How many pounds does a child have to beable not to sit in a child safety seat?

Depends on the state, here in Massachusetts: "Massachusetts state law requires that all children under age 8 and less than 57 inches tall be properly fastened and secured in a federally approved car seat or booster seat, according to the manufacturer's instructions. Children who have reached age 8 but who are not yet 4'9" tall are still affected by this law and should still be in an appropriate child restraint. Children who are old enough or tall enough to be out of a child restraint should use a vehicle seat belt."