How to measure for a retractable security gate

Inside mounted

When measuring use the same plastered or brick opening as that of what your window or door is fitted to.

Measure the plaster/brick opening horizontally to the inside of the opening: top, centre and bottom.
This will be the width measurement. Use the widest of the 3 measurements. Provide the true measurement.
Measure the length left, center & right to the inside of the wall.
Use the shortest of the 3 measurements. This will be the length measurement. Provide the exact measurement.