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What thread is my kitchen tap aerator, M22 or M24?

Most kitchen tap aerators are either a male M22 x 1 or a female M24 x 1, and the two are easy to confuse until you know where to look.

The two standard sizes

The aerator is the small threaded insert at the tap spout that mixes air into the stream. The two standard threads are a male M22 x 1, sometimes quoted as 15/16 inch-27, and a female M24 x 1, sometimes quoted as 55/64 inch-27. Male means the threads are on the outside of the aerator; female means they are on the inside.

Both use a 1mm pitch, so the difference between them is the diameter and, crucially, whether the thread is external or internal.

Male or female first

Before you measure, decide which type you have. If the aerator has threads on its outer surface that screw up into the spout, it is male. If the aerator has threads inside a recess and the spout has an external thread, it is female. This one distinction rules out half the parts immediately.

A male M22 aerator pairs with a female tap seat, and a female M24 aerator pairs with a male tap spout, so the tap and the aerator are always opposite genders.

Measuring to confirm

Measure the thread diameter with a caliper. A male aerator reads close to 22mm across the outside of its thread for the M22 size. A female aerator reads close to 24mm across the inside for the M24 size. Watch for the less common M18 and M20 aerators, which turn up on smaller and some bathroom taps.

Because the sizes are close, measuring is more reliable than eyeballing, and it saves buying the wrong adapter.

Adapting for a hose or filter

To connect a hose, a filter or a spray to an aerator thread, buy an adapter that names the aerator size and gender on one side and the hose fitting on the other. Keep the washer that seals the aerator, because like other hose-style threads it seals on the face rather than on the thread.

Related

Sources: Neoperl aerator thread reference