Significant figures and calculations
Learn the rules for determining significant figures and applying them correctly in mathematical operations like multiplication and division.
Why this shows up in the exam
NEET checks your precision in reporting answers and handling significant figures in calculations.
How NEET tests this
Learn the idea
Significant figures indicate the reliable digits of a measured quantity; in any calculation the final answer must be reported with as many significant figures as the least‑precise quantity used.
🧠 Memory hook: SIG = Significant = 'S' for Significant, 'I' for Important, 'G' for Good digits – remember: non‑zero, sandwiched zeros, and trailing zeros only when a decimal is shown.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- All non‑zero digits are always significant
- Zeros between non‑zero digits are significant
- Leading zeros are never significant
- Trailing zeros are significant only if a decimal point is present
- In multiplication or division the result must have the same number of significant figures as the factor with the fewest significant figures
- When rounding, if the first discarded digit is 5 or more round up, otherwise keep the previous digit
How to approach it
- 1Count the significant figures in each given number
- 2Carry out the multiplication or division using the full numbers
- 3Identify the smallest significant‑figure count among the numbers used
- 4Round the obtained result to that many significant figures using standard rounding
Worked example — watch it click
The area of a rectangular field (in m²) of length 55.3 m and breadth 25 m after rounding off the value for correct significant digits is:
- A)138 × 10¹
- B)1382
- C)1382.5
- ✅14 × 10²
The concept behind this problem
The worked example requires you to count the sig‑figs of length (3) and breadth (2), multiply to get 1382.5, and then round to the lowest count (2) giving 1.4×10³ m² (written as 14×10²).
Step by step
- 1Area = 55.3 × 25 = 1382.5 m².
- 2The least precise measurement is 25 m (2 significant figures).
- 3Rounding to 2 significant figures: 1400 m² = 14 × 10² m².
Watch out
Students often round 1382.5 to 1380 or 1.38×10³, forgetting that only two significant figures are allowed, so the correct rounded value is 1.4×10³.
Common slip-ups that cost marks
- •Treating 25 as having one significant figure because it lacks a decimal point – it actually has two
- •Rounding intermediate results instead of the final answer, which changes the sig‑fig count
- •Mixing up significant figures with decimal places; they are not the same thing
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Practise it
These are real questions from past NEET papers that test this exact idea.
The area of a rectangular field (in m²) of length 55.3 m and breadth 25 m after rounding off the value for correct significant digits is:
Push further
More challenging5 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.
A rectangular plate has a measured length of 2.5 cm and a width of 1.25 cm. What is its area, expressed with the correct number of significant figures?
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