Tipping in Sweden isn't as established as in many other countries. It's always welcome but not expected.
When eating out in Sweden it is common to to round up to the nearest big number. For example if your bill comes to 280 SEK, you can round up to 300 SEK. Sometimes a service charge is included in the bill, but this will be clearly stated on the bill.
Giving a couple of 'kronor' extra is a nice way of saying thanks for friendly service. However, even if you choose not to leave a tip, it’s highly unlikely anyone would be offended.
Have you decided to tip but are unsure of the amount?
A nice tip (and one that is pretty easy to calculate) is to add 10% of the final bill.
See Also
Cost of Living in Sweden