Air Hose Fitting Sizes: Coupler and Connector Chart
Air hose fitting sizes come down to the NPT thread on each end and the quick-connect style, with 1/4 inch and 3/8 inch NPT the sizes in the Milosline set and a 300 PSI maximum working pressure.
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A new air tool arrives, the hose is ready, and the coupler will not fit: the threads look right, the plug slides in partway, and air hisses out at the joint. Air hose fitting sizes are the reason, and they come down to the thread size on each end, the coupler style, and whether the plug and the coupler match. This guide explains the sizes the listings publish, how to read them, and how to match a coupler to a plug without another trip to the store.
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Below: what the sizes mean, the NPT thread sizes in the listings, a chart of the sizes in the Milosline set, coupler and plug styles, and how to match them.
Air hose fitting sizes: what the numbers mean
A fitting size names the thread that screws into the hose or the tool, and the quick-connect style that joins the hose to the tool. The thread size is written as a fraction of an inch with NPT after it, and the style is written as a letter code, such as I/M for the industrial style. The size on the hose end and the size on the tool end can differ, and the plug must match the coupler in size and style, or air leaks at the joint.
The listings in this guide publish 1/4 inch and 3/8 inch NPT sizes. The 1/4 inch size is the common size for air tools and hoses, and the 3/8 inch size is the high-flow size in the Milosline set. The set also states a 300 PSI maximum working pressure, a galvanized steel build, and 6 locking balls inside each coupler.
NPT thread sizes: 1/4 inch and 3/8 inch
NPT is the tapered pipe thread used on air fittings in the US, and the fraction before it names the thread size: 1/4 inch NPT on the smaller end, 3/8 inch NPT on the larger end. A 1/4 inch fitting threads into a 1/4 inch port, and a 3/8 inch fitting threads into a 3/8 inch port; the sizes do not mate with each other, so the thread size on the hose end must match the thread size on the tool end. The Milosline set includes couplers and plugs in 1/4 inch and 3/8 inch NPT.
Thread gender matters too. FNPT means a female NPT thread, cut inside the fitting, and MNPT means a male NPT thread, cut on the outside. The Milosline set includes FNPT and MNPT couplers and plugs in each size, so a hose end with a male thread takes an FNPT coupler, and a tool inlet with a female thread takes an MNPT plug. Read the gender letters on the listing before ordering, and match them to the threads on your hose and tool. For the hose side of the match, see our air hose sizes chart.
Air hose fitting sizes chart
| Size | Couplers in the set | Plugs in the set | What it fits |
|---|---|---|---|
| 1/4 inch NPT | 3 FNPT, 1 MNPT | 2 FNPT, 6 MNPT | Air tools and hoses |
| 3/8 inch NPT | 3 FNPT, 1 MNPT | 2 FNPT, 6 MNPT | High-flow air tools and hoses |
The full Milosline set holds 24 pieces: 8 couplers and 16 plugs, split across the 1/4 inch and 3/8 inch sizes, with a stated 300 PSI maximum working pressure. Read the chart by size first, then by gender. A 1/4 inch FNPT coupler threads onto a 1/4 inch male hose end, and a 3/8 inch MNPT plug threads into a 3/8 inch female tool inlet. When the size and the gender match, the fitting threads in by hand without binding; when they do not, the fitting binds or leaks, and no amount of tightening fixes a wrong size.
Coupler and plug styles: I/M quick connect
A quick-connect fitting joins a coupler to a plug: the coupler is the female body that stays on the hose, and the plug is the male end that pushes into the coupler and locks in place. The Milosline coupler has 6 built-in locking balls that hold the plug in place, and the set uses the I/M style named in the listing. Plugs and couplers must be the same style to connect: an I/M plug locks into an I/M coupler, and a plug from another style will not seat, even when the thread size matches. For the full range of connection types, see our air hose connection types guide.
The Milosline fittings are made of galvanized steel, which the listing calls durable and corrosion resistant, and the maker states the fittings are standardized and connect with air fittings from other brands. That means a 1/4 inch NPT I/M plug from the set locks into a 1/4 inch NPT I/M coupler from another brand, as long as the size and the style match. Check the style code on every fitting you buy, because thread size alone does not make fittings connect.
How to match a coupler to a plug
- Match the thread size. Read the size on the hose end and the tool end: 1/4 inch NPT takes a 1/4 inch fitting, and 3/8 inch NPT takes a 3/8 inch fitting.
- Match the gender. A male hose end takes an FNPT fitting, and a female tool inlet takes an MNPT fitting.
- Match the style. An I/M plug goes into an I/M coupler; a plug from another style will not lock in, even when the thread size matches.
- Check the maximum working pressure. The Milosline set states a 300 PSI maximum working pressure: never run a fitting above its stated maximum, and check the stated maximum on every fitting in the line.
- Thread by hand first. A fitting that matches threads in by hand; a fitting that binds or cross-threads is the wrong size or the wrong gender, so back it out and recheck the size instead of forcing it.
When the size, gender and style all match, and every fitting is rated above the working pressure, the joint holds air without leaks. When the size, gender or style differ, the fitting either will not thread in, will not lock, or will leak at the joint, and the fix is the right fitting, not more force.
The Milosline set states a 300 PSI maximum working pressure. Never run a fitting above its stated maximum, and check the stated maximum on every fitting in the line, not just the coupler.
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FAQ: garage tools
What is the standard air hose fitting size?
The Milosline set includes 1/4 inch and 3/8 inch NPT fittings. The 1/4 inch size is the common size for air tools and hoses, and the 3/8 inch size is the high-flow size in the set.
Are 1/4 inch and 3/8 inch air fittings interchangeable?
No. A 1/4 inch NPT thread does not mate with a 3/8 inch NPT thread, so each end needs the fitting that matches its own thread size.
What does NPT mean on an air fitting?
NPT is the tapered pipe thread standard used on air fittings, and the fraction before it names the thread size, such as 1/4 inch or 3/8 inch NPT.
What is the difference between a coupler and a plug?
The coupler is the female quick-connect body that stays on the hose, and the plug is the male end that pushes in and locks. The Milosline coupler has 6 built-in locking balls that hold the plug in place.
What pressure can air hose fittings handle?
The Milosline set states a 300 PSI maximum working pressure. Check the stated maximum on every fitting in the line, and never run a fitting above it.
Do air fittings from different brands connect?
Yes, when the size and the style match. The Milosline listing states its fittings are standardized and connect with air fittings from other brands.
Related: air compressor fittings types and sizes, best air hose fittings that do not leak, best universal air hose fittings and best quick connect air hose fittings 1/4 inch NPT. Also see our air tools hub.
