Bonsai pot size calculator — the 2/3 rule, done for you

Measure honestly before you trust the output: height runs from the soil line to the apex — not from the table — and caliper is the trunk's thickness just above the nebari (the visible surface roots). Those two numbers drive everything.

Pots come in fixed sizes, so treat the result as a target, not a spec. Within a half inch is a match. Between sizes, we'd take the smaller pot for a finished tree and the larger one for a tree still in development — extra root run speeds a trunk up.

The pot swap itself is a spring job. The repotting guide covers the how and the when.

The whole method is one old rule plus its exceptions:

• Pot length ≈ 2/3 of the tree's height for oval and rectangular pots. For a wide, spreading tree, use 2/3 of the spread instead — the rule follows the tree's biggest dimension.
• Depth ≈ the trunk's caliper at the base, with a 1.5-inch floor — shallower than that isn't a home for roots.
• Width ≈ 2/3 of the length, which is roughly how manufacturers size pots anyway.
• The exceptions: cascade needs 2–3× the caliper in depth as a counterweight to the falling line; semi-cascade sits at 1.5–2×; literati shrinks to a shallow drum about a third of the height, because the empty space is the point.

It's an aesthetic rule as much as a horticultural one: 2/3 keeps the tree's visual weight above the pot without dwarfing it. Growers bend it constantly — but they bend it knowingly, and now you can too.