Counterweights hidden inside a sash window's frame help the glazed panels glide effortlessly up and down. The weights hold the panels or "sashes" in position when you open them. Without counterweights, you would find the sashes hard to shift. If you did manage to push a sash up, it could come crashing straight down again, with finger-breaking force. You can establish how heavy the counterweights should be with simple equipment and calculations.
Lift the window sash onto a pair of bathroom scales. The sash is heavy and you may need someone to help you move it safely. Record how much the window sash weighs, using a notebook and pencil.
Divide the weight of the window sash by two. You may wish to use a calculator to help you with this, especially if the window's weight is an awkward fraction. Record the result on your notepad. Your answer shows how heavy one of the counterweights should be. A typical sash, weighing around 9.9k g/22 lb, would need two counterweights, each weighing 4.95 kg, 11lb.
Measure the thickness of the window sash with a ruler. Record the result on your notepad. This shows how much space will be available for the counterweight when you place it into its cavity in the window frame. The thickness of the counterweight should never be greater than the thickness of the sash, or the weight for the lower panel will knock against that for the upper panel, which hangs alongside it.
Purchase suitable counterweights for the weight and thickness you have calculated. Choose counterweights that are long and slim in shape for sash windows that are taller than they are wide. Choose the shortest, thickest counterweights that will fit the sash cavity for windows that are wider than they are tall. This choice of shape allows a weight to drop the greatest distance possible in its cavity, so the windows open fully.
If you replace the glass in an existing sash with thicker panes, this increases the weight of the panel and you will need to fit heavier counterweights. This often happens when you fit double-glazed sashes in place of old single glazing.
Counterweights are always used in pairs. The two weights together should exactly match the weight of the panel you intend them to balance, or the window may be hard to open and close.