UPSC prelims explained: GS Paper I, CSAT and the 66-mark rule
The Civil Services Preliminary Examination is the first stage of UPSC's selection for the IAS, IFS, IPS and other services. Many first-time candidates misunderstand how it is marked, and some fail because of it. This article explains the two papers, the qualifying rule for CSAT and what the negative marking means for your attempt. The facts are from the 2026 notice, as summarised on our UPSC CSE page.
Two papers, two very different jobs
| GS Paper I | GS Paper II (CSAT) | |
|---|---|---|
| Questions | 100 | 80 |
| Marks | 200 (2 per question) | 200 (2.5 per question) |
| Time | 2 hours | 2 hours |
| Penalty | One third of the question's marks | One third of the question's marks |
| What it does | Decides who is called for mains | Qualifying only: you need 33%, which is 66 marks |
Both papers are compulsory and set in Hindi and English. Prelims marks are not counted in the final merit; they only decide who sits the main examination.
The 66-mark rule
CSAT marks are not added to your prelims score. They only have to reach 66 out of 200. But if you fall short of 66, your GS Paper I is not considered at all, however good it is.
This catches two kinds of candidates:
- Strong GS candidates from a humanities background who neglect CSAT's comprehension, reasoning and basic numeracy, and fall just short.
- Candidates who over-attempt CSAT and lose marks to the one-third penalty, when a careful 30 correct answers out of 35 attempts would have been enough.
The safe approach: sit a few CSAT mocks early in your preparation. If you clear 66 comfortably, practise CSAT once a week to stay safe and put your time into GS. If you do not, treat CSAT as a subject until you do.
The penalty in GS Paper I
A right answer earns 2 marks; a wrong one costs one third of that, which is 0.67. With four options, the break-even chance is 25%: a blind guess gains nothing on average. Once you can rule out one option, your chance is 33% and a guess gains on average; with two options left, 50%, it gains a lot.
That is why experienced candidates attempt more questions in GS Paper I than a fear of negative marking suggests. They do not guess blindly, but they do attempt questions where they can eliminate options with real knowledge. Our negative marking guide works through the arithmetic for UPSC and other exams.
How many questions to attempt depends on how often your eliminations are right, and that is something only your own mocks can show. Mark the questions you answered with partial certainty in each mock, and check how many of them were right.
What follows prelims
The main examination has nine written papers, of which seven count for merit (Essay, four General Studies papers and two papers of one optional subject, 250 marks each), plus a personality test of 275 marks: 2,025 in all. Two language papers are qualifying. Since prelims marks do not carry over, the aim in prelims is to clear the cut-off safely, not to top it.
A note for UPSC IFoS candidates
The Indian Forest Service uses the same preliminary examination as screening. Our UPSC IFoS page has the details.
Practising for prelims
- Sit full GS Paper I mocks under two-hour timing, and track your accuracy on questions you attempted with partial certainty.
- Sit CSAT mocks until 66 is comfortable, then keep one a week.
- Analyse every mock by topic; our guide to analysing a mock test has a method.
Mockrooms has timed UPSC CSE prelims mocks for both papers, with the one-third penalty applied and CSAT scored as qualifying.