Barbed fittings are one of the simplest connection methods in fluid handling, and one of the most commonly done wrong. The failure point is almost always the same: using a barb that's the wrong size for the hose.
How Barbs Seal
A barb fitting has one or more raised ridges along a tapered shank. You push the hose over the shank and the barbs bite into the hose interior wall, creating resistance to pull-out. A clamp over the hose body then holds everything in via compression.
The seal relies on two things: the hose ID being slightly smaller than the barb OD so there's interference, and a clamp generating enough force to keep the hose from ballooning away from the barb under pressure.
Sizing
Barb fittings are sized and listed by their outer diameter, thus the inner diameter of the tubing they're designed for. So a 1/2″ OD barb fitting is designed for hose with a 1/2″ inner diameter.
The key measurement is the barb's outer diameter (OD), it should be equal to or slightly larger than the hose ID. This typically gives you enough grip without making the fitting impossible to insert. If the barb OD is the same as or smaller than the hose ID, you're relying entirely on the clamp for the seal, which isn't ideal.
For soft tubing like silicone or vinyl, you can usually push a barb that's up to 1/8″ larger than the nominal hose ID and the tubing will stretch to accommodate. For stiffer tubing, that sizing needs to be closer.
Wall Thickness Matters
Thin-walled tubing is more flexible and easier to push onto a barb, but it provides less resistance to pull-out and is more prone to kinking at the joint. For pressure applications, use hose with enough wall thickness to grip the barb properly and hold its shape under load.
Number of Barbs
Single-barb fittings are fine for low-pressure, low-stakes applications. Multiple barbs increase the surface area gripping the hose and distribute the pull-out force over a longer section of hose wall, which improves retention at higher pressures. For any pressurised application, a multi-barb fitting is preferred.