Measure the width of the attic access panel in your ceiling. Find this measurement in inches. This will let you know exactly how wide your attic access ladder needs to be.
Find out the incline angle of any attic access ladder you're thinking of purchasing (if applicable). If you already have a particular attic ladder in mind and you're measuring to see if it fits, the box the ladder comes in will tell you what angle it rests at when it's fully extended. This angle can change depending on the ladder, so keep this information in mind before proceeding.
Measure from your ceiling where your attic access panel is installed to the floor where your attic ladder will rest. If you don't already know the ladder inclination angle, measure straight down to the floor. If you do already know the ladder inclination angle, measure at this angle (example, 50 degrees) to get the most accurate possible measurement. Measuring with the angle in mind will give you an indication whether the ladder you want to purchase will fit. Measuring straight down will give you an indication of what type of ladders you should look at.