Duct Velocity Calculator
Calculate air velocity in feet per minute (FPM) through round or rectangular ducts given CFM and duct dimensions, with ASHRAE recommended velocity ranges.
CFM
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Air Velocity
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Velocity Pressure —
ASHRAE Zone —
Extended More scenarios, charts & detailed breakdown ▾
CFM
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Velocity (FPM)
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Velocity Pressure —
Cross-Section Area —
Professional Full parameters & maximum detail ▾
CFM
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Velocity Results
Air Velocity —
Velocity Pressure —
Equiv. Round Diameter —
ASHRAE Assessment
ASHRAE Limit (FPM) —
Design Status —
Noise Risk —
How to Use This Calculator
- Enter airflow in CFM (from HVAC load calc or equipment specs).
- Select round or rectangular duct and enter dimensions.
- Results show velocity in FPM, velocity pressure in inches WC, and ASHRAE assessment.
- Use Extended tabs for round, rectangular, or flexible duct detail. Professional tab adds equivalent diameter and design status.
Formula
Round: V = CFM / (π/4 × D²) where D is in feet
Rectangular: V = CFM / (W × H) where W, H are in feet
Velocity Pressure: VP = (V / 4005)² in WC
Example
Example: 400 CFM through a 10-inch round duct → Area = π/4 × (10/12)² = 0.545 ft² → Velocity = 400 / 0.545 = 734 FPM (within ASHRAE supply range).
Frequently Asked Questions
- Duct velocity is the speed of air flowing through the duct cross-section, measured in feet per minute (FPM). It equals CFM divided by the duct cross-sectional area in square feet.
- ASHRAE recommends 600–900 FPM for supply branches, 500–700 FPM for return air, and 1,000–1,600 FPM for main trunk ducts. Exceeding these limits causes noise and pressure loss.
- Velocity pressure (VP) is the dynamic pressure caused by moving air: VP = (V/4005)² in inches of water column (in WC). It is used to measure airflow with a pitot tube or in duct design calculations.
- Flexible duct has 50–100% more friction than smooth rigid duct at the same diameter. As a practical rule, size flex duct 1–2 inches larger and keep runs under 6 feet.
- Use the ASHRAE equivalent diameter formula: De = 1.3 × (a×b)^0.625 / (a+b)^0.25, where a and b are duct dimensions in inches. This gives the round duct with equal flow resistance.
Related Calculators
Sources & References (5) ▾
- ASHRAE Handbook – HVAC Systems and Equipment — ASHRAE
- ACCA Manual D – Residential Duct Systems — Air Conditioning Contractors of America
- SMACNA HVAC Duct Construction Standards — SMACNA
- Engineering Toolbox – Air Duct Velocity — Engineering Toolbox
- ASHRAE Fundamentals Handbook Ch. 21 – Duct Design — ASHRAE