# How to calculate the weight of a plastic bag when knowing the size and thickness

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a. What is specific gravity or density?

Specific gravity or density is the mass of matter in a given volume, is calculated by dividing the mass of matter by its volume.

Here we have the formula: D = m/V, with D (Density) is the density, m is the mass and V is the volume.

b. What does the density index mean?

The density of a substance helps us to calculate their weight per unit volume, knowing how much heavier one substance is than another based on their density. For example, gasoline has a density of 700 kg/m3, which means that the amount of gasoline contained in a tank with a volume of 1 cubic meter will weigh 700 kg. Iron has a density of 7800 kg/m3, which means that one cubic meter of iron will weigh exactly 7800 kg.

All chemical elements known to man have been found to have their densities by measuring and experimenting many times.

c. How to calculate the weight of a plastic bag by density

We already have the formula for density above, and when we reverse the equation, we can calculate the weight when we know the density and volume. The formula would be:

m= D x V

Density is given in kilograms per cubic meter (kg/m3) or grams per cubic centimeter (g/cm3). Given the very light weight of plastic packaging, converting to the g/cm3 system will help simplify the formula for calculating their mass.

The density (D) of each plastic material can be found easily online, or from your packaging supplier. This parameter is printed on the plastic resins packaging that they use to create the product. For example: The density (D) of POF shrink film is 0.92 g/cm3, LDPE is 0.91 and PVC is 1.4.

The volume of the bag will be calculated by length x width x height, and the height here is the thickness of the bag (of both sides of the bag combined). Don't forget to convert the thickness to cm. If the thickness is being measured in mm, divide it by 10, and if it's in microns, divide it by 10000.

Let's take an example: the size of LDPE plastic bags is 80 cm x 150 cm x 70 microns, we have the formula for weight calculation as D x V = 0.91 x 80 x 150 x 0.007 = 76.44 grams/ bag.

1 kg is equal to 1000 grams, dividing 1000 by 76.44 we get 13.08. That is, with the above LDPE 80 x 150 cm bag size 70 micron thick, one kilogram will include about 13 bags..

It should be noted that the thickness in this case is for both sides of the bag, which means that each membrane layer of a bag is only 35 microns thick.

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