How to Measure for a Kitchen Hood: A Historical Glossary of Key Terms

From Smoke Canopy to Modern Extraction: A Brief History

Before the 20th century, cooking smoke left the house through a central chimney or a simple hole in the roof. The open hearth and later the cast-iron stove were the only 'extraction' systems, and the idea of measuring a hood for fit did not exist because there was no hood at all.

The first residential range hoods appeared in the 1930s and 1940s, adapted from commercial kitchen canopies. These early units were often custom-built by sheet metal workers and hung over the stove with little regard for standard sizes. It was only in the post-war housing boom that manufacturers began offering hoods in set widths, which forced the question of how to measure a kitchen hood properly.

Today, measuring a kitchen hood is a mix of geometry and appliance standards. The key insight from history is that hoods were designed to catch rising heat and grease, so the measurement focuses on the capture area directly above the cooktop, not the decorative outer shell.

• Capture area: The horizontal rectangle directly above the cooktop where rising smoke and steam are caught. For a wall-mounted hood, this is the same width as the hood's underside opening.
• Duct collar: The round or rectangular opening on top of the hood where the exhaust pipe attaches. Its diameter (typically 6, 8, or 10 inches) affects airflow capacity.
• CFM (cubic feet per minute): A measure of airflow. For gas cooktops, the standard rule is 100 CFM per 10,000 BTUs; for electric, 100 CFM per 10,000 BTUs of equivalent heat output.

Why Width Is the First Number You Write Down

The width of a kitchen hood is the measurement most people start with, and it is also the one most tied to standard kitchen hood dimensions. Since the 1960s, residential cooktops have been built in widths of 30, 36, and occasionally 42 inches. Hoods are manufactured to match these widths, but the hood is not always the same width as the cooktop.

The general rule is that a hood should be at least as wide as the cooktop, and ideally 3 to 6 inches wider on each side for better capture. For example, a 36-inch cooktop often pairs with a 36-inch hood, but a 42-inch hood is better if the cooktop sits in a corner or near a window where drafts are common. This is why measuring the hood itself, not just the stove, matters.

To measure the hood width, place a tape measure across the front edge of the hood's underside, from one outer edge to the other. Do not include any decorative trim or side panels that extend beyond the main body, as those do not affect capture area. Write down the number in inches; this is your 'hood width' for comparison with standard kitchen hood widths.

Depth and Height: The Hidden Dimensions

Width is only one part of the equation. Depth, measured from the front of the hood to the back wall, determines how much of the cooking surface is covered. Standard depths are 18, 21, or 24 inches, with deeper hoods reserved for large cooktops or islands. A shallow hood over a deep cooktop will leave the front burners unprotected.

Height, or the vertical distance from the cooktop surface to the hood's underside, is a separate measurement that affects performance. This is often called 'mounting height' and is covered in detail by kitchen hood clearance from cooktop guides, but for measuring purposes you only need to record the actual distance you have to work with. Measure from the top of the burners to the bottom of the hood, then compare to the hood's adjustable range.

When measuring depth, use a tape measure from the back edge (where the hood meets the wall) to the front lip. For island hoods, measure from the center of the hood to the outer edge and double it, or measure the full depth directly. Write down both depth and height, because a hood that fits in width but is too shallow will not capture smoke from the front burners.

Duct Collar and CFM: Matching Size to Airflow

The duct collar is the part of the hood that connects to the exhaust pipe, and its size directly affects how much air can be moved. Most residential hoods have a 6-inch round collar, while larger professional-style hoods use 8 or 10 inches. Measuring the collar is simple: use a tape measure across the opening, from inner edge to inner edge, to get the diameter.

The CFM (cubic feet per minute) rating is not a physical measurement you take with a tape, but it determines whether your hood size is adequate. A hood that is too small for the cooktop's BTU output will not clear smoke effectively, regardless of its width. For a gas cooktop, add up the BTU ratings of all burners and divide by 100 to get the minimum CFM. For electric, use the same formula but base it on the cooktop's wattage divided by 10,000.

To measure CFM without a flow meter, you can check the hood's rating plate, usually located inside the hood near the motor or on the side. This plate also lists the duct collar size, which saves you a step. If the plate is missing, measure the collar and count the number of bends in the ductwork; each 90-degree bend reduces effective airflow by about 5%, so a longer or more curved duct may need a higher CFM hood.

Corner and Island Hoods: Special Measuring Cases

Corner hoods are mounted in a 45-degree angle across a corner, and their measuring logic differs from straight wall mounts. Instead of a single width, you need to measure the diagonal distance from the inside corner to the front center of the hood. This diagonal is typically 24 to 36 inches, but it is not the same as the width you would order for a standard hood.

Island hoods hang from the ceiling and are measured by their canopy width, which is often wider than the cooktop to compensate for the lack of a back wall. A standard island hood for a 36-inch cooktop is 36 to 48 inches wide, but the ceiling mount height and the chimney length must also be measured. Use a laser level or a plumb bob to mark the center of the cooktop on the ceiling, then measure from that point to the hood's mounting bracket.

For both corner and island hoods, always measure twice: once for the hood's physical size and once for the duct run. The duct run length from the hood to the outside vent affects the required CFM, and a longer run means you may need a larger hood or a more powerful fan. If in doubt, consult the hood's installation manual for maximum duct length, as exceeding it will reduce performance.

• Corner hood: Measure the diagonal from the corner to the front center, and the distance from the corner to each side wall.
• Island hood: Measure the canopy width, the height from cooktop to ceiling, and the distance from the cooktop center to the ceiling mount.
• Duct run: Measure the total length of the exhaust pipe, counting each bend as 5 feet of straight pipe for CFM purposes.