TryBuildCalcIsland Range Hood CFM Calculator (Island Mount, Higher CFM Rate)
Size your island range hood instantly.
🕒 Last updated: September 12, 2026
Inputs
ℹ️Measured at the actual cooking surface (burners/elements), not the cabinet width.
ℹ️Island hoods need more CFM per linear foot — no wall to help contain the rising plume.
ℹ️The linear-foot method applies the same rate to gas and electric — gas can need more CFM only when its BTU output is high enough to exceed that.
ℹ️Total burner BTU output from the range's spec sheet. Leaving this blank returns a width-only estimate — Broan's guidance switches to the BTU method above roughly 60,000 BTU, so a high-output range needs this entered for an accurate result.
Required Range Hood CFM
650 CFM
Higher of the linear-foot and BTU methods (see below)
Makeup air may be required (over 400 CFM). Once installed range hood capacity exceeds 400 CFM, IRC M1503.6 may require makeup air (outdoor air supplied back into the home to offset what the hood exhausts) — but only when qualifying non-direct-vent, non-mechanically-drafted fuel-burning appliances (such as an atmospherically-vented gas furnace, water heater, or fireplace) are located inside the home's air barrier. An all-electric home with no such appliances is not automatically subject to this requirement. Where it does apply, code permits either mechanical or passive makeup air, sized to your INSTALLED system's actual capacity, not just this calculator's estimate. Confirm your specific appliances and local code amendments with a licensed HVAC contractor.
Range & Hood
Cooktop width: 42 in
Hood mount: Island
Fuel type: Gas
Gas output: 65,000 BTU
Sizing Methods Compared
Linear-foot method: 525 CFM
BTU method: 650 CFM
Required (higher of the two): 650 CFM
Assumptions Used
Two independently-sourced sizing methods apply and, when both are computed, the HIGHER is used: the linear-foot method (HVI/Broan guidance — 100 CFM per linear foot of cooktop width for a wall-mounted hood, 150 CFM per linear foot for an island hood, since an island hood has no wall to help contain the rising plume, applied the SAME way regardless of fuel type) and — for gas cooktops with a BTU rating entered — the BTU method (Broan guidance — roughly 100 CFM per 10,000 BTU of total gas output, i.e. BTU ÷ 100). For an electric range, or a gas range with no BTU entered, only the linear-foot method is computed. Broan's own guidance treats the width-based method as reliable for gas cooktops up to roughly 60,000 BTU total output — above that, the BTU method should govern, so a high-output gas range left without a BTU entry can understate the true requirement; a warning is shown above when this applies. Broan also recommends adding CFM beyond these two methods for an integrated grill/griddle or a mounting height above the standard 30in, which this calculator does not model (see Limitations). Range hood CFM is NOT standardized into a common retail size ladder the way bathroom fans are — ratings are continuous and manufacturer-specific across a wide range, so this reports the REQUIRED CFM directly, rounded UP for readability (never down, so the displayed figure never understates the raw requirement). Once the RAW required CFM exceeds 400, IRC M1503.6 MAY require makeup air — but only when qualifying non-direct-vent, non-mechanically-drafted fuel-burning appliances are inside the home's air barrier (an all-electric home is not automatically subject to this), and code tests your INSTALLED system's actual capacity, not this calculator's estimate; code also permits either mechanical or passive makeup air. This is a planning estimate — actual duct length, elbow count, and filter type all reduce a hood's delivered CFM below its rated CFM; confirm your chosen unit's actual performance and your specific appliances/local code requirements with a licensed contractor.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Exhaust Fan / Range Hood CFM Calculator.
Island range hood CFM sizing
An island range hood needs 150 CFM per linear foot of cooktop width, versus 100 CFM per linear foot for a wall-mounted hood of the same width, since there's no wall behind an island cooktop to help contain the rising heat and grease plume.
This page defaults to a 42 in island gas range at 65,000 BTU — edit the inputs above to match your actual island cooktop.
Exhaust Fan / Range Hood CFM Formula: How Is It Determined?
Bathroom mode selects ONE sizing method based on room size and ventilation type, per HVI's own guidance — it does not compare methods and take the highest. Kitchen mode compares two independently-sourced methods and uses the higher.
Bathroom Mode
Intermittent, ≤100 sq ft: Raw CFM = max(Bathroom Sq Ft × 1 CFM/sq ft, 50 CFM code minimum)
Intermittent, >100 sq ft: Raw CFM = max(Fixture Method, 50 CFM code minimum)
Fixture Method = 50 (toilet) + 50 (if separate shower) + 50 (if standard tub) or 100 (if jetted tub)
Continuous (any size): Raw CFM = 20 CFM flat
Nominal Fan Size = next standard retail size ≥ Raw CFM, up to 300 CFM
Per HVI's own guidance, the square-footage and fixture-based methods are BOTH intermittent-ventilation rules of thumb, and are alternatives selected by room size — not two methods to compare and take the higher of. A jetted/whirlpool tub counts as one fixture (100 CFM), not an addition on top of a separate standard tub. Continuous ventilation is a flat 20 CFM code-compliant rate, not scaled by room size or fixtures. The raw requirement is rounded UP to a standard retail bathroom fan size (50, 80, 100, 110, 150, 190, 250, or 300 CFM), since bathroom fans genuinely are sold in this common size range across manufacturers (though exact model availability still varies by brand).
Kitchen Mode
Linear-Foot Method = (Width in inches ÷ 12) × 100 (wall) or × 150 (island)
BTU Method (gas only, if entered) = Total BTU Output ÷ 100
Required CFM = max(Linear-Foot Method, BTU Method) when BTU is entered for gas, otherwise Linear-Foot Method alone
Required CFM is reported directly, rounded UP for readability (never down, so the displayed figure never understates the raw requirement), since range hood CFM is NOT standardized into a common retail size ladder the way bathroom fans are. Once the RAW required CFM exceeds 400, IRC M1503.6 may require makeup air — but only when qualifying non-direct-vent, non-mechanically-drafted fuel-burning appliances are inside the home's air barrier; code permits mechanical or passive makeup air, sized to your installed system's actual capacity, not just this estimate. Broan's own guidance treats the linear-foot method as reliable for gas cooktops only up to roughly 60,000 BTU total output — above that, entering the BTU rating is necessary, since a width-only estimate can significantly understate a high-output gas range's true requirement. Broan also recommends additional CFM beyond both methods here for an integrated grill/griddle or a mounting height above the standard ~30in, which this calculator does not model.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Cooktop width | 42 in |
| Hood mount / fuel | Island / Gas |
| Gas BTU output | 65,000 BTU |
Comparing Sizing Methods
| Method | Calculation | Result |
|---|---|---|
| Linear-foot | (42/12) × 150 | 525 CFM |
| BTU | 65,000 ÷ 100 | 650 CFM |
| Required (higher of the two) | 650 CFM | |
Therefore, this 42 in island gas range needs a hood rated at approximately 650 CFM. Since this exceeds 400 CFM, IRC M1503.6 may require makeup air if qualifying fuel-burning appliances are inside your home's air barrier — check your specific situation with a licensed HVAC contractor.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.