It is hard to know what is a heavy period and what is a normal period; because it is different for every woman. It is considered a heavy period if you are loosing more than 80ml (16 teaspoons) of blood per period or if it lasts more than 7 days, or both.
There is no need to measure how much blood you are losing, you will be able to tell if you have a heavy period. Here are some points that will help you to decide if you have a heavy period or not:
You have to change your sanitary products every hour or 2
You are passing blood clots that are around 2.5cm
You have to use more than one sanitary product
You frequently leak through your clothes