To measure block and tackle window balances first you measure the depth of the metal channel, the most common widths are 5/8" and 3/8". Then you need to measure the length of the balance. When measuring the length, measure only the metal channel, don't include the plastic guides. If the stamp on your balance is easily visible, you can use that to determine the length. The first part of the stamp is one inch shorter than the physical length of the balance. For instance, if your balance has a stamp of 1440 or 14-4, your balance is 15" long.
Measure the depth of the metal channel. This balance has a channel depth of 5/8".
Measure only the metal channel
This balance is 15" long
Location of the stamp on two different balances. The stamp may be printed onto or stamped into the metal. Both of these balances are 15"
We also have a guide on installing block and tackle window balances