The hole, the spacing, the mulch and the watering schedule, sized to one tree on one site.
Why each number is what it is: the tree planting guide (coming soon).
Where these numbers come fromHole and ball geometry follows ISA and Arbor Day planting standards, with root ball sizes per trunk caliper from ANSI Z60.1. Watering rates and the year-per-inch establishment period come from Gilman and Watson at the University of Florida. Hardiness zones are estimated from ZIP and can be overridden.
Three older practices are left out on purpose, none having survived testing: amending the backfill, fertilizing at planting, and pruning the top to balance the roots.
Outside what this tool can see: local ordinances on street trees and utility easements, the invasive status of a species in your county, and what the soil does 3 ft down. A county extension office answers all three.
The reasoning behind each figure, and the four ways a young tree dies, go in the tree planting guide (coming soon).