Significant Figures Calculator

Type a number to see which digits count and why, round to any number of sig figs, and do arithmetic that follows the lab-report rules.

Count significant figures

0.004500

■ significant   0 not significant   0 ambiguous

Trailing zeros after a decimal point are significant: they record measured precision.

SF
Significant figures
4
×10
In scientific notation
4.500 × 10⁻³

Round to significant figures

≈
2 s.f.
0.0045

Sig fig arithmetic

=
Correctly rounded answer
39
Unrounded: 38.812

Multiplication/division: keep the fewest significant figures (4 vs 2 → 2 s.f.).

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0/1500

Why significant figures exist

A measurement is only as good as the instrument. A balance that reads to 0.01 g gives 12.52 g, and claiming 12.5200 g would invent precision you never had. Significant figures are a quick way to carry that honesty through a calculation: the answer cannot be more precise than the least precise measurement that went into it.

Frequently asked questions

What are the rules for significant figures?
1) Non-zero digits are significant. 2) Zeros between non-zero digits are significant (1002 has 4). 3) Leading zeros are never significant (0.0045 has 2). 4) Trailing zeros are significant if there is a decimal point (4.500 has 4). 5) Trailing zeros in a whole number without a decimal point are ambiguous (1200 has 2, 3 or 4); write it in scientific notation to be clear.
How many sig figs does 100 have?
Strictly it is ambiguous: 1, 2 or 3. Most courses count it as 1. Write 100. (with a decimal point) for 3, or 1.0 × 10² for 2.
How do sig figs work in multiplication and division?
The answer keeps as many significant figures as the input with the fewest. 12.52 × 3.1 = 38.812, rounded to 2 s.f. = 39.
How do sig figs work in addition and subtraction?
Count decimal places, not significant figures. The answer is rounded to the fewest decimal places among the inputs. 12.52 + 3.1 = 15.62 → 15.6.
Do exact numbers limit significant figures?
No. Counted quantities (3 beakers), defined conversions (1 in = 2.54 cm exactly, 1000 mL = 1 L) and coefficients in a balanced equation have unlimited significant figures.
Should I round in the middle of a calculation?
No. Keep all digits (or at least two extra) through intermediate steps and round only the final answer, or rounding errors accumulate.