Bonsai Pot Size Calculator
Get a starting pot length, depth, and width from your tree's height and trunk diameter, using classical display proportions.
These are classical display proportions, not horticulture rules. Cascade and semi-cascade styles want a deeper pot than this, a wide canopy wants a wider one, and a young tree still in training belongs in a bigger grow pot regardless of what the numbers say. When the two disagree, repot for root health first and worry about the ideal pot later.
How it works
Bonsai pot proportion comes from a handful of classical display rules, not a horticulture formula. The rule of thumb for an upright or formal tree is that the pot should run about two thirds of the tree's height in length, and the depth should roughly match the diameter of the trunk where it meets the soil (the nebari). Width usually sits a bit narrower than length, close to three quarters of it, so the pot reads as a rectangle or oval rather than a square.
Worked example: a 45 cm tall tree with a 3.5 cm trunk at the base. Two thirds of 45 cm is 30 cm, so that's the target pot length. Depth follows the trunk directly, so 3.5 cm. Width comes out to 75% of the 30 cm length, or 22.5 cm. That gives a pot roughly 30 by 22.5 by 3.5 cm, which is a fairly shallow, wide tray typical of a formal upright display pot.
FAQ
Do these ratios apply to every style?
No. They're built around upright and formal styles, where a shallow pot balances a tall, straight trunk. Cascade and semi-cascade trees need a deep pot so the trunk can drop below the rim without looking cramped, and broad, spreading canopies often look better in a wider pot than the standard 75% ratio suggests. Treat the numbers here as a starting point for upright trees and adjust by eye for anything else.
My tree is still young and growing out. Does pot size still matter?
Less so. A tree in training, still being developed for trunk thickness and branch structure, usually lives in a larger grow pot or a wooden grow box that has nothing to do with display proportions. Save this calculator for when you're moving a developed tree into its display or near-final pot.
What if the trunk diameter and height point to different pot sizes?
Root health comes first. If your tree's roots need more room than the "ideal" display proportions allow, size the pot for the roots and treat the aesthetic ratio as a secondary guide. A slightly oversized pot on a healthy tree beats a perfectly proportioned pot that starves the roots.
Should I measure trunk diameter with the bark or without?
Measure over the bark, at the soil line, where the trunk visually meets the ground. That's the widest, most relevant point for judging how a pot's depth will look against the base of the tree.
For more on picking and sizing a pot, see how to choose the right bonsai pot, how to repot a bonsai tree, and when to repot a bonsai.