Raised beds and container gardening are becoming more popular, since they are easier to maintain and allow you more control over soil quality. It can be a challenge to estimate how many bags of topsoil you'll need to fill up your bed, and to get the right amount of soil, you'll need to calculate the volume of your planter box. A little simple math can get you on your way so that you can fill up your planter and stick some geraniums or tomatoes in there.
Use a tape measure, ruler or yardstick to measure the width of your planter box. Write this number down. As an example, let's say that your planter is two feet wide.
Measure the length of your planter, and write this number down, too. Let's say the planter is four feet long.
Multiply the width by the length to find the square footage of the base of the planter. 2 feet x 4 feet = 8 square feet.
Measure the height of the planter, and write that number down. Let's say the planter is one and a half feet tall.
Multiply the height of the planter by the square footage of the base of the planter, calculated in step 3. Eight square feet x 1.5 feet = 12 cubic feet. The volume of a planter that is two feet by four feet by one and a half feet is 12 cubic feet.
Calculate the volume of a circular planter by measuring the diameter, dividing that in half to find the radius, multiplying it by 3.14 (pi, roughly), then multiplying that number by the height of the planter. For example, a circular planter that is five feet wide has a radius of two and a half feet. Multiplying 2.5 by 3.14 gives you 7.85. If the planter is two feet tall, you would multiply 7.85 by 2 to get 15.7. A circular planter that is five feet across and two feet deep has a volume of 15.7 cubic feet.