April 11, 2026
What Is a Manual J Calculation?
A Simple Guide for Homeowners and Contractors. Learn what a Manual J is, why it matters, and how it impacts your next project.
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Round ↔ Rectangle Duct Size Calculator
Accurate conversions using the ASHRAE equivalent diameter formula — the same methodology used in Manual D software.
Enter a round duct diameter — get every standard rectangular equivalent, sorted best ratio first.
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inches (filter)
Equivalent Rectangular Sizes
| Width × Height | Equiv. Round | Difference | Aspect Ratio | Area (in²) |
|---|
Tip: Aspect ratios closest to 1:1 are most efficient. Avoid ratios beyond 4:1 — they increase noise and friction loss.
Enter rectangular duct dimensions to find the exact equivalent round diameter and nearest standard sizes.
inches
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Nearest Standard Round Sizes
| Round Size | Difference | Effect on Flow | Notes |
|---|
Round up or down? Size up to maintain or increase airflow. Size down only to intentionally reduce a branch. Always verify against your Manual D friction rate.
ASHRAE Equivalent Diameter Formula
De = 1.30 × (a × b)^0.625 / (a + b)^0.25
De = equivalent round diameter (in) | a = width (in) | b = height (in) | Source: ASHRAE Fundamentals Handbook