A precis should be one-third of the original passage's word count, with a small tolerance of five to ten words either way. This rule applies equally to CSS and PMS; neither exam changes the ratio, only the length of the passage given to you on the day.
This is one of the first rules you will learn as a precis writer, and it is also one of the easiest to get comfortably wrong. Let us walk through it properly, so that on exam day, counting words is the last thing you worry about.
How the one-third rule actually works
Take the word count of the original passage and divide it by three. That number is your target.
For example, if the examiner gives you a passage of 600 words, your precis should land close to 200 words. If the passage is 270 words, your target is around 90 words. You are not expected to hit this number to the last digit; examiners allow a buffer of five to ten words above or below your target. So for a 200-word target, anywhere between 190 and 210 words is considered acceptable.
Think of the one-third rule the way you would think of packing for a short trip. You are not leaving anything essential behind, but you are also not carrying your entire wardrobe. You are choosing only what matters, and expressing it in far fewer words than the original.
What happens if you go over or under this limit
Going noticeably over the word limit tells the examiner that you were unable to separate the essential from the non-essential, which is the very skill precis writing is designed to test. Going too far under the limit is equally risky, since it often means you have left out an important supporting idea along with the padding.
Sir Syed Kazim Ali repeatedly reminds his students that length control is a matter of selection. Suppose a 300-word passage discusses the causes, effects, and possible remedies of rural poverty. Your target is roughly 100 words. A student who writes 140 words has almost certainly retained the author's examples and illustrations, which should have been removed. A student who writes 70 words has probably kept the causes and effects but quietly dropped the remedies, leaving the precis incomplete. Both lose marks, though for opposite reasons.
Does the word limit differ between CSS and PMS?
No. The one-third rule, along with the five-to-ten-word tolerance, is the same for both CSS and PMS, and for every provincial PMS commission: Punjab, KPK, Sindh, Balochistan, and the rest. What changes between exams is the length of the passage itself, not the ratio you apply to it. A PMS KPK precis passage may be shorter than a CSS passage in a given year, but the calculation you perform is identical.
This is good news for you as an aspirant preparing for both exams together: the skill you build for one transfers directly to the other. You are not learning two separate rules; you are learning one rule and applying it to passages of different lengths.