Duct Sizing by Friction Calculator

Inputs

Airflow

Target friction rate

Material roughness Roughness affects friction factor and pressure drop.

Air properties

Density ρ (kg/m³)

Viscosity μ (Pa·s)

Rectangular aspect ratio (W:H) Use 1 for square, 2 for 2:1, etc.

Velocity warning threshold (m/s)

Results

Solved round diameter 14.41 in

366.1 mm

Nearest common nominal

14 in (356 mm)

Velocity

4.48 m/s

Reynolds number

109,183

Friction factor f

0.0198

Friction rate (calculated)

0.65 Pa/m

0.080 in.wg / 100 ft

Rectangular equivalent (W×H)

481 × 240 mm

Ratio = 2.00 : 1

Common nominal size check (same flow)

Nominal (in)	Diameter (mm)	Velocity (m/s)	Re	f	Friction (Pa/m)	Friction (in.wg/100ft)
4	102	58.21	393,421	0.0223	448.64	54.899
5	127	37.26	314,737	0.0214	140.98	17.251
6	152	25.87	262,281	0.0208	55.04	6.734
7	178	19.01	224,812	0.0204	24.95	3.053
8	203	14.55	196,711	0.0201	12.62	1.544
9	229	11.50	174,854	0.0199	6.94	0.849
10	254	9.31	157,369	0.0198	4.07	0.498
12	305	6.47	131,140	0.0197	1.63	0.199
14	356	4.75	112,406	0.0198	0.76	0.092
16	406	3.64	98,355	0.0199	0.39	0.048
18	457	2.87	87,427	0.0200	0.22	0.027
20	508	2.33	78,684	0.0202	0.13	0.016
22	559	1.92	71,531	0.0204	0.08	0.010
24	610	1.62	65,570	0.0207	0.05	0.007

Assumptions: Steady incompressible flow in round ducts, Darcy–Weisbach with Swamee–Jain friction factor; roughness based on selection.

FAQs

1) What friction rate should I start with?
Many designers begin around 0.08 in.wg per 100 ft (≈0.65 Pa/m) for main trunks and lower for branches to manage noise and energy use.

2) How does roughness affect the result?
Higher roughness increases the friction factor, which raises pressure drop for the same flow, leading to larger recommended diameters.

3) Why is velocity important?
Excessive velocity can increase noise and dynamic losses. The calculator flags values above a user‑set threshold so you can upsize if needed.

4) Can I size rectangular ducts?
Yes. Enter an aspect ratio and the tool returns an equivalent rectangular width and height that match the round solution’s pressure drop.

5) Which air properties should I use?
Defaults reflect typical indoor conditions (~20 °C). For hot or high‑altitude air, adjust density and viscosity to improve accuracy.

6) What if the solver fails?
Extremely low friction targets or very high flows can fall outside the search bounds. Try relaxing the friction rate or verifying units.

7) Are minor losses included?
No. This focuses on straight‑duct friction. Account for fittings separately using loss coefficients or equivalent length methods.