If you want to buy a heavy bag the most important factor you should keep in mind is the weight of it. You can find 40, 60, 80, 100 and even 130 lbs bags.
But, how do you know what size punching bag you should get?
The rule of a thumb is to buy one which is about the half of your weight. So, if you are 150 lbs a 70-80 lbs bag is suitable for you. Check the following punching bag size chart.
Why the half of your weight?
That is the amount that provides not too much nor too little resistance. So, you can throw punches with good power. Also, its swinging imitates well the moving of a real opponent so that you can practice the defense and offense movements.
If the bag is too light, it swings away too much. It does not provide enough resistance to placing a power punch on it.
If it is too heavy, you will have a feeling as if you were hitting a wall. The swinging is minimal. Hence it will not force you to move it around.
To sum up, a good bag develops your punching and moving skills. That is why it is crucial to pick the right weight for you.
Some words about getting a lighter bag
In some case getting a heavy bag that much lighter than your weight is not a wrong choice.
For example, when you want to use it for cardio training. When you do such a workout, there is no need to hit it with massive power; just light punches are thrown. Hence, you do not need equipment that provides high resistance.
Also, when you hit a lighter bag, it swings more, and that is an excellent opportunity to practice defense and offense skills. You need to move away to be not being hit by the returning bag. But, you also need to step here and there to place the next punches on it.
For example, Mike Tyson used this technique.
To sum up
The best is to get a bag that is half of your weight. That will be suitable for any types of workout.