🪴 Repotting Pot Size Calculator

The standard rule of thumb is to size up 1-2 inches in diameter at a time — going much bigger can hold excess moisture the roots can't use. Enter your current pot dimensions to see the recommended next size and the soil volume difference.

Current pot diameter (in)

Current pot height (in)

Size increase (inches)

Next size, and what it costs in soil

The diameter goes up by your chosen step and the height is scaled to keep the pot's proportions, then both pots are measured as plain cylinders:

new diameter = current diameter + step (1 or 2 in)
new height   = current height + step × (height / diameter)
volume       = π × (diameter / 2)² × height

A 6 in × 6 in pot stepped up 2 inches becomes 8 in × 8 in. That takes the volume from 170 to 402 cubic inches — an increase of 137%. Two inches of diameter sounds modest; in soil it is nearly two and a half times the pot.

Choosing the step

Volume follows the square of the diameter. That is why the +1 inch option exists for slow growers and small plants. On the same 6 in pot it gives 7 in × 7 in, about 269 cubic inches, a 59% rise instead of 137%.
Buying mix: 231 cubic inches is one US gallon and 57.75 is a quart, so that 402 cubic inch pot needs roughly 1.7 gallons of potting mix before the root ball displaces any of it.
The figures assume straight sides. Most nursery pots taper, so a real pot of the same rim size holds around a quarter less.

Oversizing is the actual risk here, not undersizing. Soil with no roots in it stays wet for days after watering, and roots sitting in cold, airless mix are how a healthy plant is lost to rot within weeks of being repotted.

Frequently asked questions

How much bigger should a new pot be?

One to two inches more in diameter. Anything larger leaves a volume of wet soil the plant cannot dry out, which kills far more houseplants than a slightly tight pot does.

How many cubic inches in a gallon of potting soil?

231 cubic inches to a US gallon, 57.75 to a quart. Divide the tool's volume by 231 to see how many gallons of mix to buy.

Can I skip a size for a fast-growing plant?

Better to step twice a year than jump four inches at once. If a pothos or monstera fills its pot in a season, repot it again next season rather than burying it in soil now.