Garden Soil Calculator

About the Garden Soil Calculator

Filling a raised bed, building up a border, or topping off a vegetable plot always starts with the same question: how much soil is that, really? The answer is a volume, and volume is the one thing people tend to misjudge by eye. A bed that looks small can swallow a surprising amount once you multiply the footprint by a depth of ten or twelve inches. This calculator does the multiplication for you. Enter the bed length, the bed width, the depth you want to fill, and the size of the bags you plan to buy, and it returns the bed area, the volume, and the number of bags.

Under the hood the method is a straight volume calculation. The tool multiplies length by width for the area, then by the depth converted to feet, to give cubic feet. In U.S. mode it divides by 27 to show cubic yards, the unit used by landscape suppliers who deliver by the truckload. In Metric mode it shows liters and cubic meters. The bag count divides the total volume by your bag size and rounds up, since you cannot buy a fraction of a bag.

The result is useful in two ways. For small projects, the bag count tells you exactly what to load into the cart. For larger ones, the cubic yard or cubic meter figure is what you tell a bulk supplier, which is often cheaper per unit than bagged product. Either way, knowing the volume before you shop keeps you from the two common outcomes of guessing: a half-filled bed or a heap of leftover soil on the driveway.

When to Use It

How to Use It, Step by Step

1. Choose U.S. or Metric. U.S. mode uses feet for the bed, inches for depth, and cubic feet for bag size. Metric mode uses meters, centimeters, and liters.
2. Enter the Bed length of the planting area, measured on the inside of the frame, because soil only fills the space within the boards.
3. Enter the Bed width the same way. For a round or irregular bed, measure the longest length and the average width, or break the shape into rectangles and add the results.
4. Set the Soil depth. The field starts at 6 inches (15 cm), but many vegetable beds are filled to 10 to 12 inches, while a top-dressing of compost may only need an inch or two.
5. Enter the Bag size from the label on the product you will buy. Standard bags vary between roughly 0.75 and 3 cubic feet, or 20 to 100 liters, so check the product rather than assuming.
6. Calculate and read Bed area, Volume needed, the volume in cubic yards or cubic meters, and the number of bags.
7. Compare the bag total with a bulk price. If the volume is close to one cubic yard or more, delivered bulk soil is often more economical than bags.
8. Plan for a small extra. Add a little to allow for settling, and keep a bag or two to fill dips after the first watering.

Understanding Your Results

Bed area confirms that your length and width were entered correctly, since it is just the product of the two. Volume needed is the area times the depth, and it is the most important line, because the bag count and the cubic yard figure are both derived from it. If the volume seems far too high or low, the usual cause is a depth typed in the wrong unit, such as feet in place of inches.

The cubic yard figure in U.S. mode is the volume divided by 27, and it is the unit bulk suppliers use. In Metric mode, the output shows both liters and cubic meters, which is helpful because bags are labeled in liters while deliveries are in cubic meters. The bag count is the volume divided by the size of your bag, rounded up. A 4 by 8 ft bed at 10 inches deep needs about 26.7 cubic feet, which is 18 bags of 1.5 cubic feet, and close to a full cubic yard.

As a rule of thumb, once you need more than about 15 to 20 bags, price bulk delivery for comparison. A cubic yard is 27 cubic feet, so it equals 18 bags of the 1.5 cubic foot size. Remember that the number shown is the soil that fits the space, not what you must buy if you plan to mix in material you already have, such as a compost pile or leftover soil from a previous season.

More Worked Examples

Tips for Better Results

• Measure inside the frame, not outside it. The thickness of the boards on a raised bed can add up to a couple of inches on each side and inflate the bag count by a noticeable amount.
• Fill to a little below the rim rather than flush with it. A gap of an inch or two at the top keeps water and mulch from spilling over when you irrigate.
• Mix sources to save money. Many gardeners fill the bottom of a deep bed with branches, leaves, or other coarse organic matter and put quality soil in the top layers, which reduces what you buy. If you do this, enter only the depth of purchased soil.
• Ask the supplier what the bag volume really means. Some bags are labeled by weight, and some by dry quarts. One cubic foot is about 25.7 dry U.S. quarts, so convert a quart label before you enter the bag size.
• Order slightly more than the calculation shows when the bed is new. Fresh soil and compost compress as they get wet and as roots take hold, and topping up later is easier when you already have some on hand.
• Compare cost per cubic foot across bags and bulk. A bagged product that looks cheap per bag may hold less than its competitor, so divide the price by the bag volume before choosing.

Common Mistakes to Avoid

• Entering the depth in the wrong unit. Six inches is 0.5 feet, so typing 6 in a feet field would give a volume twelve times too large.
• Calculating on the outside dimensions of the frame, which counts the board thickness as planting space and leads to an over-order.
• Assuming every bag holds the same amount. Bag sizes vary by brand and product, and a switch from 1.5 to 0.75 cubic feet doubles the bag count.
• Forgetting that soil is not the only filler. If you add compost, sand, or amendments, split the total volume across them rather than ordering the full amount of each.
• Ignoring the base of a deep bed. A tall bed on a hard surface may need drainage material, and a bed on soil may need loosening of the ground first, which affects how much you actually need to buy.

Assumptions and Limits

• The calculator computes the volume of a rectangular box and nothing else. Mounded, sloped, or curved beds need to be measured by average depth or divided into sections.
• Soil is treated as an incompressible volume. In reality, loose soil and compost settle after watering, and a freshly filled bed can drop by a noticeable amount.
• The tool does not know your soil type or mix. Topsoil, compost, and potting blends differ in weight and in how well they hold water, so check with a nursery for a recipe that suits your plants.
• Bag sizes on labels can vary slightly from the actual volume, and moisture changes density. Treat the bag count as a good estimate and budget for a little more.

Key Terms

Cubic yard

A volume equal to a cube one yard on each side, or 27 cubic feet. It is the standard unit for bulk soil, mulch, and compost deliveries.

Topsoil

The upper layer of natural soil, rich in organic matter. Bagged or bulk topsoil is often blended with compost before sale.

Compost

Decomposed organic material that adds nutrients and improves soil structure. It is typically mixed into native soil or a raised bed fill.

Raised bed mix

A blend made for containers and framed beds, often combining topsoil, compost, and an aerating material such as perlite or bark.

Top-dressing

A thin layer of compost or soil spread over the surface of an existing bed or lawn, usually an inch or two deep.

Settling

The gradual sinking of loose fill as air pockets collapse after watering and rain, lowering the surface of a newly filled bed.