Imperial Si Metric Pipe Sizes

Reference data and engineering information about imperial si metric pipe sizes for piping systems applications.

imperialmetricpipesizes

Overview

Engineering reference data for Imperial Si Metric Pipe Sizes in piping systems.

Key Formulas

Continuity

$A_1 v_1 = A_2 v_2$

Mass conservation in pipe flow.

Pressure Drop

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Darcy-Weisbach equation.

Pipe Area

$A = \frac{\pi D^2}{4}$

Cross-sectional area of a pipe.

Variables

Symbol	Description	Unit
$D$	Pipe diameter	m
$v$	Flow velocity	m/s
$\Delta P$	Pressure drop	Pa
$f$	Friction factor	—

Imperial to Metric Pipe Size Conversion Table

The following table provides a comprehensive comparison between nominal imperial pipe sizes (NPS) and nominal metric pipe sizes (DN), along with common standards from major organizations. These conversions are essential for international projects and material procurement.

36 rows

Comparison of Imperial (NPS) and Metric (DN) nominal pipe sizes with common standard dimensions (mm)
Nominal Imperial Size (NPS)(inch)	Nominal Metric Size (DN)(mm)	ASHRAE(mm)	AWWA(mm)	NFPA(mm)	ASTM Copper Tube(mm)
1/8	6	-	-	-	6
3/16	8	-	-	-	8
1/4	10	8	-	-	10
3/8	12	10	-	-	12
1/2	15	15	12.7, 13	12	15
5/8	18	-	-	-	18
3/4	20	20	-	-	22
1	25	25	25	25, 25.4	28
1 1/4	32	32	-	33	35
1 1/2	40	40	-	45	38, 38.1
2	50	50	50, 50.8	51	54
2 1/2	65	65	63, 63.5	63.5, 64	67
3	80	80	75	76, 80	79
3 1/2	90	-	-	89	-
4	100	100	100	102	105
4 1/2	115	-	-	114.3	-
5	125	-	127	-	130
6	150	150	150	152	156
8	200	200	200	103	206
10	250	250	250	250	257
12	300	300	300	300	305
14	350	-	-	308	-
16	400	-	-	400	-
18	450	-	-	450	-
20	500	-	-	500	-
24	600	-	-	600	-
28	700	-	-	700	-
30	750	-	-	750	-
32	800	-	-	800	-
36	900	-	-	900	-
40	1000	-	-	1000	-
44	1100	-	-	1100	-
48	1200	-	-	1200	-
52	1300	-	-	1300	-
56	1400	-	-	1400	-
60	1500	-	-	1500	-

Source: engineeringtoolbox.com

Understanding Pipe Sizing Systems

The United States is the only major industrial country that predominantly uses the imperial (inch-based) Nominal Pipe Size (NPS) system. The rest of the world primarily uses the metric Nominal Diameter (DN) system.

It is critical to understand that nominal sizes are not the actual physical dimensions of the pipe. For instance:

A 1-inch NPS pipe has an outside diameter (OD) of 1.315 inches (33.4 mm), not 1 inch.
The DN number roughly corresponds to the internal diameter of the pipe in millimeters for smaller sizes, but this relationship becomes approximate for larger pipes.

The table includes dimensions from several key standards bodies:

ASHRAE & AWWA: Common in HVAC and water/wastewater applications in North America.
NFPA: Sizes used in fire protection systems.
ASTM Copper Tube: Specific dimensions for copper tubing used in plumbing and refrigeration.

Note: A "-" indicates that the specific dimension is not commonly defined or used for that size under that standard.