Reference the height, the width, and the glass thickness with the "Sash weight chart."
Note: Round the measured window height up to the nearest value found in the chart.
For example, a window with 4mm glass in that measures 1230mm high, will require a spring with a length of 1299mm.
If the width is 650mm and the glass thickness is 4mm a Yellow spring is required.
Spring cut at 479.
The black tube at 63mm longer.