4 significant figures
0004050
Underlined digits are significant. Leading zeros never count; zeros between non-zero digits always count; trailing zeros count only when there is a decimal point.
How it works
Counting follows the standard rules: non-zero digits always count; zeros between them count; leading zeros never count; trailing zeros count only when the number has a decimal point. A whole number such as 1200 is ambiguous — it may have 2, 3 or 4 significant figures — so the calculator flags it and suggests scientific notation.
For arithmetic, addition and subtraction keep the fewest decimal places, while multiplication and division keep the fewest significant figures. In multi-step calculations, keep extra digits in the intermediate steps and round only the final answer.
Frequently asked questions
- How many significant figures does 0.00450 have?
- Three. The leading zeros only locate the decimal point; the 4, the 5 and the trailing zero (after the decimal point) are significant.
- Why is 1200 ambiguous?
- Without a decimal point, you can't tell whether the trailing zeros were measured or are just placeholders. Write 1.2 × 10³ (2 s.f.), 1.20 × 10³ (3 s.f.) or 1.200 × 10³ (4 s.f.) to be clear.
- Do exact numbers limit significant figures?
- No. Counted numbers (3 test tubes) and defined constants (100 cm in 1 m) are exact and have unlimited significant figures.
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