Calculator
Container size and watering planner
Pots are usually sold by their rim width, but what matters to a plant is the volume of mix and how deep it is. Enter your pot's measurements and this tool works out the volume, compares it with the minimums that university extension guidance gives for different plant sizes, tells you how many bags of mix to buy and suggests how often to look at the soil in your conditions. Nothing leaves your device.
Worked example (shown when scripts are off): a straight-sided round pot 12 inches wide and 12 inches deep, filled to 1 inch below the rim, holds about 5.4 US gallons of mix. That meets the 4 gallon minimum for medium plants but not the 8 gallon minimum for large ones such as a full-size tomato.
Before you buy pots
Make sure every pot has drainage holes. Extension guidance is clear that all containers must let extra water out, and that a layer of gravel in the bottom does not fix a pot without holes.
How the planner works
The volume uses the depth of mix, which is the pot depth minus 1 inch of space under the rim so water does not spill over. A straight round pot is the area of the circle times the mix depth. A tapered pot uses the frustum formula, which accounts for the narrower base. A rectangular box is length times width times mix depth. The result in cubic inches is divided by 231 to give US gallons, then converted to litres and quarts. Real pots have thick walls, curved sides and lips, so treat the volume as close, not exact, and check a pot's stated capacity if you can.
The size minimums
University of Maryland Extension groups vegetables into three classes. Large plants such as tomatoes, peppers, eggplant, cucumbers and winter squash want at least 8 to 10 gallons of mix at 12 to 16 inches deep, one plant per pot. Medium plants, including dwarf tomatoes and peppers, summer squash, beans, beets, chard and carrots, want 4 to 6 gallons at 8 to 12 inches deep. Small plants such as herbs, radishes, salad greens, spinach and peas manage in 1 to 3 gallons at 4 to 6 inches. The planner reports "minimum" when you reach the low end of the range and "comfortable" at the top end. A bigger pot gives you more margin on hot days because there is more water in reserve.
How much mix to buy
The tool multiplies the volume by the number of pots and divides by the bag size, rounding up. A cubic foot is about 7.5 US gallons. Bagged mix compresses once watered, so buy a little extra and top up after the first week.
The checking rhythm
The planner does not tell you how many times to water, because no fixed schedule survives a change in weather. It suggests how often to look. The rule behind it is simple physics: the smaller the volume of mix, the faster it runs out of water, and heat, sun and a porous pot wall speed that up. So the tool starts from the pot's volume, moves toward more frequent checks in hot weather and full sun, and toward fewer checks in mild weather, part shade or non-porous pots. The cut-offs between rhythms (under 3 gallons, 3 to 10, over 10) are our own judgement, not a published standard. Always confirm with a finger pushed 2 inches into the mix. If it feels dry, water slowly until water runs from the bottom.
Limits
- Minimums are for one plant per pot. Mixed planters need more volume overall.
- Varieties differ. Compact patio varieties of tomatoes and peppers can manage in the medium class.
- This is general information, not a watering instruction for your plants. Their condition and your local water rules come first.
Where this comes from
The size classes, gallon and depth minimums and the drainage-hole guidance come from the University of Maryland Extension page on types of containers for growing vegetables. The gallon conversion of 231 cubic inches is the definition of a US liquid gallon. The geometry was tested in code with dozens of size combinations, checking that volume rises with size and that a tapered pot with equal top and bottom matches a straight one. The checking-rhythm cut-offs are our own and are labelled as such above.