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Room CFM Calculator
Works out the airflow a room needs in CFM two independent ways — by air changes per hour and by cooling load — and how many registers that takes.
Calculators
Nothing uploaded
No sign-up
Free, unlimited
Leave at 0 to use the typical figure for the room type.
Around 600 in a mild climate, 400 in a hot one, 350 for a sun-facing room.
Enter your figures and press Calculate.
In short
Room CFM Calculator is a free online tool on inputtesthub.com that works out the airflow a room needs in CFM two independent ways — by air changes per hour and by cooling load — and how many registers that takes. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.
How to use the Room CFM Calculator
- Enter the room length, width and ceiling height.
- Pick the room type, or set your own air changes per hour.
- Adjust the square feet per ton if your climate is hotter or milder than average.
- Take the larger of the two figures to size ductwork.
About the Room CFM Calculator
The Room CFM Calculator works out the airflow a room needs in CFM two independent ways — by air changes per hour and by cooling load — and how many registers that takes. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Sizing a duct branch when you have no design figures.
- Checking whether an existing register is anywhere near enough for the room.
- Working out how many supply outlets a large room needs.
Details
| Air change method | Volume × ACH ÷ 60 — the 60 converts per hour to per minute |
|---|---|
| Load method | Roughly 400 CFM of air per ton of cooling in residential work |
| Bedroom | Typically 5–6 air changes per hour |
| Register limit | A supply register moves 100–150 CFM before it starts to whistle |
The maths behind it
This tool uses CFM = room volume in cubic feet × air changes per hour ÷ 60, cross-checked against floor area ÷ square feet per ton × CFM per ton. Every intermediate value is shown on screen, so the result can be checked by hand.
Both methods here are rules of thumb, and the tool prints every figure it used so you can change the ones that are wrong for your climate. The square feet per ton number in particular is the one that varies most — 600 in a mild coastal climate against 350 for a west-facing room in a hot one is nearly a factor of two.
Where this tool stops
Sizes air by volume and area only. It has no idea about your insulation, glazing, shading or how many people are in the room, and it does not attempt a room-by-room heat load.
Frequently asked questions
Why does the tool give two different numbers?
They answer different questions. Air changes per hour is a ventilation figure and scales with the volume of the room, so a tall room needs more. CFM per ton is a cooling figure and scales with floor area and climate. A room where the two agree is unremarkable; a room where they are far apart is telling you something — usually that it is unusually tall, or unusually exposed.
Which one should I size the duct on?
The larger. An oversized duct is quiet and costs a little more in sheet metal; an undersized one is noisy forever and cannot be corrected without opening the ceiling.
Is this a substitute for a Manual J load calculation?
No. Manual J models insulation, glazing, orientation, infiltration and internal gains room by room, and it is what a design should be based on. This is the back-of-the-envelope check that tells you whether a proposal is roughly right before you pay for one.
Is the Room CFM Calculator free, or is there a paid tier?
It is free, with no account, no trial and no daily cap. Every calculators tool on inputtesthub.com works the same way: open the page and use it.
Is the Room CFM Calculator safe to use with private information?
No. The Room CFM Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
What is the equation behind the Room CFM Calculator?
It uses CFM = room volume in cubic feet × air changes per hour ÷ 60, cross-checked against floor area ÷ square feet per ton × CFM per ton. The full working is printed under the result, so you can follow every step rather than trusting a black box.
When would I actually need the Room CFM Calculator?
The most common reasons people reach for it: sizing a duct branch when you have no design figures. Checking whether an existing register is anywhere near enough for the room. Working out how many supply outlets a large room needs. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
What are the limits of the Room CFM Calculator?
Sizes air by volume and area only. It has no idea about your insulation, glazing, shading or how many people are in the room, and it does not attempt a room-by-room heat load. Knowing where a tool stops being trustworthy matters more than the cases where it works.
What are the reference figures here?
Air change method is Volume × ACH ÷ 60 — the 60 converts per hour to per minute. Load method is Roughly 400 CFM of air per ton of cooling in residential work. Bedroom is Typically 5–6 air changes per hour. These are printed on the page itself, so you do not have to take them on trust.
How precise is the result?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
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