liminfoPlant Watering Calculator
Free web tool: Plant Watering Calculator
Volume
1131.0 cu in
Soil Needed
16.8 dry qt
Gallons
4.90
Liters
18.53
About Plant Watering Calculator
The Pot Size Calculator computes the volume of a round or square planter so you know exactly how much potting soil to buy. For round pots, it uses the cylinder formula: π × (diameter/2)² × depth. For square pots, it uses: side × side × depth. The tool outputs four units simultaneously — cubic inches (raw volume), U.S. gallons (for liquid capacity reference), liters (metric standard), and dry quarts (the unit printed on most potting mix bags in the US).
Home gardeners, indoor plant enthusiasts, balcony gardeners, and professional horticulturists all rely on accurate volume estimates before purchasing soil. Buying too little soil means a second trip to the garden center; buying too much wastes money and storage space. The dry quart output is especially practical because a standard 8 dry quart bag of potting mix holds 537.6 cubic inches — the calculator directly tells you how many such bags you need.
All computation runs locally in your browser with no data sent to any server. The tool supports both round and square pot shapes via a toggle button, and updates results instantly as you type. Dark mode and a fully responsive layout make it easy to use on a phone while standing in a garden center aisle.
Key Features
- Round pot volume: π × radius² × depth with automatic radius calculation from diameter
- Square pot volume: side length² × depth for rectangular planters
- Output in 4 units: cubic inches, U.S. gallons, liters, and dry quarts simultaneously
- Dry quarts output matches potting mix bag labeling for direct purchase planning
- Toggle between round and square pot modes with a single click
- 100% client-side calculation — no data leaves your browser
- Real-time update as dimensions are typed
- Dark mode and mobile-responsive layout for in-store use
Frequently Asked Questions
What is a dry quart and how does it relate to potting soil?
A dry quart equals 67.2 cubic inches and is the standard unit used on bags of potting soil, compost, and mulch sold in the United States. It differs from a liquid quart (57.75 cubic inches). When you buy an 8 dry quart bag of potting mix, it contains about 537.6 cubic inches of soil — the calculator outputs this unit directly so you can match it to bag sizes.
How do I calculate how many bags of potting soil I need?
Divide the dry quart volume shown by the bag size. For example, if your pot needs 24.5 dry quarts and you are buying 8 dry quart bags, you need about 3 bags. Since soil compresses when wet, it is often wise to round up by 10–15%.
Why does the round pot formula use π × r²?
The cross-sectional area of a circular pot is π times the radius squared (where radius = diameter ÷ 2). Multiplying area by depth gives the volume of the cylinder. For a 12-inch diameter, 10-inch deep pot: π × 6² × 10 ≈ 1,130.97 cubic inches or about 4.9 gallons.
Does the calculator account for root balls or existing soil?
No, it calculates the total geometric volume of the empty pot. For repotting, subtract the volume of the existing root ball. A common rule of thumb is that a plant's root ball occupies 20–40% of the pot volume, so you may only need 60–80% of the calculated soil amount.
What is the difference between gallons and dry quarts?
A U.S. liquid gallon equals 231 cubic inches, while a dry quart equals 67.2 cubic inches (1 dry gallon = 4 dry quarts = 268.8 cubic inches). Potting soil bags use dry quarts, not liquid gallons, so 4 dry quarts of soil is less than 1 liquid gallon in volume.
Can I use this for raised garden beds?
Yes. Treat the bed as a very large rectangular "square pot": enter the width for side length and the bed depth. The output in cubic feet can be approximated from cubic inches by dividing by 1,728. For large raised beds, many garden supply stores sell soil in cubic feet — 1 cubic foot equals 29.9 dry quarts.
How much soil does a typical 12-inch round pot need?
A 12-inch diameter, 10-inch deep round pot has a volume of about 1,131 cubic inches. That equals roughly 4.9 gallons, 18.5 liters, or 16.8 dry quarts. You would need approximately 2 bags of an 8-quart potting mix.
Is the calculator accurate for tapered pots?
The calculator assumes straight vertical sides (a perfect cylinder or rectangular prism). Tapered pots (wider at the top than the bottom) will have less actual volume than the calculation suggests. For a 15–20% taper, reduce the result by about 10% for a more realistic estimate.