What size of compression fitting do I need?

It is important to understand the difference between

Compression Fitting 6mm OD 5mm ID Ferrules and Insert Crimp

Navien NPE-2 Series Condensing Tankless Water Heaters, 50% OFF

Copper Tube Size LF Brass Compression Coupling with Stop

Sioux Chief 3/8 inch Lead-Free Brass Compression Cap

How to Replace a Compression Shutoff Valve

3/4 Brass Compression Nut 2-PackUse this 3/4 brass compression nut with copper, aluminum, and thermoplastic tubing. Not recommended for steel

3/4 Compression Nut Hex Shape 1 Thread Size Brass Compression Fitting 2-Pack

What size of BC Series brass compression fitting do I need to order?

Fittings and Tubing Guide

How to Assemble and Use Compression Fittings

Brass Compression Fittings and Adaptors

40 mm Compression Waste, Waste Pipe & Fittings

Copper Pipe Compression Fittings Supplier

Compression fitting - Wikipedia