Real pots are tapered. Grow bags are lumpy. Most calculators only handle perfect cylinders. This one handles the shapes you actually own.
Calculate Your Soil VolumeUse these ratios when mixing your own soil. Each recipe is given as parts by volume. A "part" can be any measuring cup or bucket as long as you use the same one for each ingredient.
Tomatoes and peppers need rich, moisture-retentive soil with good drainage.
Most herbs prefer soil that dries out between waterings. Extra perlite helps.
Fast drainage is the goal. Never use pure garden soil for succulents.
Petunias and marigolds bloom more when the soil holds steady moisture.
This mix works for pothos, philodendrons, and most tropical foliage plants.
Container trees need weight and structure. Topsoil adds both.
The number one mistake is filling soil right to the top edge. When you water, the water overflows and takes soil with it. Leave at least one inch of space. For succulents and cacti, leave two inches. For large planters over 18 inches tall, leave two inches.
Most plastic and clay pots are wider at the top than the bottom. If you measure only the top diameter and treat the pot as a cylinder, you will overestimate the soil by 15 to 30 percent. Always measure both the top and bottom diameter for tapered pots.
Self-watering pots have a built-in reservoir at the bottom that holds water. The soil sits on a platform above it. This means the actual soil volume is less than the total pot volume. Measure the depth of the reservoir and subtract it from the total height before calculating.
Putting gravel or rocks at the bottom of a pot does not improve drainage. It actually raises the water table and can cause root rot. Use a single piece of screen over the drainage hole and fill the entire pot with your soil mix.
Fresh potting mix settles after the first few waterings. Expect the soil level to drop by about 10 percent. Add more mix after the first week if needed. This is normal and not a sign you calculated wrong.
If your container is an irregular shape, pick the closest standard shape and use the average width and depth. For example, an oval pot can be treated as a rectangle using the longest and shortest dimensions. The result will be close enough for ordering soil.
Maria has three 14-inch tapered pots for tomatoes. Each pot is 14 inches across the top, 11 inches across the bottom, and 12 inches tall. She leaves 1.5 inches of headroom. The calculator shows each pot needs 11.2 quarts of soil. For three pots, that is 33.6 quarts, or about 1.4 cubic feet. She uses the vegetable mix recipe: 40 percent compost, 30 percent coco coir, 20 percent perlite, and 10 percent vermiculite. She mixes it all in a graduated bucket so the ratios are exact. No wasted soil. No second trip to the garden center.