How do you calculate CFM for a room?

CFM = (Room Volume × ACH) ÷ 60. First calculate the room volume (Length × Width × Height in feet). Then multiply by the required Air Changes per Hour (ACH) for the room type. Divide by 60 to convert from cubic feet per hour to cubic feet per minute. For example, a 12×15 ft room with 8 ft ceilings needs about 24 CFM for general ventilation (1,440 cu ft × 1 ACH ÷ 60).

What size duct do I need for 100 CFM?

For 100 CFM at standard duct velocity (600–900 FPM), you need a 6-inch round duct. At 600 FPM velocity, a 6-inch duct delivers about 118 CFM. For lower noise (400 FPM), use an 8-inch duct. The formula is: Duct Area = CFM ÷ Velocity (FPM), then convert area to diameter.

How many CFM per ton of AC?

The standard rule is 400 CFM per ton of air conditioning. A 3-ton AC system needs about 1,200 CFM of airflow. This can range from 350–450 CFM per ton depending on climate, humidity, and system design. In humid climates, lower CFM (350/ton) helps with dehumidification. In dry climates, higher CFM (450/ton) improves efficiency.

What is a good CFM for a bathroom exhaust fan?

IRC code requires a minimum of 50 CFM for bathrooms up to 100 sq ft. For larger bathrooms, use 1 CFM per square foot. So a 120 sq ft master bathroom needs 120 CFM. For bathrooms with a jetted tub, add 50 CFM. For enclosed toilet rooms, add 50 CFM. Most homeowners prefer 80–110 CFM for standard bathrooms for better moisture removal.

How do you calculate duct velocity?

Duct velocity (FPM) = CFM ÷ Duct Area (sq ft). For a round duct, area = π × (diameter/2)². For example, 200 CFM through a 6-inch duct: Area = π × (0.25)² = 0.196 sq ft. Velocity = 200 ÷ 0.196 = 1,019 FPM. Recommended duct velocities: main ducts 600-900 FPM, branch ducts 400-700 FPM, near grilles 300-500 FPM.

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How to Calculate CFM: Complete Guide

CFM (cubic feet per minute) measures the volume of air moved. Whether you're sizing HVAC ducts, selecting an exhaust fan, or designing a ventilation system, getting the right CFM is critical for air quality, comfort, and energy efficiency.

The Basic CFM Formula

For room ventilation based on air changes per hour (ACH):

CFM = (Length × Width × Height × ACH) ÷ 60

ACH (air changes per hour) is how many times the entire volume of air in a room is replaced each hour. Different rooms need different ACH rates based on their use.

Recommended Air Changes per Hour (ACH)

Room Type	Minimum ACH	Recommended ACH
Bedroom	1	2–4
Living Room	2	4–6
Office	1	4–6
Kitchen (residential)	4	6–8
Bathroom	6	8–10
Laundry Room	6	8–10
Garage (attached)	6	10–12
Workshop / Woodshop	8	12–15
Commercial Kitchen	12	15–20
Paint Booth	20	30–50
Welding Shop	15	20–30

Duct Sizing Reference

Round Duct (in)	Area (sq ft)	CFM @ 600 FPM	CFM @ 800 FPM
4"	0.087	52	70
5"	0.136	82	109
6"	0.196	118	157
7"	0.267	160	213
8"	0.349	209	279
10"	0.545	327	436
12"	0.785	471	628
14"	1.069	641	855
16"	1.396	838	1,117
18"	1.767	1,060	1,414

Exhaust Fan Sizing Rules

Bathrooms (≤100 sq ft): Minimum 50 CFM per IRC code. Prefer 1 CFM/sq ft.
Bathrooms (>100 sq ft): 1 CFM per square foot of floor area.
Kitchen range hoods: 100 CFM per linear foot of range for wall-mounted; 150 CFM/ft for island hoods.
Garage: 1 CFM per square foot, or 10 ACH, whichever is greater.
Attic fans: 1 CFM per square foot of attic floor area (minimum). Add 20% for dark roofs.

HVAC Airflow Basics

Rule of thumb: 400 CFM per ton of cooling. A "ton" of AC equals 12,000 BTU/hr of cooling capacity.

System Size	Total CFM	Sq Ft Served	Supply Duct (main)
1.5 Ton	600	600–900	10" round
2 Ton	800	900–1,200	12" round
2.5 Ton	1,000	1,200–1,500	14" round
3 Ton	1,200	1,500–1,800	14" round
3.5 Ton	1,400	1,800–2,100	16" round
4 Ton	1,600	2,100–2,400	16" round
5 Ton	2,000	2,400–3,000	18" round

5 Common CFM Mistakes

Undersizing exhaust fans. A cheap 50 CFM fan in a large master bath leads to mold. Size to 1 CFM per square foot minimum.
Ignoring static pressure. Ductwork creates resistance. Long runs, elbows, and transitions reduce effective CFM. A fan rated at 100 CFM might only deliver 60 CFM through a long duct run.
Not balancing supply and return. If your supply CFM exceeds return CFM, you create positive pressure that pushes conditioned air out through cracks. Returns should handle at least 80% of supply CFM.
Using high-velocity ducts everywhere. Above 900 FPM, ducts get noisy. Keep bedroom and living area branches under 600 FPM.
Forgetting makeup air. Large exhaust systems (kitchen hoods, shop ventilation) need makeup air to replace what's being exhausted. Without it, you get negative pressure, backdrafting, and doors that won't close.

Round vs. Rectangular Duct: When to Use Each

Factor	Round Duct	Rectangular Duct
Airflow efficiency	Better — less friction	More friction per CFM
Noise	Quieter at same CFM	Can be noisier
Space needed	More vertical clearance	Fits in tight spaces
Cost	Less material, cheaper	More material, labor
Insulation	Easier to wrap	Harder to seal corners
Best for	Branch runs, flex duct	Trunk lines, low ceilings

Aspect ratio rule: Keep rectangular ducts at 4:1 aspect ratio or lower (width to height). Higher ratios waste material and increase friction. A 12×6 duct (2:1) is far more efficient than a 24×3 duct (8:1) even though both have similar area.

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