Bathroom Fan Duct Length by Duct Size and Elbows

Under Table M1504.2 of the International Residential Code (IRC), a bath fan rated 80 CFM at 0.25 in. of static pressure can run 4 ft of 4 in. flex duct or 31 ft of 4 in. smooth duct, and you take 15 ft off for every elbow. At 100 CFM, 4 in. flex isn't allowed at any length and 4 in. smooth stops at 10 ft.

The table reads the same in the 2018, 2021 and 2024 IRC, and the 2015 edition printed the same numbers as Table M1506.2. Size the fan first on the free bathroom exhaust fan calculator, which leaves duct losses to you, then check the run you're planning against the numbers below.

What's the maximum duct length for a bathroom fan?

It depends on the fan's rated airflow, the duct diameter and whether the duct is flex or smooth-wall. Section M1504.2 caps exhaust duct length at the lengths in Table M1504.2. For an 80 CFM fan, 4 in. flex tops out at 4 ft and 4 in. smooth at 31 ft. Going to 5 in. raises those limits to 81 and 152 ft, and 6 in. duct has no limit at 80 CFM.

2021 IRC § M1504.2

"The length of exhaust and supply ducts used with ventilating equipment shall not exceed the lengths determined in accordance with Table M1504.2." - International Code Council, codes.iccsafe.org

Here's the part of the table that covers most bath fans. Lengths are in feet with no elbows in the run.

NL means no limit on length for that size. X means the size isn't allowed, because, in the table's own words, any length of it "with assumed turns and fittings will exceed the rated pressure drop." The full table also runs to 300 CFM, with rows for 7 in. duct and for 8 in. and larger.

Which fan rating do you read the table at?

The rating at 0.25 in. of water column, because that's what the column header says. The big number on the box can be taken at a different pressure. Broan's spec sheet for the AE110K rates it 110 CFM at 0.1 in. and 92 CFM at 0.25 in., so on Table M1504.2 the fan reads at 92 CFM, between the 80 and 100 CFM columns.

The header reads "Fan airflow rating (CFM @ 0.25 inch wca)," and footnote a says the rating follows ANSI/AMCA 210-ANSI/ASHRAE 51. ENERGY STAR's listings for the AE110 and AE110K show 95.5 CFM at 0.25 in., a few CFM off the spec sheet's 92, but still between the same two columns.

The table has no 92 or 95 column, and the code text doesn't say how to read a fan between columns. We read it at the next column up, here 100 CFM, because allowed length only drops as airflow rises, so the higher column is the stricter one. If you want to use the lower column, that's a question for your inspector.

That same 0.25 in. number matters for the fan itself. In the 2021 IRC, Table M1505.4.4 sets local exhaust for a bathroom at 50 cfm intermittent or 20 cfm continuous, and its footnote says the listed rate has to meet that at a minimum static pressure of 0.25 in. The 2024 IRC moves the rule into Section M1505.5. The 2018 table doesn't have that footnote.

How many elbows can each duct size take?

Take 15 ft off the table length for each elbow, per footnote c. At 80 CFM, 4 in. smooth duct drops from 31 ft straight to 16 ft with one elbow and 1 ft with two, and a third elbow rules it out. 4 in. flex has no room for even one. At 100 CFM, 6 in. flex still leaves 128 ft after two elbows.

2021 IRC § Table M1504.2, note c

"This table assumes that elbows are not used. Fifteen feet of allowable duct length shall be deducted for each elbow installed in the duct run." - International Code Council, codes.iccsafe.org

We worked the deduction through for the common bath fan columns. "None" means the elbows use up the whole allowance.

Sizes marked NL in the code table aren't in this one. Footnote d puts no limit on their length, and by our reading elbows don't change that. The 3 in. rows and the 150 CFM column are left out too, so for those take 15 ft per elbow off the first table.

A rectangular wall duct gets a diameter from footnote b, four times the area divided by the perimeter. A 3-1/4 x 10 in. duct works out to 4 x 32.5 / 26.5 = 4.9 in. The table has no 4.9 in. row, so by our reading you use the 4 in. row.

Why does a new 110 CFM fan pull so weakly on flex?

If it's on 4 in. flex, the table leaves almost no room. In the source thread, a homeowner with new Broan 110 CFM fans in two bathrooms found accordion flex duct and weak suction in both. The post doesn't name the model or duct size. A fan like the AE110K, rated 92 CFM at 0.25 in., gets 4 ft of 4 in. flex at the 80 CFM column and none at 100 CFM or more.

The poster on r/hvacadvice couldn't hold a piece of toilet paper up to the grille. They snaked the duct and blew it out with a leaf blower, and very little debris came out. Each fan has its own duct, and the poster's guess was a small diameter plus the accordion flex. A clean duct doesn't change what the table allows, so the length check comes before a new fan.

Here's the same check for a job you'd quote, a 12 ft run to a wall cap with two elbows.

1. Airflow needed. An 8 x 10 bathroom with an 8 ft ceiling comes out at 80 CFM on our calculator, from HVI's 1 CFM per sq ft.
2. Fan. The AE110K moves 92 CFM at 0.25 in., which covers the 80 CFM. Its spec sheet calls for 4 in. round ductwork, so the 4 in. rows apply.
3. Column. 92 CFM sits between 80 and 100, so by our reading use the 100 CFM column and look at the 80 CFM column as the best case.
4. Check. 4 in. smooth allows 10 ft at 100 CFM, and two elbows take 30 ft off, so nothing's left. At 80 CFM it's 31 - 30 = 1 ft. Either way the 12 ft run fails, and 4 in. flex fails worse.
5. Fix. Switch to a straight run of 4 in. smooth-wall duct, 10 ft or less, measure the installed airflow under the exception below, or switch to a fan rated for 6 in. duct and run 6 in. duct. ENERGY STAR lists Broan's XB110 at 110.2 CFM at 0.25 in. with a 6 in. duct. By our next-column-up reading that's the 125 CFM column, where 6 in. flex leaves 91 - 30 = 61 ft after two elbows and 6 in. smooth leaves 138 ft, so both cover the 12 ft run.

When doesn't the length table apply?

When the duct system follows the fan maker's design criteria, or when the installer or an approved third party measures the installed airflow with a flow hood, flow grid or other airflow measuring device. That's the exception written into M1504.2. Broan's AE110K spec sheet prints a fan curve but no duct length chart, so check the installation instructions before you count on the first route.

2021 IRC § M1504.2, Exception

"Duct length shall not be limited where the duct system complies with the manufacturer's design criteria or where the flow rate of the installed ventilating equipment is verified by the installer or approved third party using a flow hood, flow grid or other airflow measuring device." - International Code Council, codes.iccsafe.org

The exception doesn't say what number the measured flow has to hit. By our reading it's the local exhaust rate the room needs, which for a bathroom is 50 cfm intermittent or 20 cfm continuous in Table M1505.4.4 of the 2018 and 2021 IRC and Table M1505.5 of the 2024 IRC.

The 2015 edition had the same exception under M1506.2. Local amendments can change any of this, so check the edition and amendments your jurisdiction enforces.

Checking a bath fan run with BuildSolver

Get the airflow from the free bathroom exhaust fan calculator, then check the duct against Table M1504.2 yourself. The calculator sizes the fan from floor area, ceiling height and fixtures, rounds up to a common fan size of 50, 70, 80, 110 or 150 CFM and tells you to size to the fan's CFM at 0.25 in. It doesn't compute duct losses, so the length check is yours.

1. Enter the bathroom length, width and ceiling height, plus the toilets, showers and tubs. Fixture counts only change the result for a bathroom over 100 sq ft. Above 150 CFM the calculator shows the raw number and points you to two fans or an inline fan.
2. Pick a fan whose airflow at 0.25 in. meets the calculator's number. Take it from the spec sheet or the ENERGY STAR listing, since the number on the front of the box can be taken at a different pressure.
3. Find that rating's column in Table M1504.2, or, by our reading, the next column up, and the row for your duct diameter and type.
4. Take 15 ft off for each elbow and compare what's left with the run you measured on site.
5. If it doesn't fit, shorten the run, cut elbows, go to a fan and duct size the table allows, or plan to measure the installed airflow.

Run the numbers for your next bathroom on the bathroom exhaust fan calculator, then hold the duct plan up against the elbow table above before you cut the ceiling.

Sources

• International Code Council, 2021 International Residential Code, Chapter 15, Section M1504.2, Table M1504.2 and Table M1505.4.4: codes.iccsafe.org
• International Code Council, 2018 International Residential Code, Chapter 15, Section M1504.2, Table M1504.2 and Table M1505.4.4: codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Chapter 15, Section M1504.2, Table M1504.2 and Section M1505.5: codes.iccsafe.org
• International Code Council, 2015 International Residential Code, Chapter 15, Section M1506.2 and Table M1506.2: codes.iccsafe.org
• Broan, AE110K Flex Series fan specification sheet 02C 1102372B, HVI performance at 0.1 and 0.25 in.: lightingsupply.com
• ENERGY STAR Certified Ventilating Fans dataset, Broan AE110, AE110K and XB110 listings: data.energystar.gov
• Home Ventilating Institute, "Bathroom Ventilation," 1 CFM per sq ft for bathrooms up to 100 sq ft: hvi.org
• Source question, r/hvacadvice, "New 110 CFM bathroom fans - weak suction": reddit.com

For estimation purposes only. The code edition and amendments your jurisdiction has adopted, and the fan maker's installation instructions, govern how the exhaust duct goes in.