Bags and Weight
The weight of a punching bag directly impacts on the amount of resistance training it offers in a workout. The heavier the bag, the more force it requires to move it but also the greater risk of injury the bag might pose to the hands and wrists.
These are the classic punching bag. The standard example weighs about 70 lbs., but there are heavier models that weigh 100 to 200 lbs.
Water bags offer increased safety for the hands and wrists. The normal bag is 70 or 100 lbs., but these bags are made in weights up to 150 lbs.
Specially shaped bags exist to work on particular forms of attack such as the uppercut bag or teardrop bag. These often weigh 60 to 80 lbs.
A freestanding bag will often have 150 to 200 lbs. of sand or water-filled weights to anchor it to the floor, but this has nothing to do with the resistance of the bag. That is instead determined by the bag's spring.
The final category of punching bag is the double-end bag, which has an air-filled bladder. These will weigh ounces and rarely much more than a pound.