BWG to MM Tube Gauge Conversion

Quick Answer

BWG (Birmingham Wire Gauge) to mm conversion is straightforward using the standard chart below. Each BWG gauge number corresponds to a fixed wall thickness in millimeters. BWG 18 = 1.245 mm, BWG 14 = 2.108 mm, BWG 10 = 3.404 mm. These values are fixed and do not change with tube outside diameter.

The BWG to mm conversion chart is one of the most frequently referenced tables in heat exchanger and boiler tube specifications. Engineers and procurement professionals use this conversion to translate gauge designations into metric dimensions for manufacturing, inspection, and international trade.

BWG to MM Conversion Chart

BWG Gauge	Thickness (inch)	Thickness (mm)	Common Application
G. 7	0.180	4.572	Heavy-wall boiler tubes
G. 8	0.165	4.191	Boiler and superheater tubes
G. 9	0.148	3.759	High-pressure boiler tubes
G. 10	0.134	3.404	Boiler tubes, thick-wall exchangers
G. 11	0.120	3.048	Heat exchanger tubes (heavy duty)
G. 12	0.109	2.769	Heat exchanger tubes (standard)
G. 13	0.095	2.413	Heat exchanger tubes
G. 14	0.083	2.108	Heat exchanger tubes (common)
G. 15	0.072	1.829	Condenser tubes
G. 16	0.065	1.651	Condenser and exchanger tubes
G. 17	0.058	1.473	Thin-wall condenser tubes
G. 18	0.049	1.245	Instrumentation tubing, condensers
G. 19	0.042	1.067	Thin-wall instrumentation tubing
G. 20	0.035	0.889	Light-duty tubing
G. 21	0.032	0.813	Light-duty tubing
G. 22	0.028	0.711	Very thin wall tubing

How to Use the BWG to MM Conversion

The BWG gauge number defines only the wall thickness. A tube is fully specified by its outside diameter (OD) and its BWG gauge (wall thickness). The inside diameter (ID) is calculated as:

ID = OD - (2 x wall thickness)

For example, a 1” OD tube with BWG 14 wall:

• OD = 25.4 mm
• Wall thickness (BWG 14) = 2.108 mm
• ID = 25.4 - (2 x 2.108) = 21.184 mm

BWG to MM: Quick Reference for Common Gauges

BWG Gauge	mm	Fraction Reference
BWG 10	3.40	Approx. 1/8” + 1/128”
BWG 12	2.77	Approx. 7/64”
BWG 14	2.11	Approx. 5/64”
BWG 16	1.65	Approx. 1/16”
BWG 18	1.24	Approx. 3/64”
BWG 20	0.89	Approx. 1/32”

Pro Tip

When ordering tubes internationally, always specify the wall thickness in both BWG gauge and millimeters on the purchase order. Many non-US manufacturers work in metric and may not be familiar with BWG designations. Stating “BWG 14 (2.11 mm)” eliminates ambiguity and prevents manufacturing errors. For tube-to-fitting connections, verify compatibility with compression fitting specifications.

For complete tubing weight tables organized by OD and BWG gauge, see the full BWG/SWG tube size charts.

Read the full guide to BWG/SWG tube sizes