How to Calculate Raised Bed Soil in Five Simple Steps

Raised bed soil in one line

Raised bed soil is simple: bed area, times depth in feet, divided by the bag size, plus waste, rounded up. A 12 ft by 4 ft bed at 18 in deep is 48 square feet, 1.5 feet deep, 72 cubic feet or 2.67 cubic yards — 36 bags exact at 2 cubic feet per bag, 40 bags with 10% waste. The rest of this guide is about getting those inputs honest: the length, the width, the depth, and the bag size.

Step 1: Measure the bed, length and width

Measure the bed in feet, length and width, on the inside of the frame, not the outside. The worked example is 12 ft long and 4 ft wide, the inside measurement is the one that holds soil: a thick frame steals inches off each dimension, and a bed measured outside over-orders. If the bed is not a rectangle, measure each part separately and add the areas.

Step 2: Multiply length by width for the bed area

Multiply the length by the width for the bed area, since soil fills the bed, not the path around it. The example is 12 times 4, or 48 square feet. Width matters as much as length: a foot wider is another 12 square feet of bed, or 18 cubic feet at 18 in deep.

Step 3: Turn the depth into feet

Turn the soil depth into feet before multiplying, since the length and width are in feet and the depth is usually given in inches. Divide the depth in inches by 12: 18 in is 1.5 ft, 12 in is 1 ft, and 6 in is 0.5 ft. Skipping it is the most common way to over-order: 48 times 18 is 864 cubic feet, twelve times the true 72. Measure depth to the top of the soil, not the frame.

Step 4: Multiply area by depth for the volume

Multiply the bed area by the depth in feet for the volume in cubic feet, and divide by 27 for cubic yards, since bulk soil is sold by the cubic yard and bags by the cubic foot. The example is 48 times 1.5, or 72 cubic feet, and 72 divided by 27 is 2.67 cubic yards. At 2.67 cubic yards, bags are still reasonable; a second bed the same size takes the order past 5 cubic yards, where bulk usually wins.

Step 5: Divide by the bag, add waste, round up

Divide the volume by the bag size for the exact count, add the waste for spillage and settling, then round up to whole bags, since bags are sold by the bag. Ten percent on the example is 36 times 1.10, or 39.6, rounded up to 40 bags, and the spare bag is not wasted — it is the bag the spillage and the first settling consume. Round up at the end, not at the start. Then fill, water, and top up a week later.

Raised bed soil in numbers

Suppose the bed is 12 ft long and 4 ft wide, the soil depth is 18 in, the bags are 2 cu ft, and the waste is 10%. Bed area: 12 × 4 = 48 sq ft. Depth in feet: 18 ÷ 12 = 1.5 ft. Volume: 48 × 1.5 = 72 cu ft. Cubic yards: 72 ÷ 27 = 2.67. Bags exact: 72 ÷ 2 = 36. Bags with waste: 36 × 1.10 = 39.6. Bags to buy: 40, rounded up. Order: 40 bags.

Common mistakes

The most common mistake is multiplying by the depth in inches instead of feet, which over-orders twelve times over. The second is measuring the outside of the frame, which buys soil for timber as well as bed. The third is forgetting the waste, which buys for a perfect fill and runs short at the first spill and the first settling. The fourth is rounding up before adding waste, which double-counts the rounding and over-orders. The fifth is forgetting that soil settles.

Worked example, start to finish

A vegetable bed, 12 ft long and 4 ft wide inside the frame, 18 in of soil, 2 cu ft bags, 10% waste. Bed area: 48 sq ft. Depth in feet: 1.5 ft. Volume: 72 cu ft. Cubic yards: 2.67. Bags exact: 36. Bags with waste: 39.6. Bags to buy: 40. Result: buy 40 bags, and keep one bag back for topping up. If the same bed were 16 ft long instead, the need would rise to 53 bags, but the depth would be unchanged.

What changes the answer most

The depth changes the answer most, because it is the figure most often guessed and it multiplies everything: moving from 18 in to 12 in cuts the order from 40 bags to 27 bags, while the bed never changes, and moving to 24 in lifts it to 53 bags. The length is second: every foot at 4 ft wide and 18 in deep is another 6 cubic feet. The width is third, and it moves the order exactly like the length. The bag size is fourth: it changes only the bag count, never the volume. The waste is fifth, and it changes only the top of the bag order, never the soil itself.

Frequently asked questions