HVAC CFM Calculator
The new system is the right size on paper, then the callback comes: a back bedroom that never cools, a register that whistles, a coil that ices up. More often than not the cause is airflow, not capacity. This free CFM calculator for HVAC gives you the CFM from BTU/h and temperature split, from tonnage, from a room's air changes or from a duct's size and velocity, and shows the duct that carries it.
Airflow needed
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Same job at other values
Quick answer: the HVAC CFM formula is CFM = BTU/h ÷ (1.08 × ΔT). A 24,000 BTU/h sensible load with a 20°F temperature split needs 24,000 ÷ (1.08 × 20) = 1,111 CFM. The other three ways to get CFM are below.
How to calculate CFM in HVAC
CFM is cubic feet per minute: how much air moves past a point each minute. The calculation of CFM for HVAC work comes down to four questions, depending on what you have in front of you. The calculator has a tab for each.
- You have the heat load. Use BTU/h and the temperature split between supply and return. This is the one the load calculation feeds.
- You have the equipment size. Multiply the tonnage by the airflow per ton, about 400 CFM.
- You have a room and a ventilation target. Use the volume and the air changes per hour.
- You have a duct. Use the velocity and the cross-section area, either to find the CFM it carries or, working backwards, the size it needs.
HVAC CFM calculation formula
The 1.08 in the first formula is not magic: it is 0.075 lb/ft³ (the density of standard air) × 0.24 BTU/lb·°F (its specific heat) × 60 minutes. It is built for sensible heat at sea level, so treat the answer as a starting point at altitude, where thinner air needs more CFM for the same load.
The calculation, step by step
This is the job the calculator starts with. Every figure is an example to replace with your own.
- Take the sensible load. 24,000 BTU/h.
- Take the temperature split. 20°F between return and supply air.
- Multiply the constant by the split. 1.08 × 20 = 21.6.
- Divide the load by it. 24,000 ÷ 21.6 = 1,111 CFM.
- Turn it into a duct. At a design velocity of 700 FPM the area is 1,111 ÷ 700 = 1.587 sq ft, a 17.1-inch round duct. The next even size up is 18 inches, which runs at about 629 FPM. As a rectangle 10 inches high it is about 22.9 inches wide.
CFM per ton: the quick check
Before you trust any CFM figure on a system, check it against the tonnage. The usual rule of thumb is about 400 CFM per ton, and 350 to 450 is the commonly quoted range. Many techs go toward the low end in humid climates and the high end in dry ones, but the manufacturer's data for the unit is what counts.
| System size | At 350 CFM / ton | At 400 CFM / ton | At 450 CFM / ton |
|---|---|---|---|
| 1.5 ton | 525 | 600 | 675 |
| 2 ton | 700 | 800 | 900 |
| 2.5 ton | 875 | 1,000 | 1,125 |
| 3 ton | 1,050 | 1,200 | 1,350 |
| 3.5 ton | 1,225 | 1,400 | 1,575 |
| 4 ton | 1,400 | 1,600 | 1,800 |
| 5 ton | 1,750 | 2,000 | 2,250 |
HVAC duct CFM calculator: what a duct carries
A duct's capacity is set by its area and how fast the air moves. Push too much air through a small duct and the velocity goes up, and with it the noise and the pressure the blower has to overcome. Go too big and you pay for sheet metal and space you did not need. The table shows what a round duct carries at 700 FPM, a figure often used as a design speed for branch runs; use the velocity your own design calls for.
| Round duct | Area | CFM at 700 FPM |
|---|---|---|
| 6 in | 0.196 sq ft | 137 |
| 8 in | 0.349 sq ft | 244 |
| 10 in | 0.545 sq ft | 382 |
| 12 in | 0.785 sq ft | 550 |
| 14 in | 1.069 sq ft | 748 |
| 16 in | 1.396 sq ft | 977 |
| 18 in | 1.767 sq ft | 1,237 |
| 20 in | 2.182 sq ft | 1,527 |
This is a velocity check, not a duct design. It does not account for friction rate, length, fittings or flex duct, so for a full layout use a ductulator or your duct design software.
CFM for a room from air changes
When the question is how much air to move through a room, not how much heat to carry, use air changes per hour. A 20 × 15 × 8 ft room holds 2,400 cubic feet. Replacing that air 6 times an hour takes 2,400 × 6 ÷ 60 = 240 CFM, or 0.80 CFM per square foot of floor. The air changes you need depend on the room and the code or spec you work to.
An HVAC CFM calculator for your phone
There is nothing to download. This HVAC CFM calculator runs in the browser on a phone, so you can use it in the attic, at the air handler or at the customer's door. Add the page to your home screen and it opens like an app. The result stays at the bottom of the screen while you type, and "Copy a link with these numbers" saves the whole calculation in a link you can text to a coworker.
What this calculator does not do
It does not calculate a heating or cooling load, so it is not a Manual J. It does not select equipment against a manufacturer's performance data or design a duct system with friction losses and fittings. It also takes your numbers at face value, so a wrong ΔT or a wrong BTU figure gives a wrong CFM. Use it for the airflow arithmetic and keep your design tools for the design.
From the airflow to the quote
The airflow is one number on a job. The rest is the price, the schedule and the paperwork. OwnDesk HVAC is the business side of the work: you build the estimate from your own price book, with a takeoff that counts the feet of line set, pounds of refrigerant and feet of duct, rounds them up to whole units and adds an install labor line from your own hours. Then it puts the job on a dispatch board, keeps the maintenance agreements and the equipment installed at each customer, tracks the job's costs and prints the invoice.
It is a one-time payment, not a monthly subscription, and it runs offline on one computer. To be clear about what it is not: it is not load calculation software. It does not run a Manual J, size equipment or design ductwork, and it does not read plans, sync between devices, track vans by GPS or send texts. Keep the calculator and your design tools for sizing, and use OwnDesk HVAC for what happens before and after.
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Frequently asked questions
How do you calculate CFM in HVAC?
What is the HVAC CFM formula?
How many CFM per ton of cooling?
What does the 1.08 in the CFM formula mean?
How do I calculate CFM for a duct?
What size duct do I need for a given CFM?
How do I calculate CFM for a room?
Is this a Manual J load calculation or a duct design?
Is this HVAC CFM calculator free, and does it work on a phone?
Size the airflow here, run the job in OwnDesk HVAC
OwnDesk HVAC builds the estimate from your price book, schedules the job, keeps the customer's equipment on file and sends the invoice. One payment, no monthly fee, runs offline on your computer.