Potting Soil Calculator
Compute potting soil using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Formula
Round: V = pi x r^2 x h | Rectangular: V = L x W x H | Tapered: V = (pi x h / 3)(r1^2 + r1r2 + r2^2)
Volume is calculated based on container shape. For round pots, multiply pi by the radius squared by the depth. For rectangular containers, multiply length by width by depth. For tapered pots, use the frustum formula where r1 is the top radius and r2 is the bottom radius (estimated at 70% of top). Results are converted to liters, gallons, quarts, and cubic feet.
Worked Examples
Example 1: Standard Round Pot
Problem: Calculate the potting soil needed for 6 round pots, each 30 cm diameter and 25 cm deep.
Solution: Volume per pot = pi x (15)^2 x 25 = 17,671 cm3 = 17.67 liters\nTotal for 6 pots = 17.67 x 6 = 106.03 liters\nIn quarts = 106.03 x 1.057 = 112.1 quarts\nIn cubic feet = 106.03 x 0.0353 = 3.74 cubic feet\nBags needed: 2 bags of 2 cu ft, or 7 bags of 16 qt
Result: 106 liters | 28 gallons | 3.74 cu ft | ~$18.70 estimated cost
Example 2: Rectangular Window Box
Problem: How much soil for a 60 cm x 20 cm x 18 cm window box?
Solution: Volume = 60 x 20 x 18 = 21,600 cm3 = 21.6 liters\nIn gallons = 21.6 x 0.264 = 5.7 gallons\nIn quarts = 5.7 x 4 = 22.8 quarts\nIn cubic feet = 0.76 cu ft\nDry weight = 21.6 x 0.6 = 13.0 kg (28.6 lbs)\nWet weight = 21.6 x 1.0 = 21.6 kg (47.6 lbs)
Result: 21.6 liters | 5.7 gallons | Wet weight: 47.6 lbs
Frequently Asked Questions
How much potting soil do I need for my container?
The amount of potting soil depends on your container dimensions. For round pots, use the formula: Volume = pi x radius^2 x depth. For rectangular planters, multiply length x width x depth. A standard 12-inch (30 cm) round pot that is 10 inches (25 cm) deep needs about 17.7 liters or 4.7 gallons of soil. Keep in mind that potting soil compresses over time, so it is good practice to fill containers slightly above the rim and firm gently. Buy about 10% extra to account for settling and spillage.
What is the difference between potting soil and garden soil?
Potting soil (or potting mix) is a specially formulated growing medium designed for containers. It typically contains peat moss or coir, perlite or vermiculite, and composted bark, but often contains no actual soil. It is lightweight, well-draining, and sterile to prevent disease. Garden soil is heavier, denser, and can contain weed seeds, pathogens, and insects. Using garden soil in pots leads to poor drainage, compaction, and root rot. Always use potting soil for containers — garden soil should only be used in the ground where it benefits from natural drainage and soil organisms.
How do I calculate soil volume for a tapered pot?
Most flower pots are tapered (wider at top, narrower at bottom), not straight cylinders. For a tapered (frustum) pot, use the formula: V = (pi x h / 3) x (r1^2 + r1 x r2 + r2^2), where r1 is the top radius, r2 is the bottom radius, and h is the height. The bottom diameter of standard nursery pots is typically 65-75% of the top diameter. Using the straight cylinder formula (top diameter only) will overestimate the volume by 15-25%. Potting Soil Calculator includes a tapered pot option for more accurate results.
How much does wet potting soil weigh?
Dry potting soil weighs approximately 0.5-0.7 kg per liter (4-6 lbs per gallon). When fully saturated with water, it weighs about 0.8-1.2 kg per liter (7-10 lbs per gallon). A 5-gallon container filled with wet soil can weigh 35-50 lbs (16-23 kg). This weight is important for balcony gardens (structural load limits), hanging baskets (hook strength), and when moving containers. Lightweight mixes with extra perlite weigh less, while mixes with more compost or moisture-retaining additives weigh more. Always consider wet weight when placing containers on elevated surfaces.
Should I fill the entire pot with potting soil?
For most plants, filling the pot completely with potting soil is recommended. However, for very deep containers (over 18 inches) growing shallow-rooted plants, you can place a filler in the bottom third — such as empty plastic bottles, packing peanuts, or a layer of gravel covered with landscape fabric. This reduces soil cost and weight while maintaining drainage. Never use rocks alone as drainage without landscape fabric, as fine soil particles wash into the gaps and create a perched water table. For most standard-sized containers (up to 14 inches), fill entirely with quality potting mix for best results.
What is soil composition and how is it measured?
Soil is composed of minerals (45%), organic matter (5%), water (25%), and air (25%). Texture is classified by percentages of sand (0.05-2mm), silt (0.002-0.05mm), and clay (less than 0.002mm) using the USDA soil texture triangle. Loam, an ideal garden soil, has roughly equal parts of each.