What is the negative marking in UPSC Prelims?

One-third of the marks of the question: −0.66 for a wrong answer in GS Paper I (2 marks a question) and −0.83 in CSAT (2.5 marks a question). Blank answers carry no penalty.

Is CSAT counted for the Prelims cut-off?

No. CSAT (Paper II) is qualifying: you need at least 33%, that is 66 out of 200. The cut-off is applied on GS Paper I alone.

Should I guess in UPSC Prelims?

A blind guess among four options earns almost nothing on average. Once you can rule out one option with confidence, attempting earns about +0.23 marks on average, and +0.67 when only two options remain.

The UPSC Prelims guide: negative marking, CSAT and how to attempt

Paper I (General Studies): 100 questions, 200 marks, 2 hours, +2 / −0.66. It alone decides the cut-off.

Paper II (CSAT): 80 questions, 200 marks, 2 hours, +2.5 / −0.83. Qualifying at 33% (66 marks).

A blind guess with four options is worth about +0.005 marks; rule out one option and it rises to about +0.23.