Significant Figures: The Rules for Zeros, Rounding & Calculations (MYP Science)
Significant figures tell the reader how precise a measurement really is. Writing 12.0 instead of 12 is a claim that you measured to a tenth; writing more digits than your equipment can justify claims accuracy you do not have. Marks in Criterion C are regularly lost by reporting results to the wrong number of figures.
Try it yourself: apply each rule in a series of worlds and see how many significant figures you can count correctly.
Which digits count?
- •All non-zero digits count: 4.62 has 3 significant figures.
- •Zeros between non-zero digits count ("sandwiched"): 5008 has 4.
- •Leading zeros never count — they only place the decimal point: 0.0045 has 2.
- •Trailing zeros count only if there is a decimal point: 1.500 has 4, and 2.30 has 3.
The tricky case: trailing zeros without a decimal point
A number like 1500 is ambiguous — it might have 2, 3 or 4 significant figures. Scientific notation removes the doubt: 1.5 × 10³ has 2, while 1.500 × 10³ has 4. When you report a measurement, write it so the number of figures reflects how precisely you measured.
Rounding in calculations
When multiplying or dividing, give the answer to the same number of significant figures as the least precise value used. When adding or subtracting, match the fewest decimal places instead. Round only at the end of a calculation, not in the middle, or small errors build up.
Why examiners care
Reporting results to a sensible precision shows you understand measurement. A stopwatch that reads to 0.01 s does not justify an answer of 3.14159 s. In Criterion C, consistent, justified precision across a data table is a mark of careful processing.
Frequently asked questions
Three. The leading zeros do not count, but the 4, 5 and the trailing 0 (after the decimal point) do.
It is ambiguous without more information — at least 2. Write 1.5 × 10³ for 2, or 1.500 × 10³ for 4.
Round to the same number of significant figures as the least precise number in the calculation.