 75–85 | 1.10 | yd³ (some ton) | New lawns, bed top-up, levelling |
| Topsoil (moist) | 95–110 | 1.40 | yd³ | As delivered after rain; weighs more |
| Fill dirt / structural fill | 90–100 | 1.25 | ton or yd³ | Raising grade, backfill, berms |
| Compost | 40–50 | 0.55 | yd³ or bag | Bed amendment, lawn top-dressing |
| Garden / raised-bed mix | 65–80 | 1.00 | yd³ or bag | Filling raised beds (soil + compost blend) |
Frequently asked questions
How much soil do I need for a 4x8 raised bed?
At 10 inches deep, 26.7 cubic feet, almost exactly one cubic yard, or 27 bags of 1 cubic foot. At 6 inches it is 16 cubic feet, about 16 bags. At 12 inches it is 32 cubic feet, or 1.19 cubic yards.
How many bags of soil for a 4x4 raised bed?
At 10 inches deep, 13.3 cubic feet: 14 bags of 1 cubic foot, 9 bags of 1.5, or 7 bags of 2 cubic feet. At 12 inches deep it is 16 cubic feet, so 16, 11 or 8 bags respectively.
How deep should a raised garden bed be?
Six inches for greens and herbs over decent soil, 10 to 12 inches for most vegetables, and 18 inches or more for root crops or any bed sitting on a patio or hard surface where roots cannot grow down.
What soil mix should I fill a raised bed with?
About 60% screened topsoil, 30% compost, and 10% coarse bark fines or perlite for aeration. For one cubic yard that is roughly 16 cubic feet of topsoil, 8 of compost and 3 of bark or perlite.
Can I fill a raised bed with just compost?
No. Pure compost holds too much water, loses structure as it finishes decomposing, and shrinks noticeably within a season. Blend it with screened topsoil at roughly one part compost to two parts soil.
How many cubic feet is a bag of garden soil?
Usually 1, 1.5 or 2 cubic feet, printed on the bag. A cubic yard is 27 cubic feet, so that is 27, 18 or 13.5 bags per yard. Soil sold by weight (a 40 lb bag) is about 0.75 cubic feet.
Is it cheaper to buy soil in bags or in bulk?
Bulk, past about ten bags. Bagged garden soil commonly works out to $40 to $70 per cubic yard before you have lifted anything; bulk blended soil is often $30 to $45 per yard plus a flat delivery fee. One 4 by 8 foot bed is already at the crossover.
Do I measure the inside or the outside of the bed?
The inside. A bed described as 4 by 8 feet built from 2-inch lumber has an inside footprint nearer 3.7 by 7.7 feet, which is about 11% less volume: roughly three bags on a 27-bag order.
Why does my raised bed soil level drop?
Organic matter in the mix keeps decomposing and the whole bed settles, typically 10 to 20% in the first season. Order about 10% extra at the start, and plan to top up with an inch or two of compost each spring.
Can I put anything in the bottom of a raised bed?
In a tall bed, yes: branches, coarse woody material or upturned turf in the bottom third saves a lot of bought soil. It breaks down over a few years so the level drops and needs topping up. Calculate only the depth you are filling with purchased mix.
How many bags of soil for a 4x8x12 raised bed?
At the nominal 4 × 8, 32 cubic feet: 32 bags of 1 cubic foot, 22 bags of 1.5, or 16 bags of 2 cubic feet. Measured inside a 2-inch frame it is 28.1 cubic feet, so 29, 19 or 15 bags. That is 1.19 cubic yards nominal, order bulk.
How much does a cubic yard of raised bed mix weigh?
About 1 ton, or 2,000 lb, for a blend at 65 to 80 lb per cubic foot. That is lighter than screened topsoil at 1.1 tons because of the compost and bark fines in it, and much lighter than wet topsoil at 1.4.
Can I reuse last year raised bed soil?
Yes. The level will have dropped 10 to 20%, so top up with an inch or two of compost and fork it into the top few inches rather than replacing the whole bed. Full replacement is only warranted after a soil-borne disease problem.
What can I put in the bottom of a raised bed to save soil?
In a bed 18 inches or deeper, branches, wood chips, leaves or upturned turf in the bottom third. It breaks down over three to five years, so the level keeps dropping and needs annual topping up. Do not do it in a 10-inch bed: there is no depth to spare.
How many bags of soil fill a 2x8 raised bed?
At 10 inches deep, 13.3 cubic feet nominal: 14 bags of 1 cubic foot, 9 of 1.5, or 7 of 2 cubic feet. A narrow 2-foot bed is the one case where bags genuinely win, since it is half a cubic yard and awkward to order bulk.