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How to Calculate Topsoil for a Raised Garden Bed

Worked examples for 4×8, 4×12, and L-shaped beds, plus how deep the soil should be for vegetables versus flowers.

Updated August 25, 2026

Why a bed is easier than a lawn

A raised bed has four walls and a known inside measurement. That is the only area that matters. Do not measure the outside of the lumber and then wonder why the bag count is short. Measure the planting surface: inside length × inside width.

Depth is the second number, and it is not the height of the boards. If you build an 11-inch box and put two inches of gravel in the bottom for drainage, you have nine inches of soil room. The calculator wants the soil depth, not the lumber height.

The formula, written for a box

Cubic yards = (length in feet × width in feet × depth in inches) ÷ 324. NIST Handbook 44 states that 1 cubic yard is 27 cubic feet; 27 × 12 inches = 324 square feet at 1 inch. One cubic yard of moist screened topsoil is treated here as about 1.35 tons (a planning factor, not NRCS oven-dry bulk density). Bags are usually 2 cubic feet; there are 27 cubic feet in a yard, so one yard is 14 bags after you round up the last partial bag.

BedInside sizeSoil depthYardsTons2 cu ft bags
Common starter4 × 8 ft8 in0.81.111
Long kitchen bed4 × 12 ft10 in1.52.020
Deep tomato bed4 × 8 ft12 in1.21.616
Two 4×8 beds64 sq ft8 in1.62.222

Yards and tons match the calculator’s one-decimal display. Bag counts use unrounded volume, then round up — the same rule as the live tool.

How deep vegetables actually want

Penn State York County Master Gardeners describe a popular framed bed as 4 × 8 ft by 1 ft deep — enough for most annual vegetable roots. Minnesota Extension says simple mounded beds are often 6–8 inches, and that a bed on pavement may want 1–2 feet so roots are not baking. This site’s 8-inch garden preset sits in that land-grant band. Flowers and perennials are happy at 8 inches if the native ground below is not a sealed clay pan.

If the bed sits on grass, strip the sod or smother it. Do not count the dead turf as soil depth. If the bed sits on a patio and has a solid bottom, every inch has to come from the bags or the bulk load. Those closed-bottom beds also weigh more than people expect. An 8-inch 4×8 is about 0.8 yards — roughly a ton at this site’s 1.35 factor, more if the mix is wet.

L-shaped and multi-bed layouts

The calculator’s L shape is two rectangles that do not overlap. Measure the long run and the short run separately, then add them. A 12×4 bed with a 6×4 wing is 48 + 24 = 72 square feet. At 8 inches that is 72 × 8 ÷ 324 = 1.8 cubic yards, about 2.4 tons, or 24 of the 2-cubic-foot bags.

If you have three identical 4×8 beds, do not invent a new shape. Multiply one bed by three. One 4×8 at 8 inches is 0.8 yards; three beds are 2.4 yards. That is one small dump-trailer load for most landscape yards, or 32 bags if you are carrying them yourself.

What to put in the bed

Screened topsoil alone is usually too dense for a raised bed. It holds water and then crusts. Minnesota Extension recommends about ½–⅔ topsoil with ⅓–½ plant-based compost. A bagged “raised bed mix” that already looks like that is fine. If you buy bulk, ask the yard whether the pile is screened and whether it has been sitting wet. Wet soil is sold by the yard, but it weighs more and shrinks when it dries.

Do not fill a food bed with fill dirt. Fill dirt is subsoil. It is for raising grade, not growing tomatoes. If the only cheap material nearby is “garden blend,” ask what the blend is. Some yards mix sand into clay and call it garden soil. That can work, but it is not the same as a composted planting mix.

Bags versus a bulk drop

Bags win when you have stairs, a locked gate, or one small bed. They lose as soon as you need more than about a yard. Twenty bags of 2-cubic-foot mix is one cubic yard plus change, and you will handle each bag twice. A bulk drop for 1.5 to 2 yards is usually cheaper even after a delivery fee, provided the truck can reach the side yard.

If the truck cannot reach the beds, budget time to wheelbarrow. A contractor wheelbarrow holds about 3 cubic feet struck, closer to 2.5 when you do not want it slopping. One cubic yard is roughly 10 to 11 loads. Two yards is an afternoon.

A full walk-through

Suppose you are building two 4×8 beds on a patio, 10 inches of soil, no gravel. Area is 64 square feet. Volume is 64 × 10 ÷ 324 = 2.0 cubic yards. Weight is about 2.7 tons if the mix is as dense as screened topsoil; a lighter compost blend will weigh less. Turn on 10% waste if you want a leftover pile for settling: 2.2 yards.

Open the topsoil calculator, choose rectangle, enter 8 and 8 if you combine the two beds into one 8×8 rectangle, set depth to 10 inches, and read yards, tons, and bags. If you prefer to keep the beds separate, run 4 × 8 × 10 once and double it. The two methods agree.

Sources

Numbers on this page are planning figures. Follow the link for the original table or standard. Full list: Sources.

  1. NIST. Handbook 44, Appendix C — General Tables of Units of Measurement. 1 cubic yard = 27 cubic feet. Coverage identity: 27 × 12 = 324 sq ft at 1 inch.
  2. USDA NRCS. Soil Bulk Density / Moisture / Aeration (Soil Quality Kit). Oven-dry bulk density varies by texture; silt loam example ≈ 1.33 g/cm³. Not the same as a wet truckload.
  3. South Dakota State University Extension. Bulk Density is an Indicator of Soil Health. Ideal bulk-density bands by texture (NRCS table). 1 g/cm³ ≈ 62.3 lb/ft³.
  4. Penn State Extension. Lawn Establishment. Minimum 4–6 inches (firmed) of quality topsoil to establish turf; blend into subsoil.
  5. University of Illinois Extension. Preparing Lawn Soil and Site. Aim for six inches or more of well-prepared soil before seed or sod.
  6. University of Nebraska–Lincoln Extension. Establishing Lawns From Sod. Replace topsoil to a minimum of about 4 inches after rough grade; sod often cut 0.25–0.5 in thick.
  7. University of Arkansas System Division of Agriculture. Establishing a Lawn From Sod (FSA-2042). About 19 cubic yards of topsoil make a 6-inch layer over 1,000 sq ft; sample the upper 4–6 inches.
  8. Angi. What Is the Cost of Topsoil? [2026 Data]. National homeowner bands: about $10–$55/yd³ by type; screened loam $18–$26; delivery often $50–$150/trip.
  9. Lawn Love. How Much Does Topsoil Cost in 2026?. Bulk topsoil commonly cited at about $10–$50 per cubic yard, plus delivery.
  10. University of Minnesota Extension. Raised bed gardens. Mounded beds often 6–8 in high. Mix about ½–⅔ topsoil with ⅓–½ plant-based compost. Deeper (1–2 ft) if the bed sits on pavement.
  11. Penn State Extension (York County Master Gardeners). Raised Bed Gardening. A common framed bed is 4 × 8 ft by 1 ft deep — enough for most annual vegetable roots.
  12. U.S. Code of Federal Regulations. 49 CFR 571.110 — Tire selection and rims; motor vehicle. Payload / vehicle capacity weight is the number on the driver’s door-jamb label — not a generic “half-ton.”

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