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Equal frictionRound + rectangularFree

Duct Sizing
Calculator

Run a bounded equal-friction geometry sensitivity using project-entered airflow and friction rate. It does not select duct size, airflow, velocity criteria, or fan duty.

Who it's for: mechanical engineers and HVAC designers sizing supply or return duct runs — you'll need the design CFM and a target friction rate at construction documents; this is a code-level sizing tool, not a rule-of-thumb estimate.

Equal-friction (ASHRAE chart fit)
D = [0.109 · Q^1.9 / (ΔP/L)]^0.199, where D = inches, Q = CFM, ΔP/L = in.wg per 100 ft (typically 0.08–0.10). Velocity = Q / area. Typical limits: supply main ≤ 1,500 fpm, branch ≤ 900 fpm, return ≤ 1,000 fpm.
Duct Size
Diameter · velocity

How to use this calculator

1
Enter the airflow (CFM)
The design airflow for this duct section — taken from a room load calculation, equipment schedule, or air-distribution layout.
2
Enter the friction rate (in.wg / 100 ft)
Target pressure drop per 100 ft of duct. ASHRAE recommends 0.08–0.10 for most commercial systems; use 0.06–0.08 for quieter residential runs.
3
Enter a rectangular height (optional)
One side of the rectangular duct in inches. Leave blank to get the round diameter only; fill it in to receive the matching rectangular width via the Huebscher formula.
4
Click Size duct
The calculator solves for the round diameter and duct velocity, and — if you entered a height — iterates the Huebscher equation for the equivalent rectangular width.
5
Read the diameter and velocity
The diameter is the calculated minimum; round up to the next standard size (e.g., 15" for a 14.5" result). Velocity is flagged against ASHRAE branch, supply-main, and return limits.

The formula

The equal-friction method fixes a target pressure loss per 100 ft and solves for the diameter that meets it.

D = [0.109 × Q1.9 / (ΔP/L)]0.199
D = round duct diameter, inches
Q = airflow, CFM
ΔP/L = friction rate, in.wg per 100 ft (typically 0.08–0.10)

Duct velocity: V = Q / A, where A = π(D/24)² ft² (converts the inch diameter to a radius in feet).

Rectangular equivalent (Huebscher): De = 1.30 × (a·b)0.625 / (a+b)0.25, solved iteratively for the unknown side b given De = D.

Worked example

Example
A supply branch serves 1,000 CFM at a target friction rate of 0.08 in.wg/100 ft. The equal-friction formula gives D = (0.109 × 10001.9 / 0.08)0.199 = 14.5 in — specify a standard 15" round duct. Velocity = 1,000 ÷ (π × (14.5/24)²) = 873 fpm, which passes the ASHRAE branch/return limit of 900 fpm.

When this estimate will be off

  • Curve fit (±5%) for galvanized round duct at standard air (70 °F, sea level). Significant altitude or air temperatures outside the normal range require a density-corrected friction chart.
  • Sizes one duct run at a constant friction rate. A balanced system requires every branch sized to the same pressure budget; the longest run (the index circuit) sets the fan static pressure.
  • The result is the mathematical minimum diameter. Always round up to the next nominal standard size — never down — to stay within the friction budget.
  • Velocity thresholds shown are simplified ASHRAE guidance. Noise-sensitive spaces such as libraries, patient rooms, and recording studios typically require lower velocities than the defaults.

Frequently asked questions

A common mistake: Curve fit (±5%) for galvanized round duct at standard air (70 °F, sea level). Significant altitude or air temperatures outside the normal range require a density-corrected friction chart.

The equal-friction method sets a fixed pressure drop per 100 ft of duct — typically 0.08–0.10 in.wg — and sizes each section to the diameter that meets it. Pressure loss is spread evenly per foot of duct, making manual sizing and field balancing more straightforward than constant-velocity alternatives.

ASHRAE recommends 0.08–0.10 in.wg/100 ft for most commercial HVAC systems. Residential systems typically run 0.06–0.08. Lower rates reduce noise and fan energy but require larger ductwork; higher rates shrink duct size but raise operating cost and can generate noise at branches and fittings.

No. The formula is a curve fit accurate to ±5% for standard air in galvanized round duct. Permit-ready HVAC design requires a full system analysis by a licensed mechanical engineer — covering total external static pressure, fan curve matching, duct leakage, and all fitting losses. This tool is for preliminary sizing and design exploration.

The Huebscher equation (1948) defines the round diameter that produces the same friction loss per foot as a rectangular duct of sides a and b. Solving for the unknown side requires iteration because both sides appear inside a product and a sum. The calculator runs 40 steps, which converges to within fractions of an inch for typical duct sizes.

Sources

  • ASHRAE Handbook — Fundamentals, Duct Design — Equal-friction method and friction-rate chart from which the 0.109 coefficient curve fit is derived
  • Huebscher (1948) — Rectangular duct equivalent diameter formula: De = 1.30 × (a·b)^0.625 / (a+b)^0.25

This free Duct Sizing Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.