Exhaust Fan Duct Sizing: Bathrooms, Kitchens, and Commercial

Exhaust fans serve a different purpose than HVAC supply and return systems, but the ductwork principles are the same: size the duct to move the required CFM at an acceptable velocity and static pressure drop. Get the sizing wrong — usually too small or too long — and the fan moves far less air than its nameplate rating. The result is inadequate ventilation, moisture accumulation, and codes violations.

This guide covers exhaust fan duct sizing for residential bathrooms, residential kitchen range hoods, and commercial exhaust applications, with the equations and reference tables you need to do it right.

Why Fan Ratings Are Misleading

Exhaust fan CFM ratings are measured at 0.1 inches WC static pressure — essentially free delivery with almost no system resistance. Real installations have duct runs, elbows, roof or wall caps, and dampers that add static pressure. The fan's actual airflow at system resistance is almost always lower than the nameplate CFM.

HVI (Home Ventilating Institute) certifies fans at 0.1 inches WC and publishes performance curves at higher static pressures. A fan rated at 80 CFM at 0.1 inches WC might deliver only 55 CFM at 0.25 inches WC — a 31% drop. If your duct system adds more than 0.1 inches WC of resistance, you need either a larger duct or a fan with a steeper performance curve to maintain the required airflow.

The lesson: always check the fan's performance curve at your calculated system static pressure, not just the nameplate CFM.

Bathroom Exhaust Fan Duct Sizing

IRC Section M1507.3 requires minimum exhaust airflow rates for bathrooms: 50 CFM intermittent, or 20 CFM continuous. ASHRAE 62.2 uses similar minimums but calculates based on floor area for whole-house ventilation strategy. Most bath fans in practice are sized at 1 CFM per square foot of bathroom area, minimum 50 CFM.

Duct diameter selection for bathroom fans:

Keep bathroom exhaust duct velocity between 500 and 900 FPM. Below 500 FPM, moisture can condense in the duct before reaching the exterior; above 900 FPM, noise increases and static pressure rises enough to significantly reduce fan output.

IRC M1507.2 prohibits exhausting bathroom fans into attics, crawl spaces, or wall cavities — they must terminate outside the building. Use an insulated flex or rigid duct with a code-compliant roof or soffit cap. Avoid through-soffit terminations that exhaust below the attic ventilation intake — the moist air will re-enter the attic.

Kitchen Range Hood Duct Sizing

Kitchen range hood ductwork carries grease-laden exhaust air, which creates requirements beyond basic sizing: the duct must maintain minimum velocity to prevent grease accumulation, and all joints must be grease-tight.

HVI and IRC M1503 require kitchen hood exhaust ducts to be:

• Smooth interior surface — no flex duct (grease accumulation in corrugations creates fire risk)
• Grease-tight construction — all joints must be sealed or continuously welded
• Minimum 26-gauge galvanized or 0.016-inch stainless steel for residential applications
• Maximum duct length: check manufacturer specifications, but generally 30-foot equivalent length limit for residential hoods
• Minimum velocity: 600 FPM to prevent grease dropout in the duct

Range hood duct sizing by airflow:

Each 90-degree elbow in a range hood duct run is equivalent to 10–15 feet of straight duct. A run with two elbows and a 15-foot straight length has an equivalent length of 35–45 feet. If your hood has a 30-foot equivalent length limit, two elbows already consume most of your budget — use smooth-radius elbows (R/D = 1.5 or greater) to reduce fitting losses.

Commercial Exhaust Duct Sizing

Commercial exhaust systems — serving restrooms, locker rooms, laboratories, or general ventilation — are designed using the same principles but to higher accuracy standards. ASHRAE 62.1 provides minimum ventilation rates by space type; SMACNA provides duct construction standards.

Commercial exhaust duct velocity targets:

• Main trunk: 1,000–1,500 FPM
• Branch ducts: 700–1,000 FPM
• Final runouts to diffusers: 500–700 FPM

For multi-branch commercial exhaust systems, the design must balance static pressure at each branch junction so each space receives its required CFM without the other branches being starved or oversupplied. This requires iterative duct sizing using the equal friction method or static regain method, both documented in ACCA Manual Q and SMACNA HVAC Systems Duct Design.

Equivalent Length Method for Duct Runs

Every fitting in an exhaust duct run adds resistance equivalent to a length of straight duct. Use equivalent length (EL) tables from ASHRAE or fan manufacturers to calculate total system resistance:

• 4-inch 90° elbow: 10–15 feet EL
• 6-inch 90° elbow (short radius): 15–20 feet EL
• 6-inch 90° elbow (long radius): 8–12 feet EL
• Roof cap or wall cap: 25–35 feet EL depending on style
• Damper (open): 5–10 feet EL
• Tee junction (branch): 15–25 feet EL

Sum straight duct length plus fitting equivalent lengths to get total equivalent length. Multiply by the friction rate per 100 feet at design velocity to get system static pressure. Compare to the fan performance curve to confirm adequate airflow at that static pressure. If not, increase duct diameter or select a fan with a higher static pressure rating.

Code Requirements Summary

Key code references for exhaust duct sizing:

• IRC M1507.3: Bathroom exhaust — 50 CFM intermittent minimum, terminate outside building
• IRC M1503: Kitchen exhaust — smooth duct, 0.016" minimum thickness, terminate outside building
• IRC M1502: Dryer exhaust — 4-inch minimum diameter, maximum 25-foot equivalent length (not applicable to HVAC fans but often confused)
• ASHRAE 62.1/62.2: Minimum ventilation rates by occupancy type and floor area
• SMACNA HVAC Systems Duct Design: Commercial exhaust duct construction and sizing standards