Closet drawings make deep shelves look wonderfully efficient. Laundry day is less impressed. If your sheets disappear behind the towels and every basket needs to be tilted sideways, the problem is usually access rather than a shortage of square footage.
Fold things the way you actually store them
Make a representative stack of towels, sweaters or sheets. Measure its footprint without squashing it into an unnatural fold. Measure baskets at their widest point, including handles, and leave usable hand space. A storage box's advertised size may describe the rim rather than the narrower base, so measure the one you plan to buy.
Use cardboard to mark the proposed shelf front and a second piece to represent the shelf above. Try putting a stack away and removing the bottom item. Check the test from your normal standing position, not with your head inside an empty closet. This reveals depth and vertical spacing problems together.
Your actual items
Stored footprint
Measure the folded stack or basket, including protruding handles.
Opening test
Clear retrieval path
Allow room for hands and for the item to pass the doorway without an awkward tilt.
Reach test
Comfortable reach
Check the farthest useful point with the shelf above in place.
Decide whether the back row has a purpose
A shallow shelf makes daily items visible. A deeper shelf can suit bulky bedding or a removable bin, but two unlabelled rows of small items become a rummaging exercise. If seasonal items live behind daily ones, put the seasonal set in a container that comes out as a unit. Do not count inaccessible space as successful storage.
Vary depth by height when it improves reach. A shallower upper shelf can keep its front edge visible; a deeper low shelf may suit a large basket if you can lift it safely. Keep heavy items where they can be handled comfortably. A tall stack of towels is not a reason to put a heavy appliance on the top shelf.
Choose the support system after the layout works
ClosetMaid's installation guide matches brackets to shelving depth, with specific exceptions for its combined shelf-and-rod products. Follow the selected system rather than hanging a deeper board on whatever supports are already there. Panel material, span, supports and loading need their own check; this measuring exercise does not establish a safe structural span.
Leave room for hinges and the door stop as well as the door opening. Where a narrow opening hides wide shelves, the linen-closet access guide helps decide which side pockets are useful. If the closet also has a bowing clothes rod, resolve its supports before treating the shelf above as part of the repair.

