The net weight of an item is its total weight (known as the gross weight) minus the weight of any containers or packaging the item is in (known as the tare weight). For example, the net weight of a tin of flour is the total weight minus the weight of the tin. In other words, the net weight is the gross weight minus the tare weight.
Using a Filled Container
Determine the Gross Weight
Place the object in its container or packaging on a scale. Write down the reading on the scale. This is the gross weight.
Determine the Tare Weight
Transfer the object – completely – from its container or packaging to a separate container and set it aside. Make sure none of the object (if you're working with items like powders or crumbly substances) remains in the first container. Place all of the original packaging on the scale and record the total weight. This is the tare weight.
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Subtract the Tare Weight From the Gross Weight
Subtract the tare weight from the gross weight. For example, say you have a tin of soup with a gross weight of 400 grams and a tare weight of 10 grams. Work out 400 – 10 = 390. The net weight of this item is 390 grams. Check your work with a calculator if you wish.
Using an Empty Container
Weigh the Empty Container
Place the empty container on a scale. Write down the reading on the scale.
Work out Your Final Reading
Calculate your final weight, which is the weight of the object plus the weight of the container. For example, say you need 500 grams of flour for a recipe and your container weighs 15 grams. The correct reading on the scale would be 500 + 15 = 515.
Fill the Container
Add your object or item to the container until the scale displays your final reading. In this example, add flour to the container until the scale reads 515 grams.
TL;DR (Too Long; Didn't Read)
Many scales have a tare button that zeros out a scale, even if a container is on it. You place the empty container on the scale, hit the tare button and then fill the container with the net weight of your object or item.