Begin with the items you carry on a typical day. A laptop, lunch container, water bottle, and change of clothes each affect the bag's required shape and capacity. Check the dimensions of your laptop against the padded compartment rather than relying on a general size label. A bag that is too large can encourage unnecessary packing, while one that is too small may leave items awkwardly exposed or difficult to retrieve.
Comfort depends on the fit and how weight is distributed. Look at strap width, padding, adjustability, and whether the back panel feels comfortable against your clothing. If you carry heavier loads, a sternum strap may help stabilize the bag. Pockets should support your routine: quick-access storage is useful for transit cards and keys, but numerous compartments can make it harder to remember where things are. Test zippers and closures for smooth operation.
Consider rain protection, cleaning, and how the bag stands when set down. Water-resistant fabric may handle light exposure but should not automatically be treated as waterproof. A separate cover or dry pouch can protect important electronics in wet conditions. Reflective details may be useful when commuting in low light. Choose a backpack that fits your commute and body comfortably, then check that the empty weight does not take too much of your carrying allowance.