Marine Hose Barb and Pipe Thread Fittings: How to Pick the Right Size

Why Fitting Size Trips People Up

Boat plumbing uses two different sizing systems side by side: hose barbs are sized by the inside diameter of the hose, while threaded ends use nominal pipe sizes that don't match any measurement you'd take with a ruler. Mix them up and you'll end up with a fitting that won't seal. Here's how to get it right the first time.

Reading a Fitting Name

A name like 1" male pipe thread x 3/4" hose barb describes each end of the fitting:

• MPT (male pipe thread): threads on the outside, screws into a threaded port.
• FPT (female pipe thread): threads on the inside, accepts a male-threaded part.
• HB (hose barb): ridged end that slides into a hose and is secured with a clamp.

So a 1" MPT x 3/4" HB adapter threads into a 1" port on one end and accepts 3/4" ID hose on the other.

How to Measure Hose

Hose barbs match the hose's inside diameter (ID), not the outside. Cut a clean end and measure straight across the opening. Common marine sizes are 5/8", 3/4", 1", 1 1/8", and 1 1/2". Many hoses also have the ID printed on the side. The barb should fit snugly; if it slides in loosely, you have the wrong size.

How to Measure Pipe Threads

Nominal pipe sizes run smaller than the actual outside diameter of the threads. Measure the outside diameter of a male thread (or inside of a female port) and use this rough guide:

• About 7/8" OD = 1/2" pipe thread
• About 1 1/16" OD = 3/4" pipe thread
• About 1 5/16" OD = 1" pipe thread
• About 1 7/8" OD = 1 1/2" pipe thread

When in doubt, bring the old fitting to a hardware store thread gauge or compare it against a known part.

Choosing the Right Shape

• Straight adapters connect a threaded port to a hose, or join two hoses of the same or different sizes. Browse our male to hose barb and hose and barb adapters.
• Elbows turn the line 90 degrees in tight spaces behind thru-hulls, pumps, and drains. See elbow fittings and adaptors.
• Tees and Y fittings split or combine lines, for example teeing two drains into one overboard. See hose Y fittings.

Installation Tips

• Wrap male threads with PTFE tape or use a marine thread sealant. Don't overtighten plastic fittings, which can crack.
• Use two stainless hose clamps on any below-waterline connection, with screws offset from each other.
• Warm stiff hose in hot water so it slides onto the barb without forcing it.
• Never use a hose barb fitting that's smaller than your hose just because it's what you have. A loose connection will leak.

Replacing an Old Fitting?

Layup is a good time to check plumbing for cracked, brittle, or corroded fittings, especially where they connect to thru-hulls and drains. For a full walkthrough, see our guide on installing or replacing a thru-hull fitting. Not sure what size you need? Email info@replacementboatparts.com with a photo and measurements and we'll help.