What size Loc-line do i have?

Apr 23, 2017

It seems like a simple question to answer, but for anyone who has tried to figure this out, it doesn’t always seem clear. You’ll find loc-line in use in just about every saltwater aquarium and it’s used as a means of directing water flow on your return line. The hobby is full of all kinds of different attachments, accessories and nozzles that snap right on to the end of your loc-line, including our own Random Flow Generator, or RFG Nozzle.

When it comes time to choosing the right size it can be hard to tell what you need – especially if you only have access to one of the two most common sizes; 1/2” and 3/4” loc-line.

To make matters worse, if you were to try and measure your loc-line, you’re likely to get a measurement that seems confusing. For instance, the 1/2in  loc-line is a little more than 3/4in across.

So, how can you tell what size you have? Here are a few tips to help you properly measure your loc-line.

Need Loc-Line?

We now offer geniune Loc-Line® plumbing parts including 1/2in and 3/4in modular hose, adapters, Y fittings and connectors.

The size of the loc-lin refers to the inner dimension or ID as opposed to the outer dimension or OD.

When measuring loc-line, use these numbers to determine what size you have.

1/2” loc-line

ID – approx. .675in, or about 17mm
OD – approx. .846in, or about 21.5mm

1/2in Loc-line

3/4” Loc-line

ID – approx. .925in, or about 23mm
OD – approx. 1.125in, or about 28.5mm

3/4in Loc-line

Compare the end to a Nickel (US).

A nickel is a good way to visually measure the end of a loc-line – just don’t stick it in your tank and accidently drop it. We would recommend you use this method out of the water.

Sizing your Loc-line

If all else fails – stick your finger in it!

It’s not the most exact method, but if you can fit your index finger in it, or at least most of the way in, it’s probably a 3/4in lock-line. If you can’t and the opening is too small, then it’s probably the 1/2in variety.


1/2” loc-line

1/2″ loc-line is pretty small, so you may not be able to fit your index finger in.

3/4in Loc-line

3/4” Loc-line

You finger will likly fit at least most of the way in – just don’t get it stuck.

3/4in Loc-line

We hope this quick-tip helps you figure you your lock-line. and the next time you have to measure loc-line, you can do it like a pro.

Happy reefing!