Compression Fitting Sizing Chart & Tube Size Reference | Apex Flow

The single most common cause of a leaking compression joint is a size mismatch — ordering a fitting that does not match the actual outside diameter of the tube. Compression fittings are sized by tube OD, not by the nominal pipe size or the thread on the body, and getting that distinction right is the difference between a leak-free seal and a failed connection. This reference gives you the inch and metric sizing charts plus the measuring method to get it right the first time.

Apex Flow Solutions stocks brass compression fittings across the full inch and metric range. Use the charts below to match your tube to the correct fitting and ferrule size.

Mixing inch and metric tubing?

Compression fittings are not interchangeable between inch and metric tube. If you are unsure which standard your tubing follows, contact our team with your measured OD and we will identify the correct fitting.

How Compression Fittings Are Sized

A compression fitting seals when a brass ferrule (also called an olive or sleeve) is compressed against the outside of the tube as the nut is tightened onto the body. Because the ferrule grips the tube's outer wall, the controlling dimension is the tube outside diameter (OD). A "3/8 inch compression fitting" accepts tube with a 3/8" (0.375") OD — it has nothing to do with the inside diameter of the tube or the NPT thread on the other end of the fitting.

This is where ordering errors happen. The compression nut thread is larger than the tube OD, so a 3/8" compression fitting has a nut thread around 7/16"–20. Do not measure the nut thread and assume it is the fitting size. Always size by the tube OD.

Exploded view of a brass compression fitting showing body, ferrule, and nut on a tube

A compression fitting seals on the tube OD: the nut drives the ferrule into the body's tapered seat, compressing it around the tube. Fitting size equals tube OD.

Inch Tube Size Chart

Match the nominal compression fitting size to the measured tube OD. Typical working pressures shown are for brass fittings on annealed copper tube at ambient temperature; verify against tube and fitting ratings.

Fitting Size	Tube OD (in)	Tube OD (mm)	Typical Max Pressure
1/8"	0.125	3.18	300 PSI
3/16"	0.1875	4.76	300 PSI
1/4"	0.250	6.35	300 PSI
5/16"	0.3125	7.94	250 PSI
3/8"	0.375	9.53	250 PSI
1/2"	0.500	12.70	200 PSI
5/8"	0.625	15.88	175 PSI
3/4"	0.750	19.05	150 PSI
7/8"	0.875	22.23	150 PSI
1"	1.000	25.40	150 PSI

Metric Tube Size Chart

Metric compression fittings are sized directly by tube OD in millimeters. The nearest inch equivalents below are reference only — metric and inch fittings are NOT interchangeable, because a 6mm tube (0.236") will not seal in a 1/4" (0.250") fitting.

Metric Fitting	Tube OD (mm)	Tube OD (in)	Nearest Inch (not interchangeable)
4 mm	4.0	0.157	~5/32"
6 mm	6.0	0.236	~1/4"
8 mm	8.0	0.315	~5/16"
10 mm	10.0	0.394	~3/8"
12 mm	12.0	0.472	~1/2"
15 mm	15.0	0.591	~5/8"
18 mm	18.0	0.709	~3/4"
22 mm	22.0	0.866	~7/8"

Compression Nut Thread Reference

If you have a loose nut and need to identify the fitting size, the nut thread (a SAE straight thread) can confirm it. Note how the thread is always larger than the tube OD it serves.

Fitting (Tube OD)	Nut Thread (SAE)	Threads per Inch
1/8"	5/16"-24	24
1/4"	7/16"-24	24
3/8"	9/16"-24	24
1/2"	11/16"-20	20
5/8"	13/16"-18	18
3/4"	15/16"-16	16

How to Measure Correctly

Use a caliper, never a tape measure or ruler — the difference between a 1/4" and 5/16" tube is 0.0625", far too small to read on a tape. Measure the outside diameter of the tube at a round, undamaged section. Take two readings 90 degrees apart to confirm the tube is not ovalized. The measured OD is your fitting size. If the reading falls between an inch size and a metric size (for example 0.236"/6mm versus 0.250"/1/4"), the tube is almost certainly the standard that matches exactly — measure precisely enough to tell them apart.

One ferrule rule worth remembering: once a brass ferrule has been compressed, it has permanently deformed to that tube. Reusing a compressed ferrule on a new joint is not recommended for pressure or critical service — install a fresh ferrule.

Caliper measuring the outside diameter of copper tubing

Measure tube OD with a caliper at a round section. The measured outside diameter is the compression fitting size — 0.375" means a 3/8" fitting.

Standards & References

Compression fitting threads are SAE straight threads per SAE J512 (automotive tube fittings). Copper tube dimensions follow ASTM B75 (seamless copper tube) and ASTM B280 (ACR tube for refrigeration). Brass fitting alloy is typically C36000 free-cutting brass per ASTM B16. For potable water service, fittings must meet NSF/ANSI 61 and the lead-free requirements of NSF/ANSI 372. Metric tube and fitting dimensions follow ISO 8434 and DIN 2353.

Frequently Asked Questions

How do I know what size compression fitting I need?

Measure the outside diameter (OD) of your tube with a caliper. The measured OD is the fitting size — a tube measuring 0.375" takes a 3/8" compression fitting. Do not size by the inside diameter or the nut thread.

Are inch and metric compression fittings interchangeable?

No. A 6mm tube (0.236" OD) will not seal in a 1/4" fitting (0.250" OD), and forcing it leads to leaks. Confirm whether your tubing is inch or metric before ordering.

Can I reuse a compression ferrule?

It is not recommended for pressure or critical service. The ferrule permanently deforms to the tube when compressed. For a reliable seal on a new joint, install a fresh ferrule.

Do compression fittings need Teflon tape?

Not on the compression (ferrule) joint itself — the seal is mechanical. Only apply Teflon tape or pipe dope to a tapered NPT thread if the opposite end of the fitting has one.

What is the maximum pressure for a brass compression fitting?

It depends on size and tube material, but brass compression fittings on copper tube typically range from about 150 PSI for 3/4"–1" up to 300 PSI for 1/8"–1/4" at ambient temperature. Always confirm against the specific fitting and tube ratings.