How do I know what sash weight I need

A home owner from morgan hill says:
I have a milliard window that I am replacing the balancer.
It does not have a stamp. How do I know what weight to buy?

Here is the product I am buying.
33" Series 380 Channel Balance
Product Code: S380-33
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Tom from SWISCO responded:
Thank you for posting. If you do not have a stamp to use as reference then you will need to weigh your sash on a scale and refer to the chart on the S380-33 store page, any common bathroom scale should do.
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