Hood Fan Size Calculator

The Hood Fan Size Calculator determines the appropriate hood fan extraction rate and duct diameter from hob width, heat output, room volume, and duct length.

What Is a Hood Fan Size Calculator?

A hood fan size calculator estimates the fan capacity and hood width needed to capture and exhaust cooking byproducts. It uses your cooktop’s fuel type, BTU output, and dimensions to compute airflow in cubic feet per minute. It also accounts for the mounting style and duct route, because elbows and long runs add resistance. That resistance reduces delivered airflow at the hood face.

In short, the tool converts your kitchen conditions into a clear recommendation. You get a target fan capacity, a minimum hood width and depth, and a suggested duct diameter. These results help you choose models, compare materials, and plan installation with the right units. Use it before you buy, so your design supports safe ventilation and comfortable cooking.

Formulas for Hood Fan Size

These sizing rules are practical formulas used by installers and referenced by trade groups. They combine cooktop power, hood coverage, and duct penalties. Use them as a baseline, then select the next standard model size above your result.

• Hood width: Hood width = cooktop width + 6 in (3 in overhang on each side is a common minimum).
• Base CFM for electric or induction: Base CFM = 100 × (cooktop width in feet). Example: 30 in = 2.5 ft → 250 CFM.
• Base CFM for gas: Base CFM = total BTU per hour ÷ 100. Example: 60,000 BTU per hour → 600 CFM.
• Island factor: If the hood is over an island, multiply Base CFM by 1.3 (30 percent more for cross drafts).
• Duct penalty adjustment: Required CFM = Base CFM × [1 + 0.1 × (duct length in feet ÷ 10) + 0.2 × (number of 90-degree elbows)].
• Capture velocity check: CFM ≈ hood opening area in square feet × 60 to 90 feet per minute. Use 90 for heavier, greasy cooking.

These are rules of thumb, not code. They align with common guidance from HVI and manufacturers. Always verify the final selection against local code, the appliance manual, and the hood’s rated performance curve. When values disagree, defer to code and the cooktop manufacturer’s instructions.

The Mechanics Behind Hood Fan Size

Cooking creates a thermal plume that carries moisture, grease aerosols, and odors. Your hood must first capture the plume, then contain it, and finally exhaust it through ducts. The fan’s rated capacity helps, but coverage, enclosure, and duct friction determine real results. That is why hood dimensions, materials, and route matter as much as the nameplate CFM.

• Coverage: Width and depth must cover front and rear burners; a small overhang improves capture.
• Containment: Deeper hoods and baffle filters slow and collect the plume before it escapes to the room.
• Duct friction: Long ducts, small diameters, and sharp elbows reduce delivered airflow.
• Make-up air: Strong fans pull air from the home; large systems may require dedicated supply air.
• Noise: Higher airflow through narrow ducts raises velocity and noise, reducing real-world use.

The calculator integrates these effects as multipliers and minimum sizes. It encourages a larger hood with moderate airflow rather than a narrow hood with a loud fan. That approach improves capture efficiency and comfort. It also reduces oil residue on cabinets by keeping the plume inside the hood body.

Inputs and Assumptions for Hood Fan Size

The calculator asks for key inputs that drive capture and exhaust. Gather these before you start. You will use actual dimensions, the fuel type, and the duct route. When the tool requests units, match the examples to avoid errors.

• Cooktop width and depth in inches; total BTU per hour for gas models, or width for electric/induction.
• Mounting style: wall, under-cabinet, or island (islands need more airflow).
• Hood mounting height above the cooking surface (typical 24 to 30 in for residential).
• Duct diameter, total duct length, and number of 90-degree elbows and 45-degree bends.
• Filter type (baffle vs mesh) and whether the hood is ducted or recirculating.
• Local code thresholds for make-up air (often 400 CFM or higher requires provisions).

The tool assumes residential use at sea level with steady cooking. High-altitude locations, wok ranges, griddles, or grills raise demand. Very long ducts or many elbows can push results above typical catalogs. In those edge cases, consider a larger duct, a remote inline blower, or a closer mount.

How to Use the Hood Fan Size Calculator (Steps)

Here’s a concise overview before we dive into the key points:

1. Measure your cooktop width and depth, and note fuel type and total BTU per hour if gas.
2. Select the mounting style: wall/under-cabinet or island, and enter the planned mounting height.
3. Enter the duct diameter, total straight length, and count the number of 90-degree elbows and 45-degree bends.
4. Choose filter type and whether the hood will be ducted or recirculating.
5. Review the calculator’s base CFM and apply the island factor if applicable.
6. Apply duct penalties to get the required adjusted CFM and compare to standard hood ratings.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

Wall-mounted, 30 in electric range. Cooktop width is 30 in (2.5 ft). Base CFM = 100 × 2.5 = 250 CFM. No island factor. Duct length is 12 ft with two 90-degree elbows. Required CFM = 250 × [1 + 0.1 × (12 ÷ 10) + 0.2 × 2] = 250 × (1 + 0.12 + 0.4) = 250 × 1.52 = 380 CFM. Hood width target = 30 + 6 = 36 in. A 400 CFM, 36 in wall hood with a 6 in duct meets the need. What this means: Choose at least a 400 CFM hood and, if space allows, prefer a 36 in width for better coverage.

Island, 36 in gas cooktop at 65,000 BTU per hour. Base CFM = 65,000 ÷ 100 = 650 CFM. Island factor: 650 × 1.3 = 845 CFM. Duct is 15 ft with two 90-degree elbows. Required CFM = 845 × [1 + 0.1 × (15 ÷ 10) + 0.2 × 2] = 845 × (1 + 0.15 + 0.4) = 845 × 1.55 ≈ 1,310 CFM. Hood width target = 36 + 6 = 42 in; depth should be 24 in if possible. This level often triggers make-up air rules. What this means: Plan a 1,200 to 1,300 CFM island hood, a 42 in body, a large duct (8 to 10 in), and code-compliant make-up air.

Accuracy & Limitations

The calculator is a planning tool, not an engineering stamp. It gives you a safe, practical size range for most homes. Actual performance depends on how you cook, your duct materials, and installation quality. Always read the hood and cooktop manuals and follow local code.

• Rules of thumb may oversize or undersize special appliances like indoor grills or woks.
• Recirculating hoods filter odors but do not remove moisture or combustion byproducts.
• Manufacturer CFM ratings are at specific static pressures; real ducts change those values.
• Noise ratings vary; a loud hood may run at lower speeds and underperform in practice.
• Make-up air and building tightness affect safety and comfort at higher airflow rates.

Use the results to shortlist models and duct sizes. Then refine with performance curves, duct calculators, and installer feedback. When in doubt, enlarge the hood body and duct, and avoid extra elbows. Those steps improve capture without relying only on more fan power.

Units & Conversions

Ventilation sizing mixes different measurement systems. You will see airflow in CFM, duct diameters in inches, cooking power in BTU per hour, and sometimes airflow in L/s. Use the conversions below to keep your units consistent and avoid errors during planning.

Multiply by the factor to convert. For example, a 600 CFM hood is about 283 L/s (600 × 0.472). A 6 in duct is 152.4 mm. When checking area, convert diameter to feet before using area = 3.1416 × (diameter ÷ 2)^2.

Troubleshooting

If your result seems too high or too low, check each input carefully. A missing elbow or a mistyped BTU number will change the answer a lot. Confirm your mounting style and cooktop dimensions. Make sure your duct length includes vertical and horizontal segments.

• Result too high: Reduce elbows, increase duct diameter, or choose a deeper hood to improve capture efficiency.
• Result too low: Recheck BTU totals, apply the island factor, and confirm you used feet, not inches, in formulas.
• Noise concerns: Look for larger ducts, smooth-wall metal materials, and a remote or inline blower.
• Code flags: If CFM exceeds your local make-up air threshold, plan a compliant make-up air system.

Re-run the calculator after each change. Small layout adjustments can save you from oversizing the fan. If results remain unusual, consult an installer with a duct calculator and the hood’s performance curve.

FAQ about Hood Fan Size Calculator

Do I size by BTU or by cooktop width?

For gas, size by total BTU per hour; for electric or induction, size by cooktop width. Then apply the island factor and duct penalties for either type.

How much wider should the hood be than the range?

A common target is 3 inches on each side, so 6 inches wider overall. If space allows, go wider, especially for island installations or heavy cooking.

What duct size should I use for a 600 CFM hood?

Most 600 CFM residential hoods recommend at least a 6 to 8 inch round duct. Check the manufacturer’s collar size and match or upsize the route to keep velocity low.

Do recirculating hoods need the same CFM as ducted hoods?

Recirculating hoods filter odors and grease but do not remove heat or moisture. They can use similar ratings, but real-world clearing is lower than a ducted system.

Hood Fan Size Terms & Definitions

CFM

Short for cubic feet per minute, it measures airflow rate. Higher CFM means more air moved through the hood and duct each minute.

BTU per hour

British Thermal Units per hour measure cooking power for gas. Total BTU per hour from all burners guides the base fan size for gas ranges.

Capture Area

The open underside of the hood where smoke and steam are collected. A larger, deeper capture area improves containment before exhaust.

Equivalent Length

A method to express duct elbows and fittings as extra feet of straight duct. More equivalent length increases resistance and reduces delivered airflow.

Make-Up Air

Air supplied to replace exhausted air. At higher CFM, dedicated make-up air prevents depressurization and supports safe combustion.

Baffle Filter

A metal filter that changes airflow direction to trap grease. It is durable, easy to clean, and common on higher-performance hoods.

Sones

A unit for perceived loudness of the hood. Lower sones are quieter, but real noise depends on duct size, materials, and speed setting.

Mounting Height

The vertical distance from cooktop to hood. Typical residential ranges are 24 to 30 inches; closer mounting improves capture and reduces required CFM.

Sources & Further Reading

Here’s a concise overview before we dive into the key points:

• Home Ventilating Institute: Choosing the Right Ventilation
• International Residential Code 2021: M1503 Range Hoods
• ASHRAE Standard 62.2 Residential Ventilation (overview and addenda)
• NFPA 96: Ventilation Control and Fire Protection of Commercial Cooking Operations
• Broan-NuTone Range Hood Buying and Sizing Resources

These points provide quick orientation—use them alongside the full explanations in this page.