Start with the cooktop, not the hood. The cooktop tells you how much air has to move, and the room tells you where that air has to go. Walk the kitchen with the cooktop model in hand and the plan drawings open, and the rest of this follows.
Match the width to the cooktop
Look at the cooktop width first. A hood narrower than the cooktop leaves the outer burners pulling air past the edge. Plan for a hood that matches the cooktop or runs a little wider. Check the mounting height in the cooktop manual and in the hood manual. The two must agree, and the hood must sit where the rated clearance says it should.
Read the airflow rating against the way you cook
An airflow rating is a manufacturer figure, so compare it only across models that publish it on the same basis. Ask what the rating assumes about duct length and bends. High-output burners, a habit of wok cooking, or frequent searing and frying ask for more air than a kettle and a weekly pot of soup. An open plan asks for more than a closed kitchen, because cross drafts can carry smoke sideways before the hood catches it. If your cooking runs heavy, look at the upper part of the rating range, not the middle.
Measure the duct run before you choose the hood
A ducted hood is only as good as the duct behind it. Measure the path from the hood outlet to the exterior cap. Count every elbow and every change in direction. Each bend costs airflow, and a long flat run through a cold attic or a crawl space costs more. Keep the duct the same diameter as the hood outlet, and do not let an installer reduce it to fit a tight cavity. Ask whether the exterior cap has a backdraft damper and whether the duct is rigid metal. Flexible plastic sags, collects grease, and works against the fan.
Check the make-up air and the room
A powerful hood in a tight, well-sealed house can struggle to pull enough replacement air, and the strain shows up as weak capture at the cooktop and noise at the grille. Ask whether the house has a fresh air source or whether the installer expects air to come from other rooms. If the kitchen is closed off and the hood is large, plan for a make-up air path before the purchase, not after the fan is hung. Ask who will assess this and how.
Questions to put to the installer
Ask what hood size and airflow they recommend for your cooktop, and why. Ask how long the duct run will be and where the elbows fall. Ask what duct material they plan to use. Ask who runs the power and whether a separate circuit is needed. If the cooktop is gas, ask how the clearance to the hood is set and who checks the gas connection. Ask how the filters are cleaned and which replacement filters the model takes. Write the answers down next to the hood model number so the record travels with the house.
Keep the filters and the fan on the plan
Grease filters are wear items. Clean them on a schedule that matches your cooking, and replace them when the mesh stops clearing. A clogged filter cuts airflow long before the fan itself is worn. As a hood ages, the motor and controls become a planning input, the same way the cooktop and the surrounding cabinet do. Treat the age of the hood as a reason to check its airflow and its sound each season, not as a reason to expect it to stop working.
Once the width, the airflow, the duct run, and the make-up air are settled, the choice of model is the easy part.