Faucet Aerator Threads

Inch sizes

The most common aerator thread size for residential faucets in the United States is 55/64" -27.  The "-27" is the number of threads per inch.  55/64"-27 is the size of the threads for the AquaPurr diverter valve.


Fifty five sixty fourths of an inch?  Really?  Yes, really.  No,/ I don't know why. 


The next most common sizes are 15/16"-27 and 13/16"-27.  15/16" is slightly larger than 55/64" and 13/16" is slightly smaller than 55/64".


Threads have gender.  In the image below, the top of the thread adapter has female threads and the bottom has male threads.  I'll assume the reader understands the reasoning behind these labels.

Faucet thread adapter female 55/64" by male 55/64"

Beyond the sizes listed above, there are many more sizes including:


  • 11/16"-24

  • 11/16"-27

  • 5/8"-24

  • 5/8"-27

  • 3/4"-27

  • 3/8" NPSM*

*  National Pipe Straight Mechanical.  NPSM! OMG LOL

Metric sizes

There are many different sizes of metric faucet threads ranging from 16mm to 28mm.  There may be more, what do I know?


Whereas inch threads identify the number of threads per inch, metric threads identify the spacing between threads.  All the metric faucet threads I have encountered are 1mm spacing, except for one (more on that later).  The 1 mm spacing is so common, that detail is often dropped from the specification of metric thread sizes.


Most common in the United States are "hidden" aerators with metric sizes of 16.5mm, 18.5mm, 21.5mm and 24mm.  


"Hidden" aerators are designed for both aesthetics and for reducing theft.  Hidden aerators cannot be removed without the use of a key:

aerator removal tool cache hidden

The keys have notches that mate to recesses in the hidden aerators.  This is what makes them theft resistant.


The color of the key indicates the size of the aerator threads. 

  • Yellow: 16.5mm

  • Green: 18.5mm

  • Red: 21.5mm

  • Grey: 24mm

The less common metric sizes for faucets in the United States are 16mm, 18mm, 20mm and 22mm.  These sizes are common in Asia, so faucets imported from China, Japan, Taiwan, etc. are likely to have these whole number metric sizes.

Aerator O-rings

O-rings are aptly named.  They are a ring, shaped like an O.  Surprise!


The blue ring on this faucet thread adapter is an O-ring.

If you are searching for an adapter for a faucet where the aerator has an O-ring, then your adapter must also have an O-ring or it won't fit and won't seal.

Rubber washers

Rubber washers play the same role as the O-ring, but in a different way.  For an aerator or a faucet thread adapter that uses a rubber washer, it must be placed between the faucet and the aerator or adapter.  The connection is sealed by screwing in the aerator or adapter sufficiently to compress the rubber gasket.


In the photo below, the black rings are rubber washers.

If you fail to include the rubber washer in your installation, a leak is a certainty.

Removing Aerators

The threads of aerators are the standard right-hand design.  If you could put your head in the sink and look up at the aerator, you would turn it counterclockwise to remove it.  But since that is a tad difficult, it is better to look down on the faucet from above and turn the aerator clockwise.

Metric versus inch size trivia

55/64" may be more logical that it appears!  55/64"-27 is functionally identical to 22mm x 1.  They are completely interchangeable.  Perhaps someone was thinking after all.

Somebody had to be different!

Moen, inexplicably, has a few faucets with metric threads that have 1.5mm spacing!  Why?  I have no idea.


I never found an adapter for this size thread so I had one madeSee it here.

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