How much airflow does this room need?
Required CFM by room size and air changes per hour — or whole-system CFM by tonnage. Free, no email required to see the math.
Your inputs
Sets the air changes per hour (ACH) we use.
Leave at 0 to use the room-type default.
You'll need
Assumptions (tap to adjust)+
The math, in plain English.
CFM is cubic feet per minute — how much air a duct or register actually moves. ACH is air changes per hour — how many times the whole volume of the room gets replaced. To size a room, we multiply length × width × height for the volume in cubic feet, multiply by the air changes that room type needs, and divide by 60 to get a per-minute number. Bathrooms and kitchens need more turnover than a bedroom because of moisture and cooking; a garage or basement needs less.
The room-type dropdown sets a sensible ACH default. If your HVAC tech or a spec sheet gave you a different number, type it in the override box and it wins. Whole-system mode is the other direction: you know the tonnage on the outdoor unit and want to know how much air the blower should be pushing overall — 400 CFM per ton is the rule of thumb in our mild Pacific Northwest climate; drier, hotter regions run higher.
This is a quick sanity check, not a design. It will tell you if a bedroom that feels stuffy is starved for air, or whether a smart thermostat is fighting an undersized run. It does not replace a Manual J load calculation, which accounts for windows, insulation, orientation, and infiltration. CRM Services, LLC doesn't install HVAC — we coordinate it through licensed partners as part of remodels and repairs.
CFM questions
What does CFM mean?+
Cubic feet per minute — the volume of air moving through a duct, register, or fan every minute. A bathroom exhaust fan might be rated 80 CFM; a whole-house blower on a 3-ton system moves around 1,200.
What is the difference between CFM and ACH?+
ACH (air changes per hour) is a target: how many times per hour the air in a room should be fully replaced. CFM is the flow rate that hits that target for a room of a given size. Same ACH in a bigger room means more CFM.
Is there a CFM per square foot rule of thumb?+
Roughly 1 CFM per square foot for a typical 8-foot-ceiling living space. It falls apart with tall ceilings or high-moisture rooms, which is why we size by volume and room type instead.
How many CFM per ton of air conditioning?+
400 CFM per ton is the standard design number and a good default for the Vancouver, WA climate. Humid regions go as low as 350 to wring out more moisture; hot, dry regions run up to 450.
What are signs my airflow is too low or too high?+
Too low: weak air at registers, rooms that never reach setpoint, a frosted evaporator coil, and short cycling. Too high: noisy registers, drafts, and a system that runs but never dehumidifies. Either one is worth a service call before it turns into a compressor.
How is this different from a Manual J calculation?+
Manual J is the full ACCA heating and cooling load method — it weighs window area, insulation, orientation, infiltration, and local climate data to size equipment. This calculator only answers "how much air does this room need to turn over." Free Manual J tools like CoolCalc or the LennoxPros load calculator exist if you want the whole picture.
Get a free estimate for your project.
You've got the numbers — we've got the crew. Free onsite consult and a line-item quote, usually same day to 48 hours, depending on scope.