Because of the extra seams and zippers, a pair of convertibles generally weighs close to the same as a pair of plain pants plus a pair of nylon swim trunks. The advantage of the latter is that if you need to wade, you can go ahead and get them wet yet you still have a pair of dry pants. Or, if the pants get wet, you can wear the swim trunks over your base layer bottoms. (That used to be high fashion for backpackers, and some still do it.) On a recent fishing trip, my grandson kept wading yoo far into the lake, and he was wishing he had swim trunks instead of convertibles!
The ones you'll find at Big 5 or other budget sporting goods stores or at Campmor.com aren't as sturdy as the expensive ones, but they work. I do recommend stitching the crotch seam for reinforcement before you ever wear them. (Don't ask me how I learned this!
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As PDA says, your local thrift stores can be a great source for backpacking cothing. So, for that matter, can your closet! Just be sure they aren't cotton!