Quantity and material take-off
Quantity is driven by building length and on-center spacing. Material depends on member lengths: two top chords, one bottom chord and the web members.
count = ceil(length × 12 ÷ spacing) + 1
The Estimated Lumber result converts total member length into board feet using the selected material. Add 10% for waste, blocking and bracing. Gable-end frames, girders and hip sets may add to the count.
Quick answer for a common case: see how many trusses do I need. For loads on each truss use the load calculator.