Which thread do I have? Identifying 1/2", 3/8", 1/4" or M22
The connector for your reverse osmosis system is unpacked, but it won't screw on. Yet the unit said ‘3/8"’, didn't it? With threads, the inch size tells you little about what you measure with a ruler. Here you can work out, for four typical situations, which thread you have and which adapter fits.
First: thread inches are not tube inches
For PE tubing, the inch size is the outside diameter: 1/4" is 6.35 mm, 3/8" is 9.5 mm. For pipe threads, however, it is just a name that goes back to the former nominal bore of the pipe. That is why a 1/4" thread measures around 13 mm on the outside, and a 1/2" thread is not 12.7 mm but around 21 mm.
That is why our push-fit connectors state both in their name, for example ‘1/4" × 3/8" female’: 1/4" tube on one side, 3/8" female thread on the other. How to measure the tube and what female and male thread mean is explained in our guide to tube sizes.
Left: male thread, right: female thread
Measurement table: diameter → thread
Measure with a calliper. For a male thread, measure on the outside across the thread crests; for a female thread, measure the opening on the inside at the thread crests.
Measuring a male thread: place the calliper across the thread crests
| Thread | Male, outside, approx. | Female, inside, approx. | Typically found on |
|---|---|---|---|
| 1/8" | 9.7 mm | 8.6 mm | small reverse osmosis components |
| 1/4" | 13.2 mm | 11.4 mm | filter housings, inline filters |
| 3/8" | 16.7 mm | 15.0 mm | filter housings, valves |
| 1/2" | 21.0 mm | 18.6 mm | shower and bath mixers, shower hose |
| 3/4" | 26.4 mm | 24.1 mm | washing machine tap |
| 7/16"-24 | 11.1 mm | 10.0 mm | reverse osmosis taps, some appliances from the USA |
| M22 × 1 | 22.0 mm | 20.9 mm | aerator (thread inside the spout) |
| M24 × 1 | 24.0 mm | 22.9 mm | aerator (thread on the outside of the spout) |
These are nominal dimensions. On a male thread you will usually measure a few tenths less, on a female thread rather a little more – what matters is which row you are closest to.
1/2" or M22?
The two are only around 1 mm apart. Hold a ruler against the thread and count the grooves over 1 cm: the fine M22 × 1 has about 10, a 1/2" thread only about 5 to 6.
Situation 1: shower filter or diverter valve on the mixer
Shower and bath mixers usually have a 1/2" male thread (approx. 21 mm) for the shower hose. Unscrew the hose: its union nut has the thread on the inside, the mixer has it on the outside.
Matching products: the anti-limescale shower filters DF-007 and DF-008 have a 1/2" female thread at the top for the mixer and a 1/2" male thread at the bottom for the hose; washers are included. The 3-way diverter valve has one inlet with a 1/2" female thread and two outlets with 1/2" male threads.
Here the seal is made by the flat rubber washer in the nut – hand-tight, without PTFE tape. Before you start, measure the distance from the centre of the mixer outlet to the wall: DF-007 and DF-008 need at least approx. 4 cm, the slimmer DF-003 approx. 3 cm. If space is even tighter, there is a solution using the 45° elbow 1/2" female × female and a 1/2" double nipple – it is best to discuss with us beforehand how to seal and align them: kontakt@smarald.com.
Situation 2: 1/4" tube to a thread on the filter or reverse osmosis system
This applies to reverse osmosis systems, filter housings and inline filters. First check whether the component has a female or a male thread – the connector always needs the counterpart. Then measure and look it up in the table.
- Component has 3/8" female: Elbow push-fit connector 3/8" male × 1/4" tube with O-ring
- Component has 3/8" male: straight CP-019, version 1/4" × 3/8" female or as a 90° elbow CP-018, version 1/4" × 3/8" female
- Component has 1/4" female: straight 1/4" male × 1/4" with O-ring, as an elbow CP-002-A+O or as a set of 2 TWZ-001 – for example for the inlet and outlet of an inline filter
- Component has 1/4" male: straight CP-019, version 1/4" × 1/4" female or as a 90° elbow CP-018, version 1/4" × 1/4" female
- Component has 7/16"-24 male: CP-019, version 1/4" × 7/16"-24 female
- Component has 1/2" male: Push-fit connector 1/4" tube × 1/2" female
Connectors with an O-ring seal via the O-ring; the straight female connectors CP-019 have a female thread seal. Wrap threads without a seal with PTFE tape – our guide to fitting push-fit connectors shows you how.
A word of caution with appliances from the USA: NPT threads are common there. They are tapered and have a different thread form from the pipe thread usual in Europe – but the diameters are close, which is why they are easily confused. The two do not seal reliably with each other. If the appliance manual says ‘NPT’, please ask us before ordering.
Situation 3: washing machine or dishwasher
The appliance tap usually has a 3/4" male thread (approx. 26 mm), and the inlet hose has a union nut with a female thread. The WF-001 descaling filter has 3/4" female and male threads and is screwed in between – with a washer, hand-tight. It is designed for cold water only (2–20 °C); if your appliance is connected to the hot water supply, it is not suitable. More on this in our guide to the descaling filter.
Situation 4: tap with aerator (M22/M24)
Unscrew the aerator at the front of the spout. If the thread is inside the spout, it is usually M22 × 1; if it is on the outside, usually M24 × 1. These are metric fine threads – they do not fit 1/2" or 3/8". We do not currently stock an adapter for M22 or M24.
For a filter or reverse osmosis system under the sink, you can branch off at the angle valve instead. To do so, first measure the outlet of the angle valve: most angle valves in Germany have 3/8" there (often with a compression ring for a 10 mm pipe), less often 1/2". If yours has a 1/2" outlet, the 1/2" tee connector with shut-off valve fits: it is screwed between the angle valve and the connection hose and has a branch for 1/4" or 3/8" tube. If your system has its own inlet valve, the 1/2" 3-way adapter without a shut-off valve is also sufficient. With a 3/8" outlet, please ask us first.
The right adapter in five steps
- Turn off the water, undo the connection and wipe the thread clean.
- Determine whether the thread is on the outside (male) or on the inside (female). You order the counterpart.
- Measure the diameter and look it up in the table. For the tube, on the other hand, its outside diameter is what counts.
- Check what provides the seal: flat rubber washer, O-ring or PTFE tape.
- Not sure? Send a photo with the calliper held against the thread to kontakt@smarald.com.
The most common mistakes
- Applying the tube size to the thread. 3/8" is 9.5 mm for tubing, but around 16.7 mm for a thread.
- Mixing up female and male. Before ordering, check which side the grooves are on – and order the counterpart.
- Forcing it. If the thread binds after one or two turns, it does not fit, for example metric on inch. Stop immediately – plastic threads strip easily.
- PTFE tape wrapped under the flat washer. On the shower and washing machine, the rubber washer provides the seal. Fit a new washer instead of using tape.
In short: First establish whether the thread is on the inside or the outside, then measure the diameter and compare it with the table. Shower and shower filter: 1/2". Washing machine: 3/4". Reverse osmosis and filters: usually 1/4" or 3/8" – and the tube has its own inch size. On taps with an aerator, M22 or M24 is standard.