Calculating topsoil weight isn't exactly rocket science. However, it isn't exact science, either. Topsoil weight varies, depending on the type and consistency of soil and its water content. Sand is lighter than clay. Loam soil, a mixture of silt and sand, is the best soil; silt adds nutrients.
The topsoil may contain more water content due to recent rains or because of the consistency of the soil: Clay holds more water than sand, for example.
Topsoil is usually measured by the cubic yard—27 cubic feet. One cubic yard of topsoil weighs, generally, one ton—or 2,000 pounds. One ton of topsoil will roughly fill 300 square feet to a depth of 1 inch.
Things You'll Need
 Measuring tape
 Calculator
 Pencil and paper
Calculate Manually

Measure the length in feet of the area of topsoil.

Measure the width in feet of the area of topsoil.

Measure the depth in inches of the area of topsoil. Convert this measurement into fractions of a foot. For example, if the depth measures 4 inches, divide the 4 inches by 12 inches to get 0.33 feet.

Multiply the length times the width times the depth of the topsoil. For example, if you measured 10 feet by 10 feet by 0.33 feet, you would get 33 cubic feet.

Convert cubic feet into cubic yards. Multiply the answer in Step 4 by 0.037, since 1 cubic foot equals 0.037 cubic yard. For example, 33 cubic feet multiplied by 0.037 cubic yards is 1.2 cubic yards. Since 1 cubic yard is roughly one ton of topsoil, 1.2 cubic yards is a little more than one ton of topsoil.
Use a Conversion Calculator

Select an online conversion calculator to determine cubic yards.

Enter the width in feet of the area of topsoil.

Enter the length in feet of the area of topsoil.

Enter the depth in inches of the topsoil.

Press the button marked "Calculate." View the approximate total in cubic yards. Remember that 1 cubic yard is roughly one ton of topsoil.
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References
 Photo Credit Arid soil image by Igor Baryshev from Fotolia.com