Calculating Potting Mix Needs: 5 Tips

For home gardening and plant care, having a ready supply of potting mix around is extremely helpful. In the springtime, however, when people are beginning to replant and prepare for the growing season, you may have a large assortment of pots of many different sizes. If this is the case, you might want to spend some time calculating how much soil potting mix you need so you don’t immediately buy a truckload. There are some basic calculations you can make that will tell you exactly how much potting soil you need for each pot. These tips will help you purchase the right amount of soil to start off the growing season. Whether you are using soil or soilless potting mix, the calculations will be the same.

  1. Gather your pots of all shapes and sizes before you plant. In some cases, the volume capacity of the pot is molded to the bottom of the pot. Since just about anything can be made into a pot, this trick will only work for specifically manufactured planters.
  2. To find the cubic volume of the square or rectangular shaped pots, simply multiply the length by the width by the height. For instance, a rectangular pot that is 5 inches deep, with sides of 8 and 10 inches will have a volume of 400 cubic inches.
  3. There is a simple equation to find the volume of round pots as well. Keep in mind that many round pots taper towards the bottom. This equation is most accurate for straight sided round pots, but for pots without straight sides compute the volume then, as you fill up the pot, withhold a cup or two of potting mix and eyeball it. The equation for volume of cylinder: measure the height and the radius. Remember, the radius is from the center point in the (open) top to the edge. Multiple the height by the radius squared by pi (pi = 3.14). The equation looks like this: r²h x 3.14.
  4. If you use any odd shaped pots for the planting, determining the volume is best done another way. Use the 1-cup measuring cup from your kitchen, plug the drain holes in the pot, and begin to fill it with water, one cup at a time. Don’t fill it all the way to the top; rather, stop with about an inch left from the top. Make sure you count how many cups go into the pot before you dump out the water. Now that you know how many cups go into the pot, remember that 1 cup equals roughly 14.5 cubic inches.
  5. When purchasing potting soil or soilless mix, the volume is always found on the manufacturer’s label. If you buy in bulk, you might have a harder time knowing exactly how much you are getting. The volume is usually noted in cubic inches or centimeters. 1 cubic inch equals about 16.4 cubic centimeters. That is helpful to know if you have to convert figures. Add up the total amount of volume from all your pots, convert if necessary and you will have a very accurate idea of how much potting mix you need.

    Calculating potting mix needs is very easy. It’s simply a matter of determining volume and converting figures. The best part is you get to revisit those pesky equations you learned in high school.