There are several ways to determine how much propane is left in the tank. Try one of the following methods listed below: Use a propane tank gauge to see how much propane you have left. This costs around $30, and you can find one at most hardware stores. Weigh your tank on a scale, then subtract that number with the tare weight to determine how much propane you have. You can find the tare weight marked “TW” on the tank itself.