Duct Friction Pressure Loss

Reference data and engineering information about duct friction pressure loss for fluid mechanics applications.

ductfrictionpressureloss

Overview

Duct friction pressure loss (major loss) is the energy drop caused by air flowing through ductwork. In galvanized steel circular ducts with turbulent flow, the loss per unit length depends on airflow rate and duct diameter. This page covers the standard empirical formula, reference data for common duct sizes, and practical design limits.

Key Formulas

Duct Friction Loss (Imperial)

$\Delta h = 0.109136 \times \frac{q^{1.9}}{d^{5.02}}$

where $\Delta h$ is in inches of water gauge per 100 ft of duct, $q$ is airflow in cfm, and $d$ is duct inside diameter in inches.

Air Velocity from Flow Rate

$v = \frac{q}{A} = \frac{q}{\pi (d/2)^2}$

For a circular duct with diameter $d$ in inches and flow $q$ in cfm, velocity in ft/min is:

$v = \frac{576 \, q}{\pi \, d^2}$

Rectangular Duct Equivalent Diameter

For non-circular ducts, replace $d$ with the equivalent diameter:

$d_e = 1.30 \times \frac{(a \cdot b)^{0.625}}{(a + b)^{0.25}}$

where $a$ and $b$ are side dimensions in inches.

Variables

Symbol	Description	Typical Unit
$\Delta h$	Head loss (friction)	in H₂O / 100 ft
$q$	Air volume flow rate	cfm
$d$	Duct inside diameter	in
$v$	Air velocity	ft/min
$a, b$	Rectangular duct sides	in
$d_e$	Equivalent diameter	in

Friction Loss Reference Data

Head loss and air velocity for galvanized steel circular ducts with turbulent airflow.

13 rows

Major friction head loss and air velocity for common duct sizes
Air Volume(cfm)	Duct Size(in)	Head Loss(inH₂O/100ft)	Air Velocity(ft/min)
100	4	0.65	1146
100	6	0.09	509
100	8	0.02	286
200	5	0.8	1467
200	8	0.08	573
200	10	0.02	367
400	6	1.19	2037
400	8	0.28	1146
400	12	0.04	509
800	10	0.34	1467
800	12	0.14	1019
800	16	0.03	573
1600	16	0.12	1146

Source: engineeringtoolbox.com

Air Velocity Calculator

Estimate air velocity in a circular duct from flow rate and duct size.

Duct Air Velocity

Air Volume Flow(cfm)

Duct Diameter(in)

Imperial Units Calculator

The original page included an Imperial units calculator. This migrated version calculates air velocity, straight-duct friction loss per 100 ft, and total friction loss for round galvanized steel duct using the source equation.

Air Duct Friction Loss - Imperial Units

Air volume flow(cfm)

Round duct diameter(in)

Duct length(ft)

SI Units Calculator

This SI calculator is the same equation with metric inputs converted internally to cfm and inches.

Air Duct Friction Loss - SI Units

Air volume flow(m^3/h)

Round duct diameter(mm)

Duct length(m)

Unit Converter

Duct Airflow and Pressure Unit Converter

Airflow(cfm)

Pressure(inH2O)

Diameter(in)

Length(ft)

Friction Loss Trend — 10 in Duct

The chart below shows how friction loss rises sharply with airflow in a 10-inch duct. Loss scales roughly with $q^{1.9}$ , so doubling airflow roughly quadruples the pressure drop.

Friction Loss vs Air Volume — 10 in Duct

Air Ducts - Friction Loss Diagram

The original friction loss diagram covers several round duct sizes. This interactive version keeps the same intent by plotting friction loss against air volume for common duct diameters.

Air Ducts - Friction Loss Diagram

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Air Ducts - Major Friction Head Loss

11 rows

Air Ducts - Major Friction Head Loss
Duct Size (inches)	Duct Size (inches)	Duct Size (inches)	Duct Size (inches)	Duct Size (inches)	Duct Size (inches)	Duct Size (inches)	Head Loss (inches water gauge per 100 feet duct)
4	5	6	8	10	12	16
100	0.65	0.21	0.09	0.02	0.01
1146	733	509	286	183
200		0.8	0.32	0.08	0.02	0.01
	1467	1019	573	367	255
400			1.19	0.28	0.09	0.04	0.01
		2037	1146	733	509	286
800					0.34	0.14	0.03
				1467	1019	573
1600							0.12
						1146

Source: engineeringtoolbox.com

Air Ducts - Major Friction Head Loss, Normalized Source Rows

The source table alternates friction head-loss rows and air-velocity rows under the same duct-size columns. The normalized table below preserves the same source values in one row per air volume.

5 rows

Normalized source values from Air Ducts - Major Friction Head Loss
Air Volume(cfm)	4 in head loss(inH2O/100ft)	5 in head loss(inH2O/100ft)	6 in head loss(inH2O/100ft)	8 in head loss(inH2O/100ft)	10 in head loss(inH2O/100ft)	12 in head loss(inH2O/100ft)	16 in head loss(inH2O/100ft)	4 in velocity(ft/min)	5 in velocity(ft/min)	6 in velocity(ft/min)	8 in velocity(ft/min)	10 in velocity(ft/min)	12 in velocity(ft/min)	16 in velocity(ft/min)
100	0.65	0.21	0.09	0.02	0.01	—	—	1146	733	509	286	183	—	—
200	—	0.8	0.32	0.08	0.02	0.01	—	—	1467	1019	573	367	255	—
400	—	—	1.19	0.28	0.09	0.04	0.01	—	—	2037	1146	733	509	286
800	—	—	—	—	0.34	0.14	0.03	—	—	—	—	1467	1019	573
1600	—	—	—	—	—	—	0.12	—	—	—	—	—	—	1146

Source: engineeringtoolbox.com

Original Source Images

The following original source images are preserved to avoid losing visual reference material. When an image contains chart or tabular data, its extracted values are represented in the page tables, calculators, or interactive charts; remaining images are retained as visual source references.

Air Ducts - Velocity and Friction Loss - Excel Template Air Ducts - Friction Loss Diagram

Engineering Notes

Noise limits. Air velocity should stay below roughly 600–1000 ft/min in residential applications, 1500 ft/min in quiet commercial spaces, and 2000+ ft/min only in industrial settings. Exceeding these limits causes objectionable noise.
Turbulent flow assumption. The empirical formula above assumes fully developed turbulent flow, which is typical for HVAC duct sizing. It does not apply to laminar or transitional regimes.
Material. Reference data applies to galvanized steel ducts. Flex duct, lined duct, and fiberglass duct have higher friction factors and require correction.
Fittings and transitions. The formulas cover major (straight-duct) losses only. Elbows, tees, transitions, dampers, and grilles add minor losses that often dominate in short duct runs. Total system pressure drop must include both.
Rectangular ducts. Always convert to equivalent circular diameter before using the friction loss formula. Undersized rectangular ducts are a common source of excessive static pressure.
Economic balance. Larger ducts reduce friction loss (and fan energy) but increase material and installation cost. The $q^{1.9} / d^{5.02}$ relationship means a modest increase in duct diameter yields large reductions in pressure drop.

References

Engineering ToolBox — Air Ducts Friction Loss

Source: engineeringtoolbox.com