What size of compression fitting do I need?

It is important to understand the difference between

PTFE Ferrule for 1/16 Diameter Thermocouple Probe Compression

40 mm Compression Waste, Waste Pipe & Fittings

40 mm Compression Waste, Waste Pipe & Fittings

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

Regusol Compression fitting for copper and precision steel pipe

Ferrule Brass Compression Fittings, For PIPE LINE, Size: 1/16'' TO

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165C-12 Dixon Valve Brass Compression Fitting - Union Elbow - 3/4

Basics of Compression Fittings

Compression Fitting 5/16 Straight w/ Ferrule