Spreading topsoil before you putting down a new lawn is the best way to root the grass well and get a good-looking yard. However, you must know how much topsoil to use, so that you don't have too much or too little. A few easy calculations will provide the information you need to start a healthy, green lawn.
Measure the length and width of the lawn. If it is a very big lawn, you may want to invest in a rolling measuring wheel to take measurements. For a smaller area, use a measuring stick or tape. For odd-shaped yards, divide the yard into easily measurable squares or rectangles. If it helps, you may want to use string to mark these areas off so no overlap occurs. Measure each square or rectangle's length and width and write them down to keep track.
Find the area of your lawn. Multiply the length and width of each square or rectangle together to get the area of each. Add all the areas together to get the total lawn area in square feet.
Decide how deep a layer of top soil you want to apply to the lawn. Multiply the depth of the top soil by the square footage of the lawn to determine the volume of top soil. Convert inches to feet, if necessary.
For example, 2 inches is 1/6 of a foot. If you want 2 inches of topsoil put down, multiply the square footage by 0.16, the decimal form of 1/6. This gives you the cubic feet of topsoil you need.
Add 10 percent to your amount, in case your measurements are slightly off or to account for any irregular shapes. If you have figured you need 1,000 cubic feet, for example, add on another 100 cubic feet.