Notice: This HVAC calculator provides estimates only and is for informational purposes. Results
should be verified by a licensed HVAC professional. Use at your own risk.
Technical Tool
HVAC Duct Size Calculator with Friction Rate
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📍 SECTION 1 — DUCT SIZE BY AIRFLOW & FRICTION
MANUAL D
⚪ Round Duct Parameters
Design CFM
Friction Rate
0.08
Duct Type
System Type
Duct Run Length
ft
Number of 90° Elbows
📊 Results —
Round Duct
Parameter
Calculated
Unit
Spec / Guideline
Status
Notes
—
▭ Rectangular Duct Parameters
Design CFM
Friction Rate
0.08
Known Height (b)
in
System Type
📊 Results —
Rectangular Duct
Parameter
Calculated
Unit
Spec / Guideline
Status
Notes
—
〰 Flex Duct Parameters
Design CFM
Friction Rate
0.08
📊 Results — Flex Duct
Parameter
Calculated
Unit
Spec / Guideline
Status
Notes
—
📋 STANDARD ROUND DUCT SIZES — CLICK TO HIGHLIGHT
⚡ Pro Tips — Duct Sizing
Equal Friction Method: Design at 0.08 in. w.g./100 ft for most residential systems.
Aspect ratio: Keep rectangular ducts at 3:1 or less; 4:1 maximum per ACCA Manual D.
📊 SECTION 2 — TOTAL EXTERNAL STATIC PRESSURE BUDGET
TESP
Enter Component Pressure Drops — in. w.g.
Equipment Nameplate TESP Rating
in. w.g.
Supply Filter (clean)
Evaporator Coil (wet)
Supply Duct System
Return Duct System
📊 Results — Static Pressure Budget
Component
Pressure Drop
Unit
Typical Range
Status
Notes
—
🏆 Residential Target
Most residential systems are rated for 0.50 in. w.g. TESP.
🔧 High Static Symptoms
High TESP causes reduced airflow, noise, and component strain.
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How to Size HVAC Ductwork Accurately
The HVAC Duct Size Calculator is an essential field tool designed for contractors and
technicians to correctly dimension ductwork. Using the Equal Friction Method derived from ACCA Manual D,
this tool calculates the optimal size for round rigid, rectangular, and flexible ductwork based on your
target Cubic Feet per Minute (CFM) and available friction rate.
Equal Friction Method
Explained
Before utilizing this calculator, you must know two values: the prescribed airflow (calculated via a
room CFM calculation) and a target friction rate. For many residential applications with standard
lengths, 0.08 in. w.g. per 100 feet is a common design friction rate for rigid round pipe.
Frequently Asked
Questions
What friction rate should I use
for duct sizing?
For most residential applications, a friction rate of 0.10" per 100 feet of duct is used. However,
for a high-performance design, you should calculate the Available Static Pressure (ASP) after
accounting for the blower capacity and component losses (filter, coil, etc.).
