There's alot of terms thrown around when dealing with getting the correct window balance. Some you may know, some you may not. This guide will help you understand the window and what you may hear when speaking to a rep helping you get the right balance for your application.
First, all window balances have some sort of weight it can hold. In the window, there are two balances, and it 'counter balances' the weight of the window so it can stay up. Each balance will hold half the total weight of the sash. So, you will need to know the weight of the window, or if you're lucky there is a balance already in the window that has numbers written on it.
If you aren't so lucky, you need to get the sash weight. We recommend taking the existing sash/window out and putting it on a bathroom scale. Make sure there is no added weight pressing on it, or your foot hindering the full weight on the scale.
In the event you cannot take the window out, with the sash in the window opening disconnect the window balances from both sides. Make sure there isn't anything else in the track that can cause any friction. Pick up the sash, it will have zero tension on it so it will be heavier, and place the scale under the sash and gently rest it on the scale. We don't recommend doing this unless you are confident in the reading you get. If your weight is 5 lbs off or more, the new balances won't work properly.