Garden Soil Calculator

How to figure garden soil for a bed

To use a garden soil calculator, multiply the bed’s length, width and fill depth to get cubic feet, then divide by 1.5, the volume of one bag. The calculator rounds up to whole bags, because the store won’t sell you 0.22 of one. It also shows the cost at the per-bag price you enter.

Measure the inside of the bed and the depth you plan to fill to, which is usually an inch or two below the rim.

The formula in plain words

Bed volume in cubic feet is length × width × depth in feet. Depth in inches gets divided by 12 first. Bags are the cubic feet divided by 1.5. Bagged garden soil comes in several sizes, and this page assumes 1.5 cu ft, so check the number printed on your bag before you trust the count.

Worked example, one raised bed

An 8 ft × 4 ft bed, filled 8 inches deep, with soil at $8 a bag:

• Depth: 8 ÷ 12 = 0.667 ft
• Volume: 8 × 4 × 0.667 = 21.33 cubic feet (0.79 cubic yards)
• Bags: 21.33 ÷ 1.5 = 14.22
• Rounded up: 15 bags
• Cost: 15 × $8 = $120

The extra 0.78 of a bag is your cushion for settling.

Worked example, a deeper bed

A 12 ft × 4 ft bed for tomatoes, filled 10 inches deep:

• Volume: 12 × 4 × (10 ÷ 12) = 40 cubic feet
• Bags: 40 ÷ 1.5 = 26.67, rounded up to 27 bags
• Cost: 27 × $8 = $216

At 27 bags you are carrying about 1.5 cubic yards of soil in 1.5 cu ft pieces, so ask a supplier what bulk costs.

Bags needed by bed size and depth

Bed size	6 in	8 in	10 in	12 in
4 × 4 ft	6	8	9	11
4 × 8 ft	11	15	18	22
3 × 6 ft	6	8	10	12
4 × 12 ft	16	22	27	32

Every figure is rounded up to a whole bag.

How to measure

• Use the inside dimensions. A bed built from 2-inch lumber is 3 to 4 inches smaller each way than the outside.
• Fill depth is measured from the bed floor, so subtract any rough fill you are putting underneath.
• For a curved bed, split it into a rectangle and a half circle and add the two volumes.
• For a garden dug into the ground with no frame, measure the footprint and the depth you will dig and amend.

Common mistakes

• Using the outside dimensions of the frame. That overbuys by a few bags on a small bed.
• Forgetting that soil settles. New beds sink an inch or more after the first heavy rain, so keep a bag or two spare.
• Mixing up bag sizes. A 1 cu ft bag and a 2 cu ft bag differ by double. Read the label.
• Filling a bed that sits on bare ground with no thought for the layer below. Loosen the ground first so roots can go deeper.

Bags or bulk?

One cubic yard is 27 cubic feet, which equals 18 of these 1.5 cu ft bags. At $8 a bag, a yard bought in bags costs 18 × $8 = $144. Your 8 × 4 ft bed at 8 inches needs 0.79 yards, so bagged soil is reasonable there. The 12 × 4 ft bed at 10 inches needs 1.48 yards, which is 27 bags and $216.

Once you pass a yard or so, ask a landscape supplier for a bulk garden soil price and compare it with 18 times your bag price. Add the delivery fee and the cost of a wheelbarrow rental if you don’t own one. A small truck bed or trailer also works if you can load it yourself. Bulk soil sits on your driveway until you move it, so the bag option is easier if you only have a short window.

Related pages

If you are sizing a frame first, the raised bed soil calculator works from the bed’s dimensions. Plain fill is cheaper per yard, and the topsoil calculator handles that. For the yearly top dressing, use the compost calculator.