Old BBQ grills used to have a weight-based meter to tell you about how much gas you have left. I have no idea why they stopped making them like that, but I recently got a new grill and had a year or so old tank of propane with an unknown amount of gas left.
I dug up my portable luggage scale, which has previously paid for itself in avoiding airline fees, and now has a second use. I found an empty tank is 15 pounds, and propane weighs about 4 pounds per gallon. Thankfully it weighed in at 24 pounds, so there was no mad rush on July 4 to find a tank exchange.