To measure for your Venetian blinds, you will need a steel measuring tape. Measure to the nearest eighth of an inch - don't round your measurements and do not take your own deductions. This will ensure proper fit of your new Venetian blinds in your window. For an inside mount, the window opening must have enough unobstructed depth to allow mounting. When measuring for an outside mount, be sure to allow for the height of the product's head rail in your measurements. If these sizes are incorrect it is impossible to change the Venetian blind, so take your time and double check your measurements We will custom make your Venetian blinds to the sizes you write on the order.