Duct Calculator: Size Any Supply or Return Duct in Under a Minute

Duct Sizing Rules of Thumb (Quick Answers)

These are the numbers I sanity-check every job against. The calculator above refines them for your exact ductwork, material, and run.

• Airflow per ton: 400 CFM per ton of cooling (350 humid climates, 450 dry).
• Airflow per room: about 1 CFM per square foot in an average US home.
• The workhorse branch: a 6 inch round metal duct carries roughly 110 CFM, enough for most bedrooms.
• Velocity limits: keep supply trunks under 900 FPM, branches under 700, returns under 700.
• Flex duct rule: size flex one diameter larger than metal, pulled tight and strapped every 4 feet.
• Return rule: total return duct area should equal or beat the supply side.

How This Duct Calculator Works

This tool uses the equal friction method, the same approach behind the ASHRAE friction chart and the spinning ductulator wheel in every senior tech’s bag. You pick a design friction rate, and the calculator solves for the smallest duct that moves your CFM without exceeding it.

Then it runs a second check on air velocity, because a duct that passes on friction can still whistle and roar if air moves too fast. The recommendation you see is the smallest standard size that passes both tests, the same logic ACCA Manual D applies across a whole duct system. Run it forward as an air duct sizing tool, or run it backward as a CFM calculator with the existing-duct mode.

For rectangular ducts, the calculator converts your dimensions using the ASHRAE equivalent diameter formula. A rectangular duct always needs more cross-sectional area than its round equivalent because corners create drag.

What Is Friction Rate and Why It Decides Your Duct Size

Friction rate is the pressure your blower loses pushing air through 100 feet of duct, measured in inches of water column. Residential design typically lands between 0.05 and 0.10 in. wc per 100 ft.

A lower friction rate means bigger, quieter ducts that are kinder to long runs full of elbows. A higher rate means smaller ducts that only work on short, direct runs.

The honest version, the one Manual D formalizes, is to take your equipment’s available static pressure, subtract the pressure drops of the coil, filter, and registers, then divide by the total effective length of the worst run. If you are replacing one duct or adding a room, the Auto presets in this calculator reflect what that math usually produces in a typical American home.

Air Velocity Limits That Keep Ducts Quiet

Velocity in feet per minute (FPM) is airflow divided by duct area. Push past the limits below and you get hiss at the registers, rumble in the walls, and extra static pressure your blower has to fight.

The existing-duct mode above also works as a duct velocity calculator: enter a size and airflow, and the FPM comes back with a noise rating. Commercial designers sometimes use the velocity reduction or static regain methods instead, but equal friction remains the residential standard.

Bedrooms deserve the quiet end of every range. Nobody complains about a duct being too quiet, but I have been called back plenty of times for ones that were too loud.

Round Duct CFM Chart (Quick Reference)

These capacities come from the same friction chart math the calculator uses, for smooth galvanized metal. Flex duct delivers roughly 70 to 80 percent of these numbers in a real-world install.

Duct Size by AC Tonnage (Trunk Sizing Chart)

Bookmark this duct sizing chart for quick trunk lookups. It assumes 400 CFM per ton, smooth metal, and a 0.08 friction rate, which is the typical American install. Run your own numbers in the calculator above if your climate or material differs.

Return trunks should match or exceed these sizes, since a starved return chokes everything downstream. Flex trunk lines are a bad idea at any of these airflows.

CFM Per Ton and Per Room: The Numbers Behind the Calculator

Air conditioners and heat pumps are designed around 400 CFM per ton of cooling, so a 3 ton system wants about 1,200 CFM through the trunk. In humid Gulf states many of us drop to 350 per ton for better moisture removal, while desert installers push 450.

At the room level, 1 CFM per square foot is the classic field estimate for an average American home with 8 foot ceilings. Sunrooms, bonus rooms over garages, and vaulted spaces need more, and a Manual J calculation is the right answer whenever an inspector or a warranty is involved.

Flex Duct vs Sheet Metal: Why the Same Size Behaves Differently

The corrugated inner liner of flex duct creates far more friction than smooth metal, and every bit of sag or compression makes it worse. Research from the Texas A&M flex duct studies showed compressed flex can lose more than half its rated airflow.

This calculator already builds in a correction for a normal pulled-tight flex install. If your flex will snake around trusses or sag between straps, go one size larger again, and support it every 4 feet with wide straps.

Sheet metal and spiral duct size to the baseline numbers. Duct board sits in between, and its interior should never be cut or scuffed, because that raises friction and can shed fibers into your airstream.

Rectangular Ducts and Equivalent Diameter

Round to rectangular duct conversion trips up more DIYers than any other step. A 10 inch round duct and a 10 x 8 rectangular duct are not equal, even though the rectangle has more area on paper. Corners and the flat aspect ratio add friction, which the ASHRAE equivalent diameter formula accounts for.

The same equivalent diameter math applies when sizing a supply plenum or a transition off the air handler. Keep the aspect ratio under 4 to 1 whenever you can. A 24 x 6 duct technically fits a shallow chase, but it wastes material, leaks at more seams, and drums when the blower kicks on.

Range Hood, Bath Fan, and Dryer Duct Sizing

Exhaust ducts play by different rules than comfort ducts. The runs are short and dedicated, so they tolerate velocities up to 1,800 FPM that would be unbearable in a living room supply.

Pick the Exhaust option in the calculator above, or use this chart based on your hood’s rated CFM. Always use smooth metal for kitchen exhaust, never flex, because grease collects in every corrugation.

Clothes dryers are the exception to everything: the International Residential Code fixes them at 4 inch smooth rigid metal with a 35 foot equivalent length cap, minus 5 feet per elbow. No screws into the airstream, no plastic flex, no calculator needed.

Total Effective Length: The Part Most DIYers Miss

Fittings cost you far more than straight pipe. A single hard 90 degree elbow can add 15 to 30 feet of equivalent length, and a poorly made takeoff even more.

SMACNA fitting tables assign every elbow, takeoff, and boot an equivalent length, and they add up fast. That is why the friction rate presets matter: a short straight branch can be sized at 0.10, while a long run with four elbows belongs at 0.05 or 0.06. When in doubt, pick the lower friction preset and accept the slightly larger duct.

Common Duct Sizing Mistakes I Get Called Out to Fix

• Sizing flex like metal. A 6 inch flex run does not deliver 6 inch metal numbers, ever.
• Starving the return. Undersized returns choke the whole system and make the blower howl. Returns should be at least as generous as the supply side.
• Reducing too early. Keep the trunk full size past the first few takeoffs, then step down.
• Ignoring static pressure. A $30 manometer reading across your air handler tells you more than any guess. Total external static should usually sit at or below 0.5 in. wc on residential gear.
• Forgetting the filter. High MERV filters add resistance, so duct generosity matters even more if you run MERV 11 or 13 for air quality.
• Crushing flex in tight framing. One flattened spot throttles the entire run.

Frequently Asked Questions

This calculator provides estimates based on the ASHRAE friction chart method and standard industry practice for planning and educational purposes. Actual duct performance depends on fitting quality, total effective length, sealing, and equipment static pressure. Consult a licensed HVAC contractor and local mechanical code for final design, and verify with measured static pressure and airflow. As an Amazon Associate, this site earns from qualifying purchases.