You’ll need to know the volume of the items you want to compress. Many products will come with this information in the product specifications.
If you don’t see the product volume, you can use this method to calculate it. You may need a friend to help out:
Put your item in a square or rectangular container – like a box
Measure the length and width of the container
Compress your item down into the container with the top as even as you can get it
Measure the average depth of the compressed item
Now multiply the container’s length and width and the items compressed depth
If you are using inches – divide the total by 61 to convert to liters
If you are using centimeters – divide the total by 1000 to convert to liters
This will give you a rough idea of what liter size compression bag you should buy. You can keep in mind items filled with down are more compressible than those filled with synthetic material.