What ACH should I use?

It depends on the purpose — an air cleaner's rating, a designer's ventilation plan or a specific guideline. The calculator leaves ACH editable rather than suggesting a universal number.

Can I turn CFM into duct size here?

No. Duct sizing depends on friction rate, fittings and blower performance, which Manual D covers. A CFM figure alone is not enough.

Home Improvement

Room-by-Room CFM Planner

Build a room airflow schedule for up to six rooms in one of two clearly separate modes. Air-change mode turns room volume and an air-changes-per-hour target into CFM — useful for ventilation or air-cleaning goals. Cooling-supply mode turns each room's sensible cooling load from a load calculation into supply CFM using the standard-air sensible heat equation. The two answers measure different things and are never mixed. Dimensions can be in feet or meters; output is in cubic feet per minute.

How do I calculate CFM for a room?

It depends on the goal. For an air-change target, CFM = length × width × height (ft³) × ACH ÷ 60 — a 14 × 12 × 8 ft room at 4 ACH needs 1,344 × 4 ÷ 60 = 89.6 CFM. For cooling supply, CFM = sensible load ÷ (1.08 × ΔT) — a 4,000 BTU/h room at a 20 °F difference needs about 185 CFM. The two answers are not interchangeable.

Key takeaways

Mode A answers 'how often is the room's air replaced'; mode B answers 'how much supply air removes this heat'.
1.08 = 0.075 lb/ft³ × 60 min/h × 0.24 BTU/lb·°F for standard air; it changes at altitude.
Duct sizes come from a Manual D design, not from CFM alone.

Enter your details

Setup

Calculation mode

Number of rooms

Dimension unit

Target air changes per hour (ACH)

Your chosen target — e.g. from an air-cleaner or ventilation goal. It is not a code value.

Room 1

Room 1 length

Room 1 width

Room 1 ceiling height

Room 2

Room 2 length

Room 2 width

Room 2 ceiling height

Room 3

Room 3 length

Room 3 width

Room 3 ceiling height

Total air-change airflow

208 CFM

3 rooms, 3,120 ft³

Calculation breakdown

Formula: CFM = ft³ × ACH ÷ 60
Total volume: 3,120 ft³
ACH target: 4
Total air-change airflow: 208 CFM

Air-change airflow is not HVAC supply airflow. Use mode B with load-calculation figures to plan cooling supply.

Room airflow schedule

Room	Volume (ft³)	ACH	CFM
Room 1	1,344	4	90
Room 2	1,056	4	70
Room 3	720	4	48
Total	3,120		208

Your result is an estimate based on the information entered.

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HomeCalc USA — Room-by-Room CFM Planner

Generated

Inputs

Calculation mode: A. Air-change airflow (volume × ACH ÷ 60)
Number of rooms: 3
Dimension unit: Feet
Target air changes per hour (ACH): 4
Room 1 length: 14
Room 1 width: 12
Room 1 ceiling height: 8
Room 2 length: 12
Room 2 width: 11
Room 2 ceiling height: 8
Room 3 length: 10
Room 3 width: 9
Room 3 ceiling height: 8

Results

Total air-change airflow: 208 CFM

Formula: CFM = ft³ × ACH ÷ 60
Total volume: 3,120 ft³
ACH target: 4
Total air-change airflow: 208 CFM

Assumptions

Rectangular rooms with flat ceilings.
Standard air density (sea level, about 70 °F) for the 1.08 constant.
Loads entered are already calculated by a recognised method.

Estimates are for informational purposes only and may differ from actual costs, lender offers, tax assessments or contractor quotes.

How to use this calculator

Choose mode A for an air-change target or mode B when you have room cooling loads.
Select how many rooms, then enter each room's dimensions in one unit.
Enter the ACH target (A) or each sensible load and the supply ΔT (B).
Print or download the room schedule for your contractor.

What each input means

Input	What it means
ACH	Air changes per hour you want to achieve — a target you choose, not a code figure.
Sensible cooling load	Per-room sensible BTU/h from a Manual J or equivalent report.
ΔT	Difference between room air and supply air temperature in °F.

Formula used

A: CFM = (L × W × H in ft³) × ACH ÷ 60

B: CFM = Sensible BTU/h ÷ (1.08 × ΔT °F)

Meters are converted at 1 m = 3.28084 ft before volume is calculated.

Worked example

Three rooms — 14 × 12 × 8, 12 × 11 × 8 and 10 × 9 × 8 ft — hold 1,344 + 1,056 + 720 = 3,120 ft³. At 4 ACH: 89.6 + 70.4 + 48.0 = 208 CFM. In cooling mode with sensible loads of 4,000, 3,000 and 2,000 BTU/h at ΔT 20 °F (divisor 21.6): 185.2 + 138.9 + 92.6 ≈ 417 CFM.

Common mistakes to avoid

Using ACH airflow to size supply registers
Ventilation airflow says nothing about how much heat a room gains.
Entering total load instead of sensible
Latent (moisture) load is not removed by the 1.08 equation; using total load overstates CFM.
Mixing meters and feet
Pick one dimension unit for all rooms — the tool converts meters to cubic feet for you.

When the estimate may differ from reality

Measured airflow at a register often differs from the schedule: duct leakage, fittings, filter loading and blower speed change what arrives. Ventilation codes may also require specific rates that differ from your chosen ACH target.

Assumptions and limitations

What this calculator assumes

Rectangular rooms with flat ceilings.
Standard air density (sea level, about 70 °F) for the 1.08 constant.
Loads entered are already calculated by a recognised method.

What it does not account for

Does not calculate cooling loads, ventilation code requirements or duct sizes.
High-altitude homes need a corrected air-density constant.

Frequently asked questions

What ACH should I use?: It depends on the purpose — an air cleaner's rating, a designer's ventilation plan or a specific guideline. The calculator leaves ACH editable rather than suggesting a universal number.
Can I turn CFM into duct size here?: No. Duct sizing depends on friction rate, fittings and blower performance, which Manual D covers. A CFM figure alone is not enough.

Sources and references

How to Estimate Mini-Split Cooling CapacityA step-by-step room cooling estimate using the ENERGY STAR sizing chart and its published adjustments, with a worked example and when to get a Manual J.
Why Square Footage Alone Can Mis-size an Air ConditionerTwo rooms with the same floor area can have very different cooling loads. See which factors matter, why oversizing hurts comfort, and what a Manual J adds.
CFM vs ACH: Which Airflow Calculation Do You Need?Air changes per hour and supply CFM answer different questions. Learn both formulas, convert between them correctly, and avoid using ventilation airflow to size cooling.
How to Prepare a Room-by-Room Airflow ScheduleLay out rooms, volumes, loads and target CFM in one table your HVAC contractor can check. Includes a three-room worked schedule and the data each column needs.