Estimating soil volume is easier than it sounds. All you need is this simple formula.
When you’re ordering soil in bulk, getting the right amount is crucial. One or two bags of potting soil should be fine fora small number of flower pots,. But a new raised garden bed or a similar landscaping project will take a lot more than a few bags of Miracle Gro.
You might be wondering, “How much soil do I need?” Unlike the bulk section at the grocery store, ordering soil is not about weight. The answer is all about volume.
Soil is sold by cubit feet and yards, not pounds or ounces. Joe Raboine, director of residential hardscapes at Belgard describes one cubic yard as a 3-ft. x 3-ft. x 3-ft cube.
Most of the time, you won’t be working with a perfect cubic yard. To estimate soil volume for any area, all you need is a tape measure.
“The basic formula is simple: Length x Width x Height = Volume,” says Michael Dean, co-founder of Pool Research. Then divide the number of cubic feet by 27. So one cubic yard = 27 cubic feet = 1,728 cubic inches.
How To Estimate Soil Volume: Two Examples
Let’s start with a larger example from Raboine, for a 40-foot by 50-foot yard that requires six inches of soil. First, he says, calculate the volume at a depth of one foot. “To convert to six-inch depth, simply divide in half,” he says.
Dimensions
Cubic Feet
Cubic Yards
40 feet long x 50 feet wide x 1 foot deep
2,000
74
40 feet long x 50 feet wide x 6 inches deep
1,000
37
Here’s a smaller example: For this raised garden bed, with dimensions of 84 inches long x 36 inches wide x 28.25 inches deep, there are two ways to run the formula. Raboine first finds the number of cubic inches, then divides down to cubic yards.
Dimensions
Cubic Inches
Cubic Feet
Cubic Yards
84 inches long x 36 inches wide x 28.25 inches deep
85,428
49.4
1.8
Dean first converts the initial dimensions from inches to feet, then runs the formula.
Dimensions
Cubic Feet
Cubic Yards
7 feet long x 3 feet wide x 2.3 feet deep
48.3
1.8
The numbers have been rounded to the nearest tenth.
Finally, these examples are for estimating a simple rectangle. For an L-shaped raised garden bed or other unique area, divide it into rectangles or squares and find the volume of each.
Soil Calculator Tools
If math isn’t your thing, there are even easier ways to find how much soil you need. Plenty of free soil calculators can be found online. Some soil and landscape retailers build calculators into their websites.
Dean recommends the Omni Calculator. You type in your dimensions in centimeters, inches, feet or several other units of measurement. It will also give you a cost estimate ifyou enter a price per unit.
Soil can settle over time, so you may need to add more after a year or two. This is where bags from the garden center again come in handy. A bag of garden soil from The Home Depot is usually between one and two cubic feet.
Length in feet x Width in feet x Depth in feet (inches divided by 12).Take the total and divide by 27 (the amount of cubic feet in a yard). The final figure will be the estimated amount of cubic yards required.
Convert all measurements to the same unit. For example, if your bed is 6 feet long, 3 feet wide and 16.5 inches high, convert the inches to feet so your new measurements are 6 x 3 x 1.4 feet. Multiply all the measurements together: 6 x 3 x 1.4 = 25.2 cubic feet. Divide the answer by 27 to get the number of cubic yards.
If your garden is a rectangle or square, doing some basic math to find the volume of soil, mulch, or compost you need isn't all that difficult. Using the graphic below as a reference, you can see we want to multiply in feet the length x width x height ÷ 27 to get the volume in cubic yards of soil that you need.
Establish the depth – thickness of the topsoil layer. Let's say it's d = 0.5 yd . Multiply the area and dirt layer thickness to obtain its volume: 56 * 0.5 = 28 yd³ . The volume of soil required is equal to this value.
To calculate how much topsoil you need, simply measure the dimensions of the area you wish to cover and multiply this by the depth required. Measuring in metres is simplest, as this will give you the volume of topsoil you need in cubic metres. For example: A site has an area of 10m x 15m and needs a coverage of 150mm.
To install a new lawn, plan to spread a layer of 3 to 6 inches of topsoil before sowing seed or laying sod, depending on the quality of your existing soil.
They should have at least 8 inches of soil depth to accommodate the root systems of plants, because the majority of plant roots require 6 – 8 inches of soil for healthy root growth. A depth of 8 – 12 inches will suffice for most gardening situations.
Explained: Calculate the volume of the soil by multiplying Pi by the Radius Square by the Hole Height. Most likely the soil desired amending depth with be between 12″-18″ so we'll need to convert this to feet. We do so by dividing the depth in inches by 12. For example 18″ soil depth = 18/12 = 1.5 ft.
A standard 5-gallon bucket typically holds approximately 0.67 cubic feet of soil. That's about 2/3 of a cubic foot bag of soil, and each bag will weigh approximately 40 pounds of soil. This estimation assumes the bucket is filled to the brim without any compaction or settling of the soil.
Soil capacity is measured in pounds per square foot (psf). The load-bearing capacity of soils range from 4,000 psf for sedimentary rock to as little as 1,500 psf for clay and silt. Soils that are a predominantly sand or gravel typically fall in the middle, from 2,000-3,000 psf.
For these measurements, ppm is converted to lb/acre by the following formula: lb/acre = ppm x 0.3 x depth increment in inches. For example, a 10 ppm nitrate N test on a soil sample taken to a 24 inch depth would convert to 72 lb/acre (10 ppm x 0.3 x 24 inches).
How much will 1 tonne of topsoil cover? Topsoils can vary in screening sizes, therefore some topsoils may be denser than others. Generally speaking, 1 tonne of topsoil will cover 0.63 cubic metres or 22 cubic feet or 0.81 cubic yards.
The basic method for measuring soil fertility consists of mixing a soil sample with water and chemically extracting the N, P, and K as nitrate, phosphate, and potassium. The N, P, and K amounts in the sample are determined by comparing the solution to a color chart.
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