How much weight stack do I need on a functional trainer?

For most home users, dual stacks of 150–200 lb per side (with a 2:1 pulley ratio yielding 75–100 lb of actual resistance) cover the vast majority of isolation work like flyes, rows, curls, and tricep pushdowns. Serious lifters doing heavy lat pulldowns or cable rows will want 200 lb+ stacks. Note that machines with 2:1 ratios feel lighter than the stack number suggests, while 1:1 ratios deliver the full weight.