Understanding your result
The headline is the hood size to buy, with the required airflow and the makeup-air verdict in the sub-line. The table lists the width rule, the gas burner rule (marked not applicable for electric), the HVI minimum, the requirement (the largest) and the hood size, then the makeup-air row that applies the 400 CFM threshold, and the duct’s equivalent length and velocity. The stats repeat the requirement, the hood size and the makeup-air answer.
The makeup-air row deserves a second look: a hood rated exactly 400 CFM does not trigger the requirement, a 500 CFM hood does, and the difference between them is a motorised damper, a makeup-air duct and often a heater. If the cooktop’s burners push the requirement just past 400, a 400 CFM hood with better capture (wider, lower) is often the wiser choice, and the notes say so.
The bathroom equivalent is the bathroom exhaust fan calculator; the whole-house fresh air that a kitchen exhaust is not is sized by the whole-house ventilation calculator; the velocity in the hood duct, or any other, can be checked in the duct velocity calculator; and the air changes calculator turns the hood’s CFM into air changes for the kitchen.
How we calculate this
The gas and width rules are the Home Ventilating Institute’s recommended rates for effective capture; HVI’s published minimums (40 and 50 CFM per foot) are also shown so you can see how far below real cooking needs they are. The 400 CFM threshold is the International Residential Code’s, section M1503.6, which requires makeup air at the hood’s rated airflow. The duct allowances are typical hood-maker figures, and the velocity is airflow divided by the duct’s cross-section, flagged above 2,000 FPM where hood ducts become loud. The rules are collected on the ventilation and exhaust minimums page; the unit conversions page covers CFM to m³/h and FPM to m/s for metric mode.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Gas rule | 100 CFM per 10,000 BTU/hr of burners | Home Ventilating Institute recommended rate |
| Width rule | 100 CFM per ft (wall), 150 per ft (island) | HVI recommended rates |
| HVI minimums | 40 CFM per ft (wall), 50 (island) | HVI minimum rates |
| Makeup air threshold | Hoods rated above 400 CFM | International Residential Code M1503.6 |
| Hood sizes | 200 to 1,500 CFM | Residential range hoods sold in the US |
| Duct allowances | 15 ft per 90° elbow, 30 ft for the cap | Typical hood manufacturer installation guidance |
| Velocity warning | Above 2,000 FPM | Noise and restriction in hood ducts; makers’ duct size recommendations |
Assumptions last reviewed October 8, 2026.
The calculator does not model capture efficiency, which depends on the hood’s width, depth and mounting height and matters as much as airflow, nor the hood’s fan curve against the duct, nor the makeup-air system itself, which must be designed for the house and may need a heater in cold climates. Local codes may differ from the IRC; some jurisdictions set the makeup-air threshold lower or require it for any hood in a house with atmospherically vented appliances. The guide to bathroom and kitchen exhaust rules covers capture, makeup air and the code in detail, and the guide to whole-house ventilation explains how kitchen exhaust fits into the house’s air balance.
Two worked examples
A 30 inch wall hood over a standard gas range
Gas, 40,000 BTU/hr of burners, 30 in hood against a wall, 10 ft of 7-inch duct with one elbow to a wall cap.
- Width rule: 2.5 ft × 100 = 250 CFM; gas rule: 40,000 ÷ 10,000 × 100 = 400 CFM; HVI minimum 100 CFM
- Required: 400 CFM → hood size 400 CFM
- Makeup air: not required (not above 400)
- Duct: 10 + 15 + 30 = 55 ft equivalent; 1,497 FPM in 7-inch duct
The burners, not the width, set the size, and 400 CFM lands exactly on the threshold without crossing it, which is why 400 CFM is the most common size sold. A 6-inch duct would run at 2,037 FPM and be noticeably louder; 7-inch is the better match for a 400 CFM hood on this run.
A 36 inch island hood over a pro-style range
Gas, 65,000 BTU/hr of burners, 36 in hood over an island, 18 ft of 8-inch duct with two elbows to a roof cap.
- Width rule: 3 ft × 150 = 450 CFM; gas rule: 650 CFM; HVI minimum 150 CFM
- Required: 650 CFM → hood size 750 CFM
- Makeup air: required (above 400 CFM)
- Duct: 18 + 2 × 15 + 30 = 78 ft equivalent; 2,149 FPM in 8-inch duct
Everything about this installation is harder: the hood crosses the makeup-air threshold, the duct is at the edge of most makers’ allowed length, and 8-inch duct is loud at 750 CFM, so the notes recommend 10-inch. Island hoods also capture worse than wall hoods because there is no wall to contain the plume, which is the reason for the 150 CFM per foot rule and for choosing a hood a size wider than the range.
Where to find your inputs
Burner output. On the range’s rating plate (inside the oven door or drawer) and in its manual, as BTU/hr per burner; add them up. Typical 30-inch ranges total 40,000 to 60,000; pro-style ranges 80,000 and up.
Hood width. The hood’s nominal width, 30, 36 or 48 in; it should be at least as wide as the cooktop.
Duct run. From the hood’s outlet to the cap, counting elbows; the hood’s manual gives its outlet size and maximum equivalent length.
Mounting. Wall for a hood against a wall or in an alcove; island for any hood with open space behind it, including peninsulas.
Common mistakes
- Buying by width alone over a gas range. The burners usually set the size; 40,000 BTU needs 400 CFM whatever the width.
- Crossing 400 CFM without makeup air. It is a code requirement and a backdrafting hazard, not an option.
- A narrow hood with a big fan. Capture matters more than CFM; the hood should extend past the cooktop and sit 24 to 30 in above it.
- Flex or plastic duct. Grease collects in ribs; use smooth rigid metal.
- A long, bent run. Every elbow costs 15 ft of equivalent length and the cap 30; the hood delivers far less than its rating at the end of a long run.
- Recirculating hoods for gas. They remove some grease and odour, no moisture and no combustion products; vent gas cooktops outdoors.
Questions people ask
- How many CFM does a range hood need?
- For a gas cooktop, 100 CFM for every 10,000 BTU per hour of burner output, so 400 CFM for a typical 40,000 BTU range and 600 to 800 for a pro-style one; and in any case 100 CFM per linear foot of hood width against a wall or 150 per foot over an island, so 250 CFM for a 30-inch wall hood and 450 for a 36-inch island hood. Use the larger of the two. The Home Ventilating Institute's minimums are lower, 40 and 50 CFM per foot, and are rarely enough for real cooking.
- What is the 400 CFM makeup air rule?
- Section M1503.6 of the International Residential Code requires any kitchen exhaust hood rated above 400 CFM to have a makeup-air system that brings in at least as much outdoor air as the hood removes, automatically, when the hood runs. The reason is backdrafting. A 900 CFM hood in a tight house depressurises it enough to pull flue gases back down a water heater or furnace chimney and to pull soil gases through the slab. Makeup air means a motorised damper and a duct, often with a heater, and it adds materially to the cost.
- Is a bigger range hood better?
- Only up to the capture it needs. Capture depends on the hood being wider than the cooktop (3 inches past it on each side is the usual advice), mounted 24 to 30 inches above it and deep enough to cover the back burners; a 400 CFM hood that captures well beats a 900 CFM hood that is too narrow or too high. Above 400 CFM the makeup-air requirement and the noise rise quickly. Size for the cooktop and the width, then buy the quietest hood of that size.
- What duct size does a range hood need?
- Follow the hood maker's specification, typically 6-inch round up to about 400 CFM, 7 or 8-inch to about 900 and 10-inch above, and never smaller than the hood's outlet. Keep the run short with few elbows; each 90° elbow counts as about 15 ft of duct and a roof or wall cap about 30 ft, and many makers limit the total equivalent length to 50 to 100 ft. Use rigid smooth metal duct, never flex or plastic, because grease collects in ribs and is a fire hazard.
- Do I need a hood for an electric or induction cooktop?
- Yes, for moisture, grease and odours, though no combustion products. The gas rule does not apply, so the width rule governs, 100 CFM per foot against a wall and 150 over an island; a 30-inch wall hood over an induction cooktop needs about 250 CFM. Induction cooktops produce less stray heat, so capture is easier and a quieter, lower-speed setting does most of the work.
- Why does my range hood not clear the smoke?
- Usually because its rated airflow is measured at zero static pressure and the duct takes 20 to 40% of it, or because the hood is too narrow or too high above the cooktop to capture the plume, or because the house is tight enough that the hood cannot exhaust without makeup air. Opening a window near the kitchen while cooking is the quick test of the last cause; if the hood suddenly works, the house needs makeup air.