Significant Figures Calculator
Count the significant figures in any number and round to a specified number of sig figs. Explains which digits are significant and why, with scientific notation output.
How to use
- Enter a number in the input box.
- Set how many significant figures to round to.
- Click Calculate to see the sig fig count, the rounded value, and the scientific notation.
- The explanation shows which digits count and which don't.
Related tools:
Significant figures (sig figs) communicate the precision of a measurement. 4.5 cm has 2 sig figs - we know the value to the nearest 0.1 cm. 4.50 cm has 3 sig figs - we know it to 0.01 cm (the trailing zero is meaningful). 0.0045 has 2 sig figs - the leading zeros are just placeholders, not precision.
Rules for arithmetic with sig figs: for multiplication and division, the answer has as many sig figs as the input with the fewest. For addition and subtraction, the answer has as many decimal places as the input with the fewest decimal places. These rules prevent false precision from creeping into calculations.
Frequently Asked Questions
Are trailing zeros after the decimal point significant?
Yes. 3.400 has 4 sig figs. The trailing zeros indicate measured precision. But 3400 (no decimal point) is ambiguous - could be 2, 3, or 4 sig figs. Scientific notation resolves this: 3.4 x 10^3 clearly has 2 sig figs; 3.400 x 10^3 has 4.
Are leading zeros significant?
No. In 0.0042, the zeros before the 4 are just placeholders to set the decimal position. Only the 4 and 2 are significant - this number has 2 sig figs.
Why do sig figs matter in practice?
They prevent reporting more precision than your measurement actually has. If you measure a room as 4.2 m x 3.1 m, area = 13.02 m^2 by raw calculation. But since both measurements have 2 sig figs, the correct reported area is 13 m^2. Reporting 13.02 implies a precision you do not have.