There is already a question about whether tips are mandatory in Germany, but how much should I tip?
How much ?
In general, tipping is optional in Germany. While not tipping is not the typical case, there are people who don't tip at all. Thus it does not necessarily imply that something bad happened. It is different from the US, where you can expect the manager to ask you what kind of catastrophy the server has caused.
It is hard to give a universal percentage that is always applicable because you usually round up to the next 50 ct for low amounts and to the next euro for higher amounts. You would also probably avoid paying 89€ and round it to 90€.
I "crunched some numbers" and found that 5%-10% is a good approximation of a common tip percentage. Marked in green are values in columns that felt right. There are four important columns based on a 5%, 10%, 15% or "other" tip.
As you can see, sometimes a neat number will be prioritized. Or for 15,10€ you would take 10% and round down a little because it is rounded to 50 ct but is closer to 10%.
Obviously this is a subjective assessment but may be useful to foreigners to get a feel for what is the right amount. I found that e.g. people from the US tip too much. While no server will complain of course, you are "wasting" money and might come across as a show-off.
What to say?
If you don't say anything and just hand the server your money, they should give you the change to the cent. If they don't, this would be very rude and worth a complaint.
If you want to tip, you just say the desired amount while you are handing him the money, e.g. Achtzehn, bitte. So, simply the amount followed by a "Bitte". He will give you change for the amount specified by you.
If you have the desired amount handy, you just give him the 18 € and say "Stimmt so!" which informs the server that this is the correct amount you want to give and he does not have to give you anything back.
I have been living in Germany for 27+ years.
Generally speaking, 10% sounds good, often rounded to a "neat" value.
- If the bill is €17 or €17.50, one might pass an €20 note and say "Stimmt so." Literally translated it means "that's right as it is," indicating that one does not want any change.
- If the bill is €9.80, one might pass an €20 note and say "Auf elf." Literally "to eleven," this means one wants to pay €11 and get €9 back. (Possibly a northern idiom.)
- Passing money in a way that makes no sense if one expects change is an indication that one wants to tip. If the bill is €9.50 and one passes an €10 note plus an €2 coin, this indicates an €2.50 tip unless one says something like "Auf elf."
- Raising a hand as the waiter gets the change indicates that one wants no change. This can be a quite subtle gesture if the amount looks right for an included tip.