Calculator Skills
Rounding and Significant Figures
SPM often asks for an answer to a set number of decimal places or significant figures. The calculator helps, but the judgement is yours.
Round only at the end
Keep full precision through the working and round once, at the final step, to the accuracy the question asks. Rounding partway through and then again at the end can push the last digit off, costing an accuracy mark.
A worked keystroke sequence for rounding
- Compute 47 ÷ 9 by pressing 47, ÷, 9, = . The display reads 5.2222…
- To round to 2 decimal places, look at the third decimal digit. It is 2, which is below 5, so you round down: 5.22.
- To give 3 significant figures, count from the first non-zero digit: 5, 2, 2, the same 5.22 here.
- Now try 0.04628 to 3 significant figures. The leading zeros are not significant; the significant digits are 4, 6, 2, and the next digit 8 rounds the 2 up to 3, giving 0.0463.
Keep the calculator on full display
Some calculators can be set to a fixed number of decimals, which then rounds every result on screen. This is risky: if a later step needs the full value, the calculator has already thrown away the extra digits.
Keep the display on normal, full-precision mode, carry the full value forward with the answer key, and round by hand only when you write the final answer. The judgement about decimal places or significant figures is yours, not the machine's.
Source:SPM: Format Pentaksiran mulai 2021, Matematik (1449)
Frequently asked questions
What is the difference between decimal places and significant figures?
Decimal places count digits after the point, so 5.222 to 2 decimal places is 5.22. Significant figures count meaningful digits from the first non-zero one, so 0.04628 to 3 significant figures is 0.0463.
Read the question carefully, because the two can give different-looking answers for the same number.
When exactly should I round?
Round only when you write the final answer. Keep every intermediate value at full precision inside the calculator, because rounding a middle step and reusing it introduces error that grows through the working.
Round once, at the end, to the decimal places or significant figures the question asks for.
Is 5 rounded up or down when it is the deciding digit?
In the standard rule used for SPM, a deciding digit of 5 or more rounds up, and 4 or less rounds down. So 5.225 to 2 decimal places becomes 5.23, while 5.224 becomes 5.22.
Apply this to the single digit immediately after the place you are keeping, not to the whole tail.