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A correct method can still lose marks if the final line reads "1.68804780876 g cm⁻³" or "25 m" when the question needed metres per second. This post gathers the rules from NCERT and from NIST's guide to writing SI units, then lists the slips to check for.
Why units matter more than they seem
In 1999 NASA lost the Mars Climate Orbiter. According to NASA's mission page, ground software used English units while the spacecraft's software worked in metric. The mismatch threw off the trajectory calculations and sent the spacecraft too close to Mars. Exam stakes are smaller, but the error has the same shape.
Counting significant figures: the NCERT rules
NCERT's Units and Measurement chapter defines significant figures as the reliable digits plus the first uncertain digit. Its rules:
All non-zero digits are significant.
Zeros between two non-zero digits are significant.
For a number less than 1, zeros after the decimal point but before the first non-zero digit are not significant. 0.002308 has four significant figures.
Trailing zeros in a number without a decimal point are not significant. 123 m written as 12300 cm still has three.
Trailing zeros in a number with a decimal point are significant. 3.500 and 0.06900 both have four.
Changing units does not change the count. 2.308 cm = 0.02308 m = 23.08 mm, four each.
Exact numbers, such as the 2 in s = 2πr, have an unlimited number of significant figures.
To remove any doubt about trailing zeros, NCERT recommends scientific notation: 4.700 m = 4.700 × 10³ mm, which clearly shows four significant figures.
Rules for calculations
NCERT gives two rules, and they are easy to mix up.
Multiplying or dividing: keep as many significant figures as the least precise value. NCERT's example: a mass of 4.237 g and volume of 2.51 cm³ give a density of 1.69 g cm⁻³ (three significant figures), not 1.68804780876.
Adding or subtracting: keep as many decimal places as the value with the fewest decimal places. 436.32 g + 227.2 g + 0.301 g = 663.8 g, not 663.821 g and not 664 g.
For rounding, NCERT gives this convention: when the digit you drop is exactly 5, leave the previous digit if it is even and raise it by 1 if it is odd. So 2.745 becomes 2.74 and 2.735 also becomes 2.74. In multi-step working, keep one extra digit in intermediate steps and round only at the end.
You can read the rule in the JEE (Main) 2026 information bulletin. Rounding too early in a long calculation can push your answer to the wrong integer, so carry extra digits until the last line.
Writing units correctly
NIST's guide on writing SI units sets out conventions that keep your answers unambiguous:
Put a space between the number and the unit: 7 m, 31.4 kg.
Unit symbols are never pluralised: 250 mm, not 250 mms.
Symbols are lowercase unless the unit is named after a person: m, s, but W (watt) and Pa (pascal). The litre (L) is an exception.
Prefixes for a million or more are capitals and those below are lowercase: M for mega, m for milli. So mW and MW differ by a factor of 10⁹.
No space between prefix and unit: km, mg.
No full stop after a symbol except at the end of a sentence.
Write a leading zero for numbers below one: 0.038 g, not .038 g.
A five-point check before you box an answer
Is there a unit? Every physical answer needs one, written in symbols.
Is it the unit the question asked for? cm or m, g or kg, kJ or J.
Did you convert before substituting? Mixing cm and m inside one formula is an easy slip to make.
Is the precision right? Match the least precise data for products and quotients, and the fewest decimal places for sums.
Is it the right size? A quick estimate catches powers of ten.
Common slips, with fixes
Writing 1.68804780876 because the calculator showed it. Round to the data's precision.
Using the multiplication rule for a sum, or the reverse. Ask: am I adding or multiplying?
Writing 5 m s for 5 m/s, or mixing up ms (milliseconds) and m s.
Rounding in step two of a five-step problem, then getting the wrong integer at the end.
If a unit mismatch keeps creeping into the same kind of problem, compare your working with a complete solution. The Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer, so you can see the line where the units went wrong.
For a related check, read dimensional analysis: a 30-second check for physics answers. For chemistry, see mole concept and stoichiometry: the errors that cost marks.
Take your last practice test and circle every answer without a unit. That count is your starting point.
