TryBuildCalcExhaust Fan / Range Hood CFM Calculator (Bathroom Fan & Kitchen Hood Sizing)
Find your exhaust fan CFM instantly.
🕒 Last updated: September 12, 2026
Inputs
ℹ️Floor area of the bathroom only.
ℹ️A jetted/whirlpool tub is one fixture, not an addition to a separate standard tub.
ℹ️Sets the ASHRAE 62.2/IRC code-minimum floor — 50 CFM intermittent, 20 CFM continuous.
Recommended Bathroom Fan Size
50 CFM
Next standard retail size ≥ 50 CFM raw requirement
Bathroom
Square footage: 50 sq ft
Ventilation type: Intermittent (fan switch)
Fixtures: None beyond toilet
Sizing Method Used
Method: square-footage method (bathroom ≤100 sq ft)
Square-footage method: 50 CFM
Fixture-based method: 50 CFM (reference only)
Raw requirement: 50 CFM
Assumptions Used
Per HVI's own guidance, ONE sizing method is selected — not compared and maxed — based on room size and ventilation type: for intermittent ventilation in a bathroom of 100 sq ft or less, the square-footage method (1 CFM per sq ft) applies; for intermittent ventilation in a bathroom over 100 sq ft, the fixture-based method applies instead (50 CFM each for a toilet — always assumed present — a separate shower, and a separate standard tub; 100 CFM for a jetted/whirlpool tub, which counts as ONE fixture, not an addition on top of a separate tub). Continuous ventilation uses a flat 20 CFM code-compliant rate regardless of room size or fixtures, per ASHRAE 62.2/IRC. The raw requirement (floored at the 50 CFM intermittent / 20 CFM continuous code minimum) is rounded UP to the next standard retail bathroom fan size (50, 80, 100, 110, 150, 190, 250, or 300 CFM) — bathroom fans are commonly sold in sizes across this range, the same way central AC is sold in standard tonnage sizes, though exact model availability still varies by manufacturer. A raw requirement beyond 300 CFM exceeds what a single residential fan typically covers and is flagged rather than capped. A known limitation of HVI's own method: once a bathroom exceeds 100 sq ft, its actual floor area no longer factors into the result at all — only fixture count does, so an unusually large, sparsely-fixtured bathroom may need more capacity than shown here. This is a planning estimate — actual duct length and elbow count reduce a fan's delivered CFM below its rated CFM; confirm your chosen unit's actual (ideally HVI-certified) performance with a licensed contractor.
What Is an Exhaust Fan / Range Hood CFM Calculator?
An exhaust fan / range hood CFM calculator estimates the right airflow capacity (in CFM, cubic feet per minute) for a bathroom exhaust fan or kitchen range hood — two genuinely different fixtures with their own code-referenced sizing methods, combined here in one calculator with a Bathroom/Kitchen mode toggle.
This calculator computes a planning estimate from established formulas (HVI guidance for bathrooms, HVI/Broan guidance for range hoods). It does not model duct length, elbow count, or filter resistance, all of which reduce a unit's delivered CFM below its rated CFM.
Why bathroom and kitchen exhaust need different formulas:
- A bathroom fan removes moisture — sized from floor area and fixtures (toilet, shower, tub) that produce that moisture
- A range hood removes heat, moisture, grease, and (for gas) combustion byproducts — sized from cooktop width, mount type, and fuel type/BTU output instead
- Bathroom fans are sold in a common standard retail size ladder (like AC tonnage); range hood CFM is continuous and manufacturer-specific (like furnace BTU) — so this calculator rounds one up to a standard size and reports the other as a required raw number
- Only range hoods cross a code-relevant makeup air threshold (400 CFM) worth checking — a bathroom fan never approaches this scale
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 |
|---|---|
| Square footage | 50 sq ft |
| Ventilation type | Intermittent (fan switch) |
| Fixtures | None beyond toilet |
Sizing Method Used
This bathroom is 50 sq ft (≤100 sq ft), so HVI's square-footage method applies — the fixture-based method below is shown for reference only.
| Method | Result |
|---|---|
| Square-footage method (1 CFM/sq ft, ≤100 sq ft only) — used | 50 CFM |
| Fixture-based method (>100 sq ft only) | 50 CFM |
| Code minimum (Intermittent (fan switch)) | 50 CFM |
| Raw requirement | 50 CFM |
| Nominal fan size, rounded up | 50 CFM |
Therefore, this bathroom needs a fan rated at approximately 50 CFM.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Inputs & Method Selection+-
- Correct calculator mode selected (Bathroom vs. Kitchen) for the actual room being sized
- Bathroom: square footage measured for the actual room, and separate shower/tub fixtures accurately identified (a jetted tub selected as jetted, not also marked as a separate standard tub)
- Kitchen: cooktop/range width measured at the actual cooking surface, not the cabinet or countertop width around it
✓Before Installation+-
- An HVI (Home Ventilating Institute) certified fan/hood selected, with independently verified CFM — not just the manufacturer's advertised rating
- If kitchen required CFM exceeds 400, checked whether IRC M1503.6 makeup air applies to this home's specific fuel-burning appliances, and planned for it if so
- Duct confirmed to terminate outdoors through a proper wall/roof cap, not into an attic, soffit, or wall cavity
✓Installation+-
- Duct diameter sized appropriately for the selected fan/hood's actual CFM, not left at whatever diameter was on hand
- If a makeup air system was installed, confirmed it's wired to activate automatically and simultaneously with the range hood, not as a separate manual switch
Full QC Checklist+−
Verification checklist for a bathroom exhaust fan or kitchen range hood sizing estimate — covering input/method selection, pre-installation planning, installation, and post-installation verification. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Inputs & Method Selection+-
- Correct calculator mode selected (Bathroom vs. Kitchen) for the actual room being sized
- Bathroom: square footage measured for the actual room, and separate shower/tub fixtures accurately identified (a jetted tub selected as jetted, not also marked as a separate standard tub)
- Ventilation Type (Intermittent vs. Continuous) selected deliberately for how the fan will actually be operated, not left at the default
- Kitchen: cooktop/range width measured at the actual cooking surface, not the cabinet or countertop width around it
- Kitchen: fuel type and BTU rating confirmed against the actual appliance's spec sheet or nameplate, not guessed
✓Before Installation+-
- An HVI (Home Ventilating Institute) certified fan/hood selected, with independently verified CFM — not just the manufacturer's advertised rating
- Duct routing planned with the shortest run and fewest elbows practical, sized against the manufacturer's own duct table and equivalent-length limits for the selected CFM
- If kitchen required CFM exceeds 400, checked whether IRC M1503.6 makeup air applies to this home's specific fuel-burning appliances, and planned for it if so
- Bathroom: fan location confirmed to actually ventilate the wet zone (near the shower/tub), not just centered in the room for symmetry
- Duct confirmed to terminate outdoors through a proper wall/roof cap, not into an attic, soffit, or wall cavity
✓Installation+-
- Duct diameter sized appropriately for the selected fan/hood's actual CFM, not left at whatever diameter was on hand
- Electrical circuit and wiring confirmed adequate for the selected unit, with GFCI protection where required near wet areas
- Fan/hood securely mounted to structural framing (ceiling joists or cabinet framing), not drywall or cabinet material alone
- Exterior vent cap installed with a functioning backdraft damper and insect screen
- If a makeup air system was installed, confirmed it's wired to activate automatically and simultaneously with the range hood, not as a separate manual switch
✓After Installation+-
- Actual delivered airflow verified at the register/vent with a flow hood or anemometer, not just assumed from the unit's rated CFM
- Noise level (in sones) confirmed acceptable in practice, especially for a bathroom fan sized larger than the room previously had
- If installed, makeup air system activation verified in practice (turns on automatically with the hood, not just on paper)
- Maintenance schedule established (range hood grease filter cleaning, bathroom fan housing/blade cleaning), not treated as a one-time installation task
- Bathroom: mirror fogging, lingering odor, or visible condensation confirmed to actually clear within a reasonable time after showering, not just assumed resolved because a correctly-sized fan was installed
Exhaust Fan / Range Hood Reference Tables
Code minimums, fixture allowances, and range hood sizing rates.
| Bathroom Fixture | CFM Allowance |
|---|---|
| Toilet (always counted) | 50 CFM |
| Separate shower | +50 CFM |
| Separate bathtub | +50 CFM |
| Jetted/whirlpool tub | +100 CFM |
Bathroom Code Minimums & Standard Fan Sizes
| Ventilation Type | Code Minimum |
|---|---|
| Intermittent | 50 CFM |
| Continuous | 20 CFM |
Standard retail bathroom fan sizes: 50, 80, 100, 110, 150, 190, 250, 300 CFM.
Range Hood Sizing Rates
| Method | Wall-Mounted | Island |
|---|---|---|
| Linear-foot (per ft of width) | 100 CFM | 150 CFM |
| BTU method (gas only) | BTU ÷ 100 | BTU ÷ 100 |
When should you use this exhaust fan / range hood CFM calculator?
- Getting a ballpark bathroom fan or range hood CFM before requesting contractor quotes or shopping for a unit.
- Checking whether a proposed or existing fan/hood seems adequately sized for your room.
- Determining whether your kitchen range hood crosses the 400 CFM threshold where makeup air may need to be checked before finalizing a design.
- Comparing how bathroom fixtures or a range's fuel type/BTU rating change the required CFM.
Quick Sizing Tips
- Choose an HVI-certified fan/hood — independently verified CFM ratings are more trustworthy than a manufacturer's own uncertified advertised rating.
- Keep duct runs short with as few elbows as possible — every foot and every elbow reduces delivered CFM below the unit's rated CFM.
- If your range hood calculation exceeds 400 CFM, check early whether IRC M1503.6 makeup air applies to your specific appliances and jurisdiction, rather than as an afterthought.
- For a bathroom, mount the fan near the actual moisture source (shower/tub), not just centered in the room.
- Check the sone (noise) rating before buying — a fan that's too loud often gets switched off, defeating the point of sizing it correctly.
Common Mistakes
- Venting a bathroom fan or range hood into an attic or wall cavity instead of outdoors — causes mold and moisture damage.
- Trusting an uncertified manufacturer's advertised CFM rating instead of HVI-certified independent test data.
- Installing a high-capacity range hood (over 400 CFM) without checking whether IRC M1503.6 makeup air applies to your home's specific appliances.
- Measuring range width from the cabinet or countertop instead of the actual cooking surface.
- Ignoring duct length and elbow count, then being surprised the installed system underperforms this calculator's estimate.
Limitations
- This is a planning estimate based on established rule-of-thumb formulas, not a full mechanical ventilation design.
- Does not model duct length, elbow count, or filter resistance — all reduce a unit's delivered CFM below its rated CFM.
- Range hood CFM is not standardized across manufacturers the way bathroom fan CFM is — the kitchen mode's number is a required target, not a guaranteed purchasable model size.
- Does not account for Broan's recommended CFM add-ons for an integrated grill/griddle or a hood mounted above the standard ~30in height — its own worked examples show these adjustments can add hundreds of CFM beyond the linear-foot and BTU methods alone (e.g. reaching 900 CFM where this calculator's two methods alone would return 600). Add capacity manually for either condition.
- A gas range with no BTU entered returns a width-only estimate — Broan's guidance treats this as reliable only up to roughly 60,000 BTU total output; above that, entering the actual BTU rating is necessary for an accurate result (see the warning shown when this applies).
- Does not model laundry rooms or other utility exhaust fans, which use a different (air-changes-per-hour) sizing method.
- Does not size exhaust ductwork (diameter/material) — bathroom and range hood exhaust duct sizing follows manufacturer duct tables and equivalent-length limits specific to that equipment, not this site's Duct Size Calculator, which models HVAC supply/return ducts at different velocity targets.
- The 400 CFM makeup air flag is informational only — IRC M1503.6 makeup air is conditional on qualifying non-direct-vent, non-mechanically-drafted fuel-burning appliances being inside your home's air barrier (an all-electric home may not be subject to it at all), and code tests your INSTALLED system's actual capacity, not this calculator's estimate. Confirm your specific appliances and jurisdiction's amendments with a licensed HVAC contractor.
- For a bathroom over 100 sq ft with few fixtures, HVI's fixture-based method no longer scales with the room's actual floor area — an unusually large, sparsely-fixtured bathroom may need more capacity than this calculator's result; consult a licensed contractor for large or unusual layouts.
- Does not verify local permit or electrical code requirements for a specific installation.
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.