Bonsai Wire Size Calculator
Find the right aluminum wire gauge for a branch or trunk based on its diameter, so you get enough hold without overdoing it.
If one wire of that gauge doesn't hold the bend, double-wire with two strands of the same size instead of jumping straight to the next gauge up. Copper holds roughly one gauge better than aluminum at the same thickness, so a copper user can often go one size down. Check the wire every few weeks through the growing season. Bark grows fast in summer, and a scar left by wire that bit in can take years to heal.
How it works
The classic rule for choosing bonsai wire is that the wire diameter should be about a third of the diameter of the branch or trunk it needs to bend. Too thin and the wire won't hold the shape once you let go; too thick and it's awkward to apply and can crush thin bark. This calculator takes your branch diameter, divides it by three, and rounds to the nearest size in the common aluminum lineup: 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, and 6 mm.
Worked example: a 10 mm branch. A third of 10 mm is about 3.3 mm, which sits closer to 3.5 mm than to 3 mm on the available list, so the calculator recommends 3.5 mm wire. For a thinner 6 mm shoot, a third is exactly 2 mm, an exact match on the list, so it recommends 2 mm without any rounding needed.
FAQ
What if my branch measurement falls right between two sizes?
The calculator rounds up in a tie. A slightly thicker wire that holds well beats a slightly thinner one that springs back overnight, so when the math is genuinely ambiguous, err toward more holding power.
Should I use copper or aluminum?
Aluminum is easier to apply and remove and is the standard choice for most beginners and deciduous trees. Copper work-hardens as you bend it, so it holds a set shape more firmly, roughly one gauge better than aluminum at the same thickness. Many growers reserve copper for conifers and for final, more permanent styling once a tree's structure is set.
What if one wire wrap isn't enough to hold the bend?
Don't jump straight to the next gauge up. Try applying two wires of the same size side by side first. Doubling up spreads the bending force over more bark contact and often holds just as well as a single thicker wire, with less risk of the wire cutting in at a sharp bend.
How long can I leave the wire on?
Check it every few weeks during the growing season, since branches can thicken surprisingly fast and wire that looked loose a month ago can already be biting into the bark. A scar from wire left on too long can take years to grow out, so it's always better to remove and rewire early than to leave it and hope.
For more on applying and removing wire, see how to wire a bonsai tree, how to apply wire without scarring the bark, and when to remove bonsai wire before it bites in.