Can I measure the sash to determine the right balance length?

Is it possible to measure the window sash to determine the correct balance length? The reason I ask is I have several windows that have the 2630C balance measuring 26 inches long. I found a 2630 balance measuring 27 inches loose in the garage. I have checked every window in my home and can't find another 2630 measuring 27 inches.

Since I've only been in my home for 3 years, I have no idea if the balances marked 2630C (26 inches) are the correct length for the windows. It is possible that the balances were replaced years ago and that the 27 inch balances are what should be used.

I just wanted to see if there was another way to measure the correct balance length needed and not assume the existing balances are the correct length. I will be weighing a sash tonight per Donna's instructions.

Thanks

I will measure the glass went I get home today.

When a window sash is hung, should the channel balance shoe be very close to the bottom of the sash channel? With a few of my windows, the channel shoe is about an inch up in the channel.

When I weighed the sash last night, it was 18 pounds. If is is determined that I do need 26 inch balances, would the 2530 be my best bet?

Thanks and sorry if I'm making this too confusing

The visible height of the glass is 25 inches and these 2630C balances that I'm trying to replace are 26 inches.

With the sash weighing 18 pounds, which replacement balance do you suggest? The 2530, 2540, or something else?

From the pictures, can you confirm that these are series 390 balances?

Thank you