Duct CFM Calculator
Calculate HVAC airflow from duct size, air velocity, AC tons or cooling BTU — instantly in your browser.
HVAC duct airflow calculator
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Duct CFM Calculator Quick Answer
Duct CFM is the amount of air moving through a duct each minute.
For a known duct size and velocity, CFM = duct area × velocity.
For AC tonnage, CFM = tons × the CFM-per-ton value you select.
For cooling BTU/hr, tons = BTU/hr ÷ 12,000, then CFM = tons × the selected CFM-per-ton value.
400 CFM per ton is a common planning assumption, not a guarantee of actual blower airflow. Equipment manufacturer data and full HVAC design should govern final design.
How to Use the Duct CFM Calculator
- Choose the type of calculation.
- Enter CFM, duct dimensions, velocity, tons or cooling BTU.
- Choose the duct and application assumptions when relevant.
- Calculate.
- Read the recommended size/airflow and the supporting values.
- Copy or share the result.
How Duct CFM Is Calculated
Round duct. Area in square feet = π × diameter² ÷ 4 ÷ 144, with diameter in inches. CFM = area × velocity in FPM. Working backward, area = CFM ÷ FPM, and diameter in inches = 12 × √(4 × area ÷ π).
Rectangular duct. Area in square feet = width × height ÷ 144. CFM = area × FPM. When one side is fixed, the calculator solves the other side from the required area, then rounds that side up to the next even inch.
CFM from velocity. Velocity is speed in feet per minute. The same CFM needs less area when the air moves faster, and more area when it moves slower.
CFM from tonnage. CFM = cooling tons × the selected CFM-per-ton ratio. The choices are 350, 400, 450, or a custom ratio.
BTU to CFM. Tons = cooling BTU/hr ÷ 12,000. At 400 CFM per ton, CFM = cooling BTU/hr ÷ 30.
CFM per ton. CFM per ton = airflow CFM ÷ cooling tons.
Equivalent diameter. De = 1.30 × (width × height)^0.625 ÷ (width + height)^0.25, with width and height in inches. Equal area does not mean equal friction. A very flat rectangle has more surface, and more friction, than a round duct carrying the same CFM.
Friction rate. The friction method uses a galvanized-steel approximation of the ASHRAE friction chart for standard air: inches of water per 100 ft ≈ 0.109136 × CFM^1.9 ÷ diameter^5.02. Diameter is solved from that equation. It is a straight-run chart approximation, not a Manual D fitting schedule.
This calculator is a planning and reference tool. Final HVAC duct design should consider equipment blower data, available static pressure, friction rate, fittings and effective length, duct leakage, balancing, noise, applicable standards, and the actual system. It is not a code-compliant Manual D design.
Worked Examples
400 CFM duct
Input. 400 CFM at a 700 FPM planning velocity, round metal duct
Calculation. Area (ft²) = CFM ÷ FPM. Diameter (in) = 12 × √(4 × area ÷ π).
Result. Recommended round duct: 12 in. Exact diameter 10.2 in. Recommended standard size 12 in. Airflow 400 CFM. Actual velocity 509 FPM.
Meaning. A 400 CFM branch is a common single-room or short-trunk planning number. The exact round size is smaller than the next manufactured size, so the air slows once you step up to that standard duct.
800 CFM duct
Input. 800 CFM at a 700 FPM planning velocity, round metal duct
Calculation. Area (ft²) = CFM ÷ FPM. Diameter (in) = 12 × √(4 × area ÷ π).
Result. Recommended round duct: 16 in. Exact diameter 14.5 in. Recommended standard size 16 in. Airflow 800 CFM. Actual velocity 573 FPM.
Meaning. 800 CFM is enough for a short supply trunk or more than one room. The recommended size is the next standard round duct at or above the exact diameter, not a silent rounding of the decimal.
1,000 CFM duct
Input. 1,000 CFM at a 700 FPM planning velocity, round metal duct
Calculation. Area (ft²) = CFM ÷ FPM. Diameter (in) = 12 × √(4 × area ÷ π).
Result. Recommended round duct: 18 in. Exact diameter 16.2 in. Recommended standard size 18 in. Airflow 1,000 CFM. Actual velocity 566 FPM.
Meaning. 1,000 CFM is also what 2.5 cooling tons produce at 400 CFM per ton. Sizing that airflow at 700 FPM is a planning start. A higher allowed velocity would use a smaller duct and more pressure.
2.5 ton AC
Input. 2.5 cooling tons at 400 CFM per ton
Calculation. Tons = cooling BTU/hr ÷ 12,000. CFM = tons × 400, which is the same as cooling BTU/hr ÷ 30.
Result. Planning airflow: 1,000 CFM. Cooling tons 2.5 tons. Cooling BTU/hr 30,000. CFM per ton 400.
Meaning. 2.5 tons × 400 CFM per ton is 1,000 CFM of planning airflow. That is not the blower’s measured CFM. Use the CFM → duct size mode if you want a duct for this airflow.
36,000 BTU/hr cooling
Input. 36,000 cooling BTU/hr at 400 CFM per ton
Calculation. Tons = cooling BTU/hr ÷ 12,000. CFM = tons × 400, which is the same as cooling BTU/hr ÷ 30.
Result. Planning airflow: 1,200 CFM. Cooling tons 3 tons. Cooling BTU/hr 36,000. CFM per ton 400.
Meaning. 36,000 cooling BTU/hr is 3 tons because 36,000 ÷ 12,000 = 3. At 400 CFM per ton the airflow is 1,200 CFM, which is the same as 36,000 ÷ 30. Do not put heating BTU into this mode.
CFM to Duct Size Chart
Every row uses a 700 FPM planning velocity on smooth round metal duct. Calculated diameter is the exact round size. Recommended standard size is the next common manufactured diameter at or above that exact size. Approximate FPM is the speed inside that larger standard duct, so it is lower than 700 whenever the duct steps up. Typical use is a planning description, not a code classification. Change the velocity in the calculator if your target is different.
| CFM | Calculated diameter | Recommended standard size | Approximate FPM | Typical use |
|---|---|---|---|---|
| 50 | 3.6 in | 4 in | 573 | Bath fan or a very small register branch |
| 75 | 4.4 in | 5 in | 550 | Bath fan or a very small register branch |
| 100 | 5.1 in | 6 in | 509 | Single-room supply branch |
| 150 | 6.3 in | 7 in | 561 | Single-room supply branch |
| 200 | 7.2 in | 8 in | 573 | Larger room supply |
| 300 | 8.9 in | 9 in | 679 | Larger room supply |
| 400 | 10.2 in | 12 in | 509 | Short trunk or more than one room |
| 500 | 11.4 in | 12 in | 637 | Short trunk or more than one room |
| 600 | 12.5 in | 14 in | 561 | Supply trunk |
| 800 | 14.5 in | 16 in | 573 | Supply trunk |
| 1,000 | 16.2 in | 18 in | 566 | Main trunk |
| 1,200 | 17.7 in | 18 in | 679 | Main trunk |
| 1,600 | 20.5 in | 22 in | 606 | Large trunk or light commercial main |
| 2,000 | 22.9 in | 24 in | 637 | Large trunk or light commercial main |
At the same 700 FPM, a 6 inch round duct carries about 137 CFM, an 8 inch duct about 244 CFM, a 10 inch duct about 382 CFM, a 12 inch duct about 550 CFM, and a 14 inch duct about 748 CFM. Those are area × velocity results. A duct does not have one CFM rating by itself.
Round vs Rectangular Ducts
Round duct is generally efficient from an airflow and friction standpoint. It has less internal surface for a given area, and it is the shape the friction chart is built around.
Rectangular duct is useful where framing, a ceiling, or a wall will not accept a round pipe. A 12 × 8 trunk can fit a joist bay that a 12 inch round cannot.
Equal area does not automatically mean equal airflow performance. Equivalent diameter accounts for the extra surface of a rectangle. A very flat duct can move the same CFM only by running faster or by using more static pressure.
Duct Air Velocity and FPM
FPM means feet per minute. It is how fast the air is moving. CFM is how much air is moving. CFM = area × FPM, so the two numbers describe different things.
A higher velocity allows a smaller duct for the same CFM. It can also raise noise and pressure loss. A lower velocity needs a larger duct and is usually quieter.
Application matters. A supply branch is often planned slower than a supply trunk. A return is often planned slower than the supply that feeds the same room, because return noise and filter pressure add up. The application choices in the calculator are design starting points. Verify them against the actual system requirements. There is no single residential FPM that this page treats as code.
What Is Duct Friction Rate?
Friction rate is pressure loss along the duct, in inches of water column per 100 feet. A lower friction rate generally means a larger duct for the same CFM, because the air is asked to move with less resistance.
The pressure budget comes from the blower’s available static pressure after the coil, filter, and other fixed devices. Fittings matter: elbows, boots, and takeoffs add effective length, so a short straight pipe is not the whole run. Full duct design is more than one friction number on a straight section.
Does Flex Duct Change CFM?
Flex duct behaves differently from smooth metal because its inner liner and the way it is installed change resistance. This calculator does not apply a universal percentage correction. When you choose flex, the size and CFM are still calculated as a smooth duct of the same inside dimension, and the result says so.
That is an assumption, not a field measurement. Actual design should use the duct manufacturer’s data and Manual D methodology. Install flex taut, with full support and gentle bends. Sagging or compressed flex raises resistance and can cut the airflow the blower actually delivers.
Common Duct CFM Mistakes
- Using a CFM number by itself and calling that a complete duct design.
- Confusing CFM, the volume of air, with FPM, the speed of that air.
- Entering heating BTU into the tons/BTU mode. That mode expects cooling BTU/hr.
- Treating 400 CFM per ton as the blower’s actual airflow on every system.
- Ignoring friction rate and available static pressure.
- Ignoring elbows, fittings, and effective length.
- Using rectangular dimensions without checking the aspect ratio.
- Ordering a decimal diameter, such as 11.4 inches, when the product is sold in standard sizes.
- Treating this calculator as a code-compliant Manual D design.
Related HVAC calculators
- Continue with our Duct Size Calculator to size round or rectangular duct from CFM, velocity, or friction rate.
- Need cooling load before airflow? Use the BTU Room Calculator for room BTU/hr and AC tonnage.
- Heating output belongs in the Furnace BTU Calculator. Keep that heating BTU out of the cooling tons/BTU mode on this page.
- For heat pump capacity, use the Heat Pump Size Calculator.
- More planners are listed with the HVAC and calculator tools.
Frequently Asked Questions
What is a duct CFM calculator?
A duct CFM calculator estimates airflow in cubic feet per minute, or the duct size needed to move that air. This one can start from CFM and a velocity or friction rate, from a round or rectangular duct plus velocity, or from cooling tons or BTU/hr.
How do I calculate CFM for a duct?
If you know the inside area and the air speed, CFM = duct area in square feet × velocity in feet per minute. For cooling capacity, CFM = tons × the CFM-per-ton ratio you select. Tons = cooling BTU/hr ÷ 12,000.
How do I calculate CFM from duct size?
Convert the inside dimensions to square feet, then multiply by velocity in FPM. A round duct uses area = π × diameter² ÷ 4 ÷ 144. A rectangular duct uses area = width × height ÷ 144. The Duct size → CFM mode does that math.
How many CFM per ton of AC?
A common planning assumption is 400 CFM per cooling ton. Some systems are planned nearer 350 or 450. At 400 CFM per ton, 2.5 tons is 1,000 CFM. That ratio is not measured blower airflow, and it is not a rule for every furnace or air handler.
What size duct do I need for 400 CFM?
At a 700 FPM planning velocity, 400 CFM has an exact round diameter of 10.2 in. The next common standard size is 12 in. Velocity in that larger duct is about 509 FPM. A higher target velocity produces a smaller duct. This is not a complete duct design.
What size duct do I need for 800 CFM?
At 700 FPM, 800 CFM calculates to 14.5 in exact and a 16 in standard round duct, with about 573 FPM in that standard size. Change the velocity or friction rate if your design target is different.
How much CFM does a 6 inch duct carry?
A 6 inch round duct does not have a single CFM rating. At 700 FPM it carries about 137 CFM. An 8 inch duct at the same speed carries about 244 CFM, and a 10 inch duct about 382 CFM. Lower the velocity and the CFM drops with it.
What is the difference between CFM and FPM?
CFM is volume: how much air moves each minute. FPM is speed: how fast that air is moving. CFM = area × FPM. The same CFM is slower in a larger duct and faster in a smaller one.
Does 400 CFM per ton always apply?
No. 400 CFM per ton is a common planning assumption for cooling airflow. Manufacturer blower data, the selected fan speed, altitude, and the cooling load can call for a different ratio. At 400 CFM per ton, CFM = cooling BTU/hr ÷ 30.
Is this calculator a replacement for Manual D?
No. Manual D looks at available static pressure, the longest effective length, every fitting, duct material, leakage, and room-by-room CFM. This page sizes one straight-run planning case and labels its assumptions. Use it to understand the math, then confirm the design.