Potting Soil Calculator
Estimate the soil volume needed for rectangular planters, straight-sided round pots, or tapered flower pots, then convert the result into common bag sizes.
Container details
Soil estimate
For 5 rectangular planters filled to 100%.
Purchase planning
| Package size | Equivalent volume | Packages needed | Estimated leftover |
|---|
How to use this potting soil calculator
What this calculator does
This calculator estimates the geometric volume of potting mix needed to fill one or more containers. It supports a rectangular planter, a straight-sided round pot, and a tapered flower pot shaped like a truncated cone. The result is shown in liters, cubic feet, and U.S. quarts, with an additional whole-bag estimate for common package sizes. It is a planning estimate, not a measurement of soil weight, nutrient content, drainage performance, or structural load. Real requirements can change because of root balls, drainage material, settling, compaction, moisture, and irregular container walls.
When to use it
Use it before repotting a group of houseplants, filling balcony planters, ordering mix for a raised container, or comparing bag sizes at a garden center. It is also useful when several containers share the same dimensions and you want one combined purchase quantity instead of calculating each pot separately.
How to calculate
- Select Container shape. Choose the closest geometric match to the usable interior.
- Select Measurement unit. Existing dimensions are converted automatically when you switch among centimeters, inches, feet, and meters.
- Enter the active dimensions. A rectangular planter uses Inside length, Inside width, and Fill depth. A round pot uses Top inside diameter and Fill depth. A tapered flower pot also uses Bottom inside diameter.
- Enter Number of containers and, when appropriate, reduce Fill percentage below 100 to account for space already occupied.
- Read Total potting soil and the secondary conversions, then review the Purchase planning table. Use Download Excel to save the current inputs and results, or Reset to restore the documented example.
Input guide
Container shape is required and controls the formula. Using a cylinder for a strongly tapered pot usually overstates the volume. Measurement unit is required; all dimensions must share the selected unit, and decimal points follow the en-US convention, such as 12.5 rather than 12,5. Inside length and Inside width are positive decimal measurements used only for rectangular planters. For example, 24 cm by 18 cm means the usable interior footprint, not the outside rim.
Top inside diameter is the full distance across a round opening, not the radius. A realistic value might be 30 cm. Bottom inside diameter is required only for a tapered pot; it must be positive and no larger than the top diameter. Fill depth is the vertical depth actually occupied by mix. Entering the full pot height when you plan to leave headspace will overestimate soil. Number of containers must be a whole number from 1 to 10,000. Fill percentage accepts a decimal percentage above 0 and up to 100; 80 means four-fifths of the geometric volume. Increasing any active dimension, quantity, or fill percentage increases the result.
Output guide
Total potting soil is the combined estimated volume in liters. Per container divides that total by the number of containers. Cubic feet and U.S. quarts are equivalent volume conversions for comparing package labels. 20 L bags is a rounded-up purchasing count, so a small leftover is expected. The summary pills repeat the active shape, quantity, fill percentage, and measurement unit. In the Purchase planning table, Package size identifies the bag format, Equivalent volume expresses its capacity in liters, Packages needed rounds upward to a whole package, and Estimated leftover is purchased volume minus calculated need. A zero result is not valid because every required active dimension must be positive.
Worked example
For five rectangular planters measuring 24 cm long, 18 cm wide, and 10 cm deep, filled to 100%, one planter holds 24 × 18 × 10 = 4,320 cubic centimeters. Because 1,000 cubic centimeters equals 1 liter, that is 4.32 liters per planter. Multiplying by five gives exactly 21.60 liters. The same volume is about 0.76 cubic feet or 22.83 U.S. quarts. Since one 20-liter bag is not enough, the purchase estimate rounds up to two bags.
Learn more
For the measurement basis behind the conversions, see the National Institute of Standards and Technology guidance on SI units of volume. For practical container-growing considerations, the University of Minnesota Extension explains soil and planting choices for raised beds.
How the formulas work
A rectangular planter is modeled as a rectangular cuboid: length multiplied by width multiplied by fill depth. A straight-sided round pot is modeled as a cylinder: π multiplied by the square of the radius and by depth. Because the input is diameter, the calculator divides it by two before squaring it. A tapered flower pot is modeled as a conical frustum: π × depth × (R² + Rr + r²) ÷ 3, where R is the top radius and r is the bottom radius. Each single-container volume is multiplied by quantity and by the fill percentage expressed as a fraction.
Measurement and purchasing tips
Measure inside dimensions whenever possible. Thick walls, flared rims, false bottoms, reservoirs, and drainage layers reduce usable volume. Potting mix can also settle after watering, so the geometric estimate should be treated as a starting point. The U.S. Department of Agriculture's soil resources explain why soil structure and pore space matter; packaged potting mixes are porous materials rather than rigid solids, so actual fill behavior can vary.