Faucet Thread Size Guide

2016-04-13

Table of Contents
→Thread Standards by Region
→ Europe / UK Thread
→ United States Thread
→ Asia Thread
→ How to Measure Faucet Threads in 3 Simple Steps
→ Frequently Asked Questions
→ Conclusion

I recently helped a friend replace a bathroom faucet—and it turned into a real lesson in plumbing standards.

He bought a British-standard G1/2 faucet and tried to install it onto a US-standard NPT connection. After a few turns, water sprayed everywhere. The reason?
Different countries use completely different thread systems.

Those differences may look minor on paper, but in practice, they can mean leaks, damaged threads, or a faucet that simply won’t fit.

1. Thread Standards by Region: Check the Origin Before You Install

1.1 Europe & the UK: G Thread (BSPP) Territory

Standard: G thread, also known as BSPP (British Standard Pipe Parallel)
Characteristics: Parallel thread (same diameter from top to bottom) / 55° thread angle
Sealing relies on a rubber washer or PTFE (Teflon) tape, not metal-to-metal contact
Most common basin faucet size: G 1/2
Outside diameter: approx. 20.955 mm
Threads per inch: 14 TPI
Pitch: approx. 1.814 mm
This is the default standard for most European, British, and Chinese faucets

1.2 United States: NPT Tapered Thread System

Standard: NPT (National Pipe Taper)
Characteristics: Tapered thread (diameter gradually changes) / Taper ratio: 1:16 / 60° thread angle
Sealing is achieved by metal-to-metal interference, usually with thread sealant
Most common basin faucet size: 1/2" NPT
Outside diameter: approx. 21.336 mm
Threads per inch: 14 TPI
Pitch: approx. 1.814 mm

A friend once tried installing a US faucet in a London apartment—it jammed after two turns. Forcing it damaged the threads permanently.

1.3 Asia: Subtle but Critical Differences

China: Uses G thread (BSPP), same as Europe
Japan: Uses PT thread (BSPT)
55° thread angle (same as G)
Tapered (like NPT)

This creates a common mistake:
A G1/2 faucet may screw into a Japanese PT outlet, but it will almost certainly leak, because a parallel thread cannot properly seal against a tapered interface.

2. How to Measure Faucet Threads in 3 Simple Steps (Don't Guess—Measure)

Step 1: Measure the Outside Diameter
Use a caliper to measure the widest part of the thread:

Around 20.95 mm → likely G 1/2
Around 21.3 mm → likely 1/2" NPT

Step 2: Count Threads Per Inch
Measure 25.4 mm (1 inch) and count the threads:

14 TPI → G1/2 or NPT1/2
11 TPI → likely G3/4

Step 3: Check for Taper
Look at the thread from the side:

Parallel (same thickness) → G thread
Clearly tapered → NPT (US) or PT (Japan)

Pro tip: I once confused G1/2 with NPT1/2 because I ignored the thread angle. If possible, use a thread gauge to confirm whether the angle is 55° (G/PT) or 60° (NPT).