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

Tube Fitting Stainless SUS316 Compression Fitting Series - NIHON

Brass Fitting (Inches Size) Compression Male Connector Compression

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

Legines Brass Compression Tube Fitting, Union, 1/8 OD

HS160 - Large Size Compression Fitting ⌀75-90-110 Manufacturer

What size of compression fitting do I need?

Compression Fitting Specifications - BWC- Butt Weld Connector

Dishwasher Supply Line - Installation

Ice Maker Water Line Brass Compression Tube Fitting, 1/4 OD x 1/4 OD (2)

785302 Compression Fitting Union

GF iJoint Compression Fittings - Hynds Pipe Systems Ltd.

Compression Fittings Plumbers Pack

1 1/4 MPT Male Pipe Thread (4) Pex-al-Pex Compression Fitting