How to choose the appropriate coil balance strength
This help guide explains how to determine the correct coil balances for your windows. Coil balances are often referred to as constant force or spring balances. Before going through the trouble of manually figuring out what coil balance you need, look for a stamp on the coil balance that you are replacing. Sometimes coil balances are stamped with the pounds it supports. For example if you find the number 4 stamped on your coil balance, then replace this coil balance with a 4 pound coil balance.
To get started you first need to figure out the weight of your window sash. While it is a good idea to get the actual weight of your window sash you can use the method below to estimate an approximate weight. Remember you should take any friction caused by weather stripping in consideration.
 Measure in inches both the height and width of your window sash
 Get the square foot by using this formula... (height * width)/144 = your window's sqf

Determine if your window is single strength insulated glass or double strength insulated glass

Single Strength Insulated Glass
 Multiply the square foot of your sash by 2.25 lb

Double Strength Insulated Glass
 Multiply the square foot of your sash by 3.25 lb
After you have the weight of your window sash divide that number by 2 and round up to the nearest pound to figure out the weight each coil balance will need to handle on each side of your window sash. So for example if your window sash weighs 7.6 pounds, after you divide it by 2 you get 3.8 pounds, which means you will need two 4 lb coil balances (one on each side of the window sash). Coil balances go up to 9 pounds. If you need a coil balance more than 9 pounds you can double up coil balances on each side of the window sash. For example, if your sash weighs 22 pounds you will need an 11 pound coil balance on each side of the sash, you can put a 5 and 6 lb coil balance together on each side of the window sash.
Along with different weights, coil balances may have slightly different ends. Take a look at
Swisco's coil balance category to determine what type of coil balance you have.