Free HVAC Duct Calculator for Accurate Duct Sizing

How to Use This HVAC Duct Calculator?

Step 1: Enter Your Parameters

• Airflow (CFM): The cubic feet per minute your space needs. Not sure? A load calculation will give you this number.

Not sure what your rooms require? Our HVAC CFM calculator breaks it down room by room.

• Duct Shape: Round or rectangular
• Dimensions: Diameter for round ducts, or width × height for rectangular (in inches)
• Material: Metal, flex, or ductboard, each has different friction characteristics
• Friction Rate: Pressure drop per 100 feet (0.08–0.1 in. w.g. is typical for homes)
• Length & Fittings: Total run length plus number of elbows

Step 2: Review Results

Hit “Calculate”, and you’ll see:

• Cross-sectional area (sq. in.)
• Air velocity (FPM)
• Recommended size rounded to standard dimensions
• Equivalent length including fittings
• Total pressure drop (in. w.g.)

What Are HVAC Ducts?

Ducts are simply the pathways that carry conditioned air from your furnace or air handler to every room and bring return air back to the system.

Common materials:

• Sheet metal has the lowest friction and lasts the longest. It’s standard for trunk lines and anywhere you need maximum airflow efficiency.
• Flexible duct is easier to snake through tight spaces, but the corrugated interior creates more resistance. You’ll typically need to go up a size compared to metal.
• Ductboard is rigid fiberglass that insulates well, though it requires careful sealing at joints.

Every section of ductwork needs to handle the airflow demands of the rooms it serves. That’s the whole point of sizing, matching duct dimensions to actual airflow needs so the system works as designed.

Why Proper Sizing Matters?

Getting duct sizes wrong causes problems you’ll hear about on callbacks.

When ducts are too small:

• Airflow gets choked, creating hot and cold spots
• Static pressure builds up, causing whistling and noise
• The blower works harder, driving up energy bills
• Equipment wears out faster than it should

When ducts are too big:

• Air velocity drops and distribution suffers
• Rooms stratify, warm air up top, cool air at floor level
• In humid climates, slow-moving air can cause condensation
• You’ve wasted material and labor

Getting sizing wrong doesn’t just hurt comfort; it hits your bottom line. Between wasted materials, extra labor, and callbacks, one bad duct job can cost more than the profit on the next three.

If you’re still guessing on job pricing, our HVAC pricing guide breaks down how to quote accurately and protect your margins.

The goal is to balance: enough capacity to deliver the required airflow without oversizing and wasting resources.

Of course, none of this works if your airflow numbers are off to begin with. If you haven’t run a load calculation yet, our HVAC load calculator gives you the BTU and CFM figures you need to size accurately.

The Math Behind Duct Sizing

You don’t need to memorize formulas; that’s what the calculator is for, but understanding the relationships helps you catch errors and troubleshoot in the field.

The Core Formula

Duct Area (sq. in.) = (CFM × 144) ÷ Velocity (FPM)

The 144 just converts square feet to square inches. Velocity typically runs 600–900 FPM, depending on whether you’re sizing a quiet bedroom branch or a main trunk line.

Working Backward: Finding Airflow from Duct Size

If you know the duct dimensions and velocity:

CFM = (Area in sq. in. ÷ 144) × Velocity

Quick example: A 10-inch round duct at 700 FPM delivers about 382 CFM.

Round Duct Diameter

Diameter = 2 × √(Area ÷ π)

Rectangular Dimensions

When one side is fixed:

Other Side = Area ÷ Fixed Side

Converting Rectangular to Round

Need to know what round duct equals your rectangular one?

Equivalent Diameter = 1.3 × (W × H)^0.625 ÷ (W + H)^0.25

Quick Reference Charts

These tables give you ballpark sizing for common scenarios. All values assume standard air density at sea level.

Round Duct Sizing

Airflow	500 FPM	600 FPM	700 FPM	800 FPM	900 FPM
50 CFM	5″	4″	4″	4″	4″
100 CFM	6″	6″	5″	5″	5″
150 CFM	7″	7″	6″	6″	6″
200 CFM	8″	8″	7″	7″	6″
300 CFM	10″	9″	9″	8″	8″
400 CFM	11″	10″	10″	9″	9″
500 CFM	12″	11″	10″	10″	10″
600 CFM	13″	12″	11″	11″	10″
800 CFM	15″	14″	13″	12″	12″
1000 CFM	17″	16″	14″	14″	13″
1200 CFM	18″	17″	16″	15″	14″
1500 CFM	20″	19″	17″	16″	15″
2000 CFM	23″	21″	20″	18″	17″

Use lower velocities (500–600 FPM) for bedroom branches where noise matters. Higher velocities (700–900 FPM) work fine for main trunks.

Rectangular Duct Sizing

Airflow	Size Options
100 CFM	6×4″, 8×3″
150 CFM	8×4″, 6×6″
200 CFM	8×6″, 10×4″
300 CFM	10×6″, 12×5″, 8×8″
400 CFM	10×8″, 12×6″, 14×5″
500 CFM	12×8″, 14×6″, 10×10″
600 CFM	12×10″, 14×8″, 16×6″
800 CFM	14×10″, 16×8″, 20×6″
1000 CFM	16×10″, 14×12″, 20×8″
1200 CFM	16×12″, 18×10″, 20×10″
1500 CFM	18×14″, 20×12″, 24×10″
2000 CFM	20×16″, 24×12″, 18×18″

Try to keep aspect ratios below 4:1 for the best airflow distribution.

Flex Duct Sizing

Flex has more friction than metal, so you need larger diameters for the same airflow:

Airflow	Metal	Flex (Fully Extended)
50 CFM	4″	5″
75 CFM	5″	6″
100 CFM	6″	7″
150 CFM	7″	8″
200 CFM	8″	9″
250 CFM	9″	10″
300 CFM	9″	10″
400 CFM	10″	12″

Important: These ratings assume the flex duct is pulled tight and installed with gentle curves. Sagging, kinks, or sharp turns can cut airflow by half. When in doubt, go up a size.

Typical Room Requirements

Room	Typical Size	Airflow Range	Suggested Duct
Bedroom	150 sq. ft.	100–150 CFM	6″ round
Master Bedroom	250 sq. ft.	150–250 CFM	8″ round
Living Room	300 sq. ft.	200–350 CFM	8–10″ round
Kitchen	200 sq. ft.	200–300 CFM	8″ round
Bathroom	75 sq. ft.	50–75 CFM	5–6″ round
Home Office	150 sq. ft.	100–150 CFM	6″ round
Basement	500 sq. ft.	350–500 CFM	10–12″ round

These are starting points. Actual needs depend on insulation, windows, sun exposure, and climate. A proper load calculation gives you exact numbers.

Common Mistakes That Lead to Callbacks

1. Using Metal Sizing for Flex Installations

A 6-inch metal duct moves about 100 CFM. A 6-inch flex duct? Maybe 75 CFM on a good day. Always bump up a size when switching to flex.

2. Ignoring Fittings in Pressure Calculations

Every 90° elbow adds roughly 5 feet of equivalent length. Four elbows in a run means 20 extra feet of pressure drop you might not have accounted for.

Callbacks from undersized ducts eat into your margins fast. If you’re still tracking jobs on paper or spreadsheets, HVAC management software can help you log equipment specs, flag repeat issues, and keep service history in one place.

3. Trusting the “1 CFM Per Square Foot” Rule Too Much

It’s a decent starting point for average rooms, but kitchens, sunrooms, and spaces with big windows often need 1.5–2 CFM per square foot. Don’t skip the load calculation.

4. Forgetting About Available Static Pressure

If your filter, coil, and other components have already eaten up most of your static pressure budget, you’ll need larger ducts to compensate. Size ducts based on what pressure you actually have to work with.

5. Sizing Only for Cooling

Cooling usually needs peak airflow, but heat pumps often need similar volumes year-round. Design for the highest demand condition.

6. Rounding Down Instead of Up

When calculations give you a 9.3-inch diameter, use a 10-inch duct. Rounding down creates noise, pressure problems, and comfort complaints.

When to Bring in an Engineer?

This calculator handles most residential and light commercial jobs. But some situations call for professional engineering:

• New construction that needs permit approval
• Complex multi-zone systems
• Commercial buildings with specific ventilation codes
• Systems running near maximum equipment capacity

For these, a licensed contractor or mechanical engineer can run complete Manual J and Manual D analyses to make sure everything meets code and performs right.