BWG vs SWG: Key Differences

BWG vs SWG is a common source of confusion when specifying tube wall thickness for heat exchangers, condensers, and instrumentation tubing. BWG (Birmingham Wire Gauge) and SWG (Standard Wire Gauge) are both gauge systems that assign a number to a specific wall thickness, but their scales differ. Specifying the wrong system can lead to incorrect pressure ratings and tube fitment issues.

BWG vs SWG Comparison

The following table shows the wall thickness assigned to each gauge number under both systems. In both cases, a lower gauge number means a thicker wall.

Gauge No.	BWG Thickness (in)	BWG Thickness (mm)	SWG Thickness (in)	SWG Thickness (mm)
10	0.134	3.40	0.128	3.25
12	0.109	2.77	0.104	2.64
14	0.083	2.11	0.080	2.03
16	0.065	1.65	0.064	1.63
18	0.049	1.24	0.048	1.22
20	0.035	0.89	0.036	0.91
22	0.028	0.71	0.028	0.71

Key Differences Between BWG and SWG

• Scale origin: BWG originated in Birmingham, England, for wire and tube manufacturing. SWG (also called Imperial Wire Gauge) was standardized by the British Board of Trade.
• Regional use: BWG is the dominant standard in the United States and in international oil and gas projects. SWG is used mainly in the United Kingdom and Commonwealth countries.
• Industry preference: Heat exchanger manufacturers (TEMA standards) predominantly reference BWG. Electrical and general engineering applications in the UK may use SWG.
• Numerical difference: For the same gauge number, BWG and SWG assign slightly different thicknesses. The difference is small (typically 0.001-0.006 inches) but compounds with larger OD tubes and affects pressure calculations.

How to Avoid Gauge Mismatch

When ordering tubes, always specify the actual wall thickness in inches or millimeters alongside the gauge number. For example, write “BWG 18 (0.049 in / 1.24 mm)” rather than just “Gauge 18.” This eliminates ambiguity between BWG and SWG scales.

Common Mistake

Assuming BWG and SWG gauges are interchangeable. A Gauge 18 tube ordered as SWG (0.048 in) instead of BWG (0.049 in) may fail to meet the minimum wall thickness required by the TEMA heat exchanger design. Always confirm the gauge system with the supplier before placing the order.

For projects governed by ASME or TEMA, BWG is the standard reference. For projects under British Standards, confirm whether SWG or BWG applies in the specification.

Read the full guide to BWG/SWG tube sizes