Duct Size Calculator

How to Use This Duct Size Calculator

Start by choosing what you are sizing. The whole system mode takes the cooling capacity in tons and the airflow per ton, and returns the CFM your main trunk has to carry. The room or zone mode takes a single space's sensible load in BTU per hour and the design temperature difference, which is what you need when you are laying out a branch run to one register.

Then set the friction rate, the pressure the duct loses per 100 feet of equivalent length. Most residential systems are designed at 0.10 inches of water column per 100 feet, and that is the default here. Lower it to 0.08 or 0.05 for long runs with lots of elbows, and raise it to 0.15 or 0.20 only when the runs are short and the blower has static pressure to spare.

The calculator returns five numbers. Required airflow is the CFM the duct must carry. Round rigid metal is the smallest standard diameter that does the job at your friction rate. Round flex is one nominal size larger, because corrugated duct carries about three quarters of the air of smooth metal. Velocity tells you whether the result will be quiet. Rectangular metal gives you the equivalent flat duct for a joist bay or a soffit, computed with the standard ASHRAE equivalent diameter formula so it matches the round size in friction.

This is a planning tool built on the same equal friction method contractors use in the field. Use it to sanity check a quote, lay out a new trunk, or find out why the back bedroom never gets cold. A full ACCA Manual D design still belongs in the hands of whoever is installing the system.

CFM First: The Number That Sizes Every Duct

Ducts do not have a size in the abstract. They have a size relative to the air you push through them. Everything starts with airflow, measured in cubic feet per minute, or CFM.

For a whole system, the quickest route to CFM is the 400 CFM per ton rule. One ton of cooling is 12,000 BTU per hour, so a 3 ton air conditioner moves roughly 1,200 CFM and a 5 ton unit moves about 2,000 CFM. The number is nominal, not law. Humid climates run 350 CFM per ton so the coil stays colder and wrings more moisture out of the air, while dry climates and high-altitude homes run 450 CFM per ton so the coil does not freeze.

For a single room, work from the sensible heat equation:

CFM = Sensible Load (BTU/hr) ÷ (1.08 × Temperature Difference)

The 1.08 is not arbitrary. It is 60 minutes per hour times the density of standard air, 0.075 pounds per cubic foot, times the specific heat of air, 0.24 BTU per pound per degree. Multiply those and you get 1.08, which is why the constant shows up in every load calculation in the trade. A room with a 6,000 BTU sensible load and a 20 F supply difference needs 6,000 ÷ 21.6, or 278 CFM. That is a comfortable single 8 inch flex run, and it is also the reason a 6 inch flex run at roughly 85 CFM is too small for that room.

The Equal Friction Method and Where 0.1 Comes From

There are three ways to size duct: equal friction, velocity reduction and static regain. Residential work almost always uses equal friction, because it is simple and it self-balances reasonably well. The idea is to size every section of duct so it loses the same pressure per foot of length, which keeps the whole system near the same pressure at each takeoff.

The design number is a pressure loss per 100 feet of equivalent length, not measured length. A 90 degree elbow is not free; it costs the same pressure as 10 to 15 feet of straight duct. A supply boot with a register adds another 20 to 30 feet. Add the real pipe plus the equivalent length of every fitting and you get a number that is usually two to three times the tape measure reading on a real house.

Where does 0.10 come from? From the blower's budget. A typical residential air handler is rated at 0.5 inches of water column of external static pressure. The filter eats 0.15 to 0.25, the evaporator coil takes 0.20 to 0.30, the grilles and registers take another 0.05 to 0.10, and balancing dampers take a little more. What is left over for the duct itself is often only 0.10 to 0.20 inches total. Divide that by the total equivalent length of the longest run, usually 200 to 400 feet, and you land near 0.05 to 0.10 inches per 100 feet. That is the whole derivation, and it is why the standard value is 0.10 and not a larger, more convenient number.

Round Metal, Flex and Rectangular Duct

Each duct type trades something for something, and the tradeoffs change the size you buy.

• Rigid round metal is the baseline. Smooth walls, low friction, long life, and the capacity numbers in every chart assume it. It is the right choice for trunks and for any run you can reach with a ladder.
• Flexible round duct is a corrugated plastic skin over a wire helix. It bends around framing in seconds, which is why it dominates branch runs in production housing. The corrugations trip the airflow and raise friction, so a flex duct carries about 25% less air than the same diameter in metal. Compress it or leave a sharp bend and the loss gets worse fast. The practical fix is to go up one nominal size.
• Rectangular metal fits where round cannot, typically in a joist bay or a soffit. The standard sizes step in two inch increments from 3.25 by 10 up to 24 by 24. Because a rectangle has more wetted perimeter than a circle of the same area, it carries less air than a round duct of the same cross section. The equivalent diameter formula in this calculator handles that conversion so your rectangular choice matches the round one in friction.

One more variable is the inside surface. Ducts lined with acoustic insulation carry slightly less air than bare metal and should be sized with the liner thickness subtracted from the inside dimensions, not added to the outside.

Duct Capacity Chart by Diameter

These are approximate capacities at the standard 0.10 inches of water column per 100 feet friction rate. Read down the table to find the airflow you calculated, then across to the diameter.

Smaller sizes follow the same pattern downward. A 3 inch duct carries about 20 CFM in metal, which is why it is only ever used as a short takeoff to a single small register and never as a run.

Velocity Limits: Quiet Air or a Whistle

Diameter alone does not determine whether a duct is right. The speed of the air does, and speed is airflow divided by cross-sectional area. Push 1,200 CFM through a 16 inch duct and the air moves at about 860 feet per minute, which is a normal trunk velocity. Push the same 1,200 CFM through a 12 inch duct and it moves at roughly 1,530 feet per minute, which sounds like a vacuum cleaner running behind the wall.

The working limits in residential work:

• Supply trunk: 700 to 900 feet per minute. Above 1,000 the noise becomes the complaint that brings the homeowner back to the contractor.
• Branch run to a register: 500 to 700 feet per minute. A branch is close to the occupant's ear, so it has the tightest limit even though it carries the least air.
• Return trunk: 600 to 700 feet per minute, and lower if the return grille holds a filter.
• Below 400 feet per minute: quiet, but the air may not throw far enough to mix the room, which leaves cold and warm pockets.

Velocity and friction move together. If you pick a higher friction rate, you get a smaller duct and a faster air stream. That is the entire reason duct designers default to a low friction rate: it buys quiet.

Return Air Is Half the System

Most duct advice covers the supply side and stops. That is a mistake, because every cubic foot of air blown into a room has to find its way back to the blower. A system with generous supply ducts and a starved return behaves exactly like a system with undersized ducts everywhere.

Size the return for the same CFM as the supply but with a larger cross section, because velocity in a return trunk should stay near 600 to 700 feet per minute and lower still if the grille holds the system filter. A common field rule is one square foot of grille face area for every 300 CFM through a filter grille, or one square foot per 400 to 500 CFM through a plain return grille.

Older homes are the usual offenders. A house built in the 1970s with a single 14 by 20 return grille in a hallway may be trying to pull 1,200 CFM through a grille that wants to see 600. The symptom is a blower that runs hot and loud, a filter that bows inward, and rooms near the return that never quite satisfy the thermostat. Adding a second return path, or a larger filter grille in a central location, often does more for comfort than upsizing any supply branch.

Static Pressure: How to Tell Your Ducts Are Too Small

You do not have to guess whether a duct system is undersized. Measure it. A technician drills a small test port in the plenum before the coil and another after the blower, connects a manometer, and reads total external static pressure while the system runs at full airflow.

Most residential air handlers are rated at 0.5 inches of water column. A reading of 0.5 or below means the ducts are doing their job. A reading of 0.8 means the system is working against a restriction that the blower was never designed for. A reading above 1.0 is a duct system in serious trouble, and the airflow chart in the manufacturer's data will show the delivered CFM falling well below what the equipment needs.

The consequences show up in specific places. On the cooling side, low airflow drops the coil temperature below freezing and the coil ices over, which further blocks airflow and makes the problem worse in a spiral. On the heating side, low airflow lets the heat exchanger run above its rated temperature, which shortens its life and, on older furnaces, is a safety concern. The blower motor draws higher amps and runs hotter. A static pressure test costs almost nothing and it settles the argument about whether the ducts need work before anyone buys equipment.

Worked Example: A 3 Ton System at 1,200 CFM

Take a 2,000 square foot house in a moderate climate with a 3 ton air conditioner, standard 400 CFM per ton airflow, and a duct layout whose longest supply run has a total equivalent length of 300 feet.

Step one, airflow. Three tons times 400 CFM gives 1,200 CFM for the trunk.

Step two, friction rate. The air handler is rated at 0.5 inches of water column. The filter and coil together take 0.40, leaving 0.10 for the duct. Divided over 300 feet of equivalent length, that is 0.033 inches per 100 feet. This house is tighter than the default, so the design should use something near 0.05 rather than 0.10, which means larger ducts than the standard table suggests.

Step three, size the trunk. At 0.05 friction, a 16 inch metal duct carries about 870 CFM, well short of 1,200. An 18 inch carries about 1,157, still short. A 20 inch carries about 1,495 and clears the requirement with margin. So the correct trunk for this tight-static house is 20 inch round metal, not the 16 inch the 0.10 table would suggest. That difference is exactly why the friction rate input matters.

Step four, branches. A 300 square foot bedroom with a 4,500 BTU sensible load at a 20 F difference needs 4,500 ÷ 21.6, or 208 CFM. At 0.05 friction, a 9 inch metal branch carries about 215 CFM, so 9 inch metal or 10 inch flex does the job. Pulling a 6 inch flex run to that room, which is what a lot of tract housing actually does, would deliver roughly 60 CFM and leave the room 4 to 6 degrees warm all summer.

5 Duct Sizing Mistakes That Cost Comfort

1. Sizing from square footage alone. "One ton per 500 square feet" ignores envelope, windows and climate. Get the real load from a BTU calculation or a tonnage calculation first, then size ducts to the airflow that load requires.
2. Running flex duct at the metal size. Flex carries about 25% less air than rigid metal of the same diameter. Installing 6 inch flex where the chart says 6 inch metal undersizes that branch by a full quarter before anyone even compresses it around a corner.
3. Compressing flex to make it fit. A flex run squeezed to 70% of its length loses a third of its capacity. If the bay is short, use a metal elbow and a short flex tail instead of crushing the whole run.
4. Ignoring equivalent length. A run with six elbows and a long boot can have twice the effective length of the straight pipe. Sizing from the tape measure alone produces ducts that are quietly one or two sizes small.
5. Upgrading equipment without touching the ducts. Replacing an 80,000 BTU furnace with a 120,000 BTU unit on the same undersized ductwork raises static pressure further and can push the blower off its performance curve. If you add capacity, check the ducts and the panel with the electrical load calculator before the install.

Matching Ducts to the Rest of the System

A duct system is the middle link in a chain that starts with a load calculation and ends at the equipment. Get the airflow requirement from the HVAC BTU calculator or the AC tonnage calculator, confirm the heating side shares the same ducts with the furnace BTU calculator, then size the ducts here so both seasons move the air the equipment needs.

Insulation belongs in the same conversation. A tighter envelope lowers the load, which lowers the CFM, which lets you use smaller ducts or the same ducts at lower velocity. The insulation calculator covers that side of the equation, and the electrical load calculator confirms the panel can carry whatever equipment you end up choosing.