THE POTTING BENCH
How much potting mix do you need?
Measure the inside of your container. Use the depth you intend to fill, leaving your chosen watering space above the mix.
YOUR MIX ESTIMATE
Measure.
Calculate.
Get growing.
Your results will appear here.
An estimate for empty containers. Subtract the volume of existing root balls or mix. Nursery gallon sizes may differ from US liquid gallons. Check the volume unit on the bag.
How the estimate works
For straight-sided round pots, volume = π × radius² × fill depth. For rectangular containers, volume = length × width × fill depth. For tapered round pots, volume = π × depth × (top diameter² + top diameter × bottom diameter + bottom diameter²) ÷ 12.
For a tapered pot, measure the top diameter at the intended fill line. The calculator assumes straight walls between that line and the bottom. It converts inches to centimeters, then cubic centimeters to liters. One US liquid gallon is 3.785411784 liters; one cubic foot is 28.316846592 liters.
A US dry quart is about 1.10122 liters; a US liquid quart is about 0.94635 liters. Choose the unit printed on your bag. Do not treat a bag marked in dry quarts as liquid quarts.
The optional allowance adds extra mix for planning; it does not model compaction. Bag counts round up. This tool estimates volume, not wet weight or balcony load capacity.
Example: a 24-by-12-inch planter
A straight-sided planter filled 8 inches deep holds about 37.8 liters. With the 10% allowance, plan for about 41.5 liters: two 20-liter bags would be short, so round up to three. Two 20-US-dry-quart bags hold about 44 liters and would cover that same estimate. These figures assume an empty rectangular container measured inside.
Conversion reference: NIST Handbook 133, conversion tables.
Choose the right potting mix ↗