Potting Soil Calculator
Buying too much potting soil wastes money; buying too little means an extra trip to the store. This calculator figures out exactly how much you need based on your container shape, dimensions, and how many pots you are filling.
Potting soil is sold by volume, typically in quarts or cubic feet. A standard large bag is 1 cubic foot (about 25.7 dry quarts or 28.3 liters). Smaller bags are usually 8 or 16 quarts. To figure out what you need, measure the inside dimensions of your container and calculate the volume.
For round pots, volume = pi x radius squared x depth. For rectangular raised beds, it is simply length x width x depth. Most pots taper toward the bottom, so the actual volume is a bit less than the calculated volume. A good rule of thumb is to buy about 10% less than the calculated amount, or just fill the bottom of deep pots with filler material like empty water bottles or packing peanuts to save on soil.
Raised beds use a lot more soil than you might expect. A standard 4 x 8 foot bed that is 12 inches deep needs about 32 cubic feet of soil mix. Many gardeners fill the bottom third with logs, branches, or leaves (a technique called hugelkultur) and top it with quality potting mix to reduce cost.