If I buy a re-entry permit. Let's say I have 60 days left on my permission to stay. And I go to singapore 10 days. Do I have 50 or 60 days left in Thailand when I come back?

50

It doesn't buy you any time it only keeps your current permission to stay date active,any time out of the kingdom is lost.

Edited November 1, 201510 yr by stoneyboy

A re entry permit adds no time

Using your example you would have 50 days remaining.

Above is good advice. Be aware you can obtain reentry permit at airport on your way out. Simple process . You can download form, just allow little extra time.

Above is good advice. Be aware you can obtain reentry permit at airport on your way out. Simple process . You can download form, just allow little extra time.

The airport system is supposed not to need form or photos, but like everything in Asia it has been known to fail.

Ensure you have some recent passport size photos with you.

In addition to the above, the Re-Entry permit is good so that you don't have to start Fresh on your Extension of Stay, which usually means getting a Non-Immigrant Visa or get a Visa Conversion done in BKK, which add costs and time

Edited November 1, 201510 yr by MJCM