How can I measure at home with an accuracy of a few millimeters? 
It sounds safer to me to start with measurements from the manufacturer, am I wrong? 
Meh, electronic calipers are also cheap from AliExpress. It’s an important tool.
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Ok. I have learn some good stuff here today… thanks! 
There’s also no information on them which pin has which function (and looking up PJ-324M in google leads to various different models).
E.g. these 5 pin versions should work, and they also have a pinout and measurements given:
Thanks! I think I will buy this model. Just what is the role of pins 3/4? Used for a microphone?
Clear plastic cases are a bonus. You can see which pins go to which connectors.
@ureichen
Generally, most cheap ancillary components such as relays, sockets, connectors are still built with pins on a 1/10 inch (100mil) or 1/20 inch (50mil) grid. A small steel ruler measuring in imperial is often all that is needed to make a footprint.
Not in this model. The extra pins are used as switches. If you connect the headphones it will disconnect the signal to the other pin.
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Or just measure them with a multimeter and a headphone jack when you get them. Or plug in a headphone and using the ohms range, see which side makes a click when you test for continuity.
Another thing with these cheap parts is you may not always get what is depicted in the photo. It may be slightly off. That’s why it’s best to have the actual part in front of you.
Obviously this is terrible supply chain practice but that’s what you have to do at the bottom end of the market.
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No you can use them to detect if a plug is inserted. Or just ignore them. (These detection pins also have the numbers 4 and 5 on their schematic, not 3 and 4).
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I have used this model with no problems on headphone amp projects 
Andy
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They seem to have different numbers in the text than in the figure.
Text 1,2,3,4,5 corresponds to figure 1,2,4,5,3.
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