How to Calculate Soil for Your Garden

The one formula that handles almost everything

Measure first, then buy

The biggest mistake first-time bed-builders make is estimating from memory. A 4 x 8 foot bed looks smaller on graph paper than it does in the yard. Measure the interior dimensions of the bed (not the outside of the boards), decide on fill depth, and write the three numbers down before you call the landscape supplier or load a cart at the garden center.

• Length and width: Measured on the inside of the frame, in feet. A 4 x 8 bed with 2-inch-thick boards has an inside footprint of about 3.83 x 7.83 feet.
• Fill depth: How deep the soil will sit. Most raised beds are filled 10 to 12 inches deep, which is 0.83 to 1.0 foot. Shallower for salad beds, deeper for root crops.
• Unit discipline: Keep all three numbers in feet. Inches in one spot and feet in another is how people end up 30% short on delivery day.

Raised beds: length x width x depth

For any rectangular raised bed, multiply the three dimensions in feet to get cubic feet. A 4 x 8 foot bed filled 12 inches deep needs 4 x 8 x 1 = 32 cubic feet of soil. That is just over one cubic yard (27 cubic feet), so you would order one yard delivered or around sixteen 2-cubic-foot bags.

• One 4 x 8 bed, 10 in deep: 4 x 8 x 0.83 = 26.6 cubic feet (just under 1 cubic yard).
• One 4 x 8 bed, 12 in deep: 4 x 8 x 1.0 = 32 cubic feet (about 1.2 cubic yards).
• Two 4 x 4 beds, 12 in deep: 2 x (4 x 4 x 1.0) = 32 cubic feet (about 1.2 cubic yards).
• Odd shapes: Break the bed into rectangles, calculate each, then add. L-shapes, hexagons, and stepped beds all work this way.
• Order 5% extra: Soil settles after the first watering. Buying a hair more than the math says keeps you from driving back to the garden center in week two.

Get exact volumes for your beds and pots with the Container Garden Soil Calculator.

Containers and pots: work in gallons or quarts

Bagged potting mix is usually labeled in dry quarts or cubic feet, while pots are labeled by gallon or diameter. Keep the math simple: 1 cubic foot of soil fills roughly 7.5 US gallons of pot volume. A typical 12-inch-wide pot is about 3 gallons, and a 5-gallon bucket planter really does take close to 5 gallons of mix.

• 1 cubic foot of mix = ~7.5 gallons of pot volume. So a 1 cu ft bag fills roughly two 3-gallon pots or one large 7-gallon tomato pot.
• Small pots (6 to 8 in): About 1 to 2 dry quarts each. A 10-quart bag fills five or six.
• Medium pots (10 to 12 in): About 3 to 5 dry quarts. A single 1 cu ft bag (~30 quarts) fills six to ten pots.
• Patio planters (14 to 18 in): 1 to 2 cubic feet each. Buy by the bag, not by the quart.
• Do not fill pots to the rim: Leave 1 inch of headspace so water does not run off before it soaks in.

Top-dressing an existing bed

When you are adding a thin layer of compost or black earth over an in-ground bed, you do not need a deep fill. You need square footage times depth in inches, divided by 12 to get cubic feet. That division is the step everyone forgets.

• The formula: (square feet x depth in inches) / 12 = cubic feet.
• 100 sq ft, 1 inch topdress: (100 x 1) / 12 = 8.3 cubic feet (about 4 bags of 2 cu ft each).
• 200 sq ft, 2 inch topdress: (200 x 2) / 12 = 33.3 cubic feet (just over 1 cubic yard).
• 500 sq ft, 2 inch topdress: (500 x 2) / 12 = 83.3 cubic feet (about 3.1 cubic yards).
• Typical topdress depths: 1 inch for established beds in spring, 2 inches for neglected beds, 3 inches if you are trying to rebuild soil over a single season.

Converting cubic feet to cubic yards and bags

Landscape suppliers sell in cubic yards. Garden centers sell in bags of 1, 1.5, 2, or 3 cubic feet. Most home gardeners end up straddling both, so keep these conversions on a sticky note.

• 1 cubic yard = 27 cubic feet. So a single yard of delivered soil fills nine bags of 3 cu ft, or fourteen bags of 2 cu ft.
• 1 cubic yard covers about 100 sq ft at 3 inches deep, or 160 sq ft at 2 inches deep, or 325 sq ft at 1 inch deep.
• One 2 cu ft bag weighs 40 to 50 lb for most topsoil and compost blends. Ten bags is already a car-bending load.
• The crossover point is around 10 to 12 bags. If you would need more bags than that, bulk delivery is cheaper per cubic foot and easier to handle.

Bags vs bulk: when to switch

Bagged soil costs more per cubic foot but is clean, dry, and easy to store in a garage. Bulk soil is 30 to 60% cheaper per cubic foot but arrives as a pile in your driveway that you have to move with a wheelbarrow before the next rain.

• Choose bags when: You need less than 1 cubic yard total, you do not have a wheelbarrow, you are mixing specialty blends (seed-starting mix, cactus mix), or the bed is far from anywhere a truck can dump.
• Choose bulk when: You need 1 cubic yard or more, you have a wheelbarrow and a free afternoon, your driveway can hold a pile, and the delivery charge plus the yard cost still comes in under the bagged price.
• Half-yard orders exist: Many suppliers deliver as little as 0.5 cu yd, which is the real break-even point for a single large raised bed.
• Cover bulk the same day: A blue tarp or even a few sheets of plywood keeps the pile from turning into mud in a summer storm.

What to buy: fill soil, topsoil, garden soil, or potting mix

These names are not interchangeable. Using the wrong one is a more expensive mistake than buying too much, because you cannot undo it without digging the bed out again.

• Fill soil: Cheap subsoil used only to bring low ground up to grade. Do not plant in it.
• Topsoil: Screened dark soil, good for lawns and top-dressing, but usually low in organic matter. Mix it with compost if you are planting in it.
• Garden soil or raised-bed mix: A blend of topsoil, compost, and sometimes peat or coir, made to plant into directly. What most raised beds actually want.
• Potting mix: Peat, coir, perlite, vermiculite. Light and fluffy. Do NOT use plain garden soil in pots; it compacts, drowns roots, and drains badly.
• The budget rule for new raised beds: Bottom two-thirds topsoil or screened loam, top one-third high-quality compost or raised-bed mix. Much cheaper than filling the whole thing with premium blend.

Account for settling and topping up

Fresh garden soil and compost settle 10 to 20% in the first season as organic matter breaks down and water compacts the pile. Plan for it so your bed does not look half-empty in August.

• First season: Fill 1 to 2 inches above the top of the boards. It will settle to flush by summer.
• Every spring after: Top up with 1 inch of compost before planting. That single habit rebuilds the bed every year.
• If a bed looks tired by year three: Pull the plants, fork the soil 8 inches deep, add 2 to 3 inches of compost, and replant. That beats replacing the soil.

FAQ

Bottom line

Three formulas cover nearly every backyard soil project. Length x width x depth in feet for beds. Pot volume in gallons (roughly 7.5 gallons per cubic foot) for containers. Square footage x inches / 12 for top-dressing. Once you have cubic feet, divide by 27 for cubic yards, switch to bulk delivery above a cubic yard, and always order a touch extra for settling.