Significant Figures Calculator
Count the significant figures in a number, round to any number of sig figs, or do arithmetic with measurements and get a correctly rounded answer.
Why significant figures exist
A measurement is only as good as the instrument. If a ruler reads to the nearest millimetre, writing 12.347 cm claims a precision you don’t have. Significant figures are a lightweight way to carry that honesty through calculations: the answer can’t be more precise than the least precise input.
In statistics the same idea applies when reporting results. A mean of 72.41866 from test scores recorded as whole numbers is usually reported as 72.4. Our other calculators show extra digits so you can round at the end, as you should. Pair this with the standard deviation calculator when reporting lab results as mean ± SD.
Frequently asked questions
What are the rules for significant figures?
1) All non-zero digits are significant. 2) Zeros between non-zero digits are significant (1002 has four). 3) Leading zeros are never significant (0.0045 has two). 4) Trailing zeros are significant if there is a decimal point (2.50 has three). 5) Trailing zeros in a whole number without a decimal point are ambiguous (1200 could have two, three or four); write it in scientific notation to be clear.
How many significant figures does 100 have?
Strictly, it is ambiguous. Most textbooks count one, because trailing zeros with no decimal point are treated as placeholders. Write 100. (with a decimal point) for three, or use scientific notation: 1.00 × 10² has three.
What is the rule for multiplying and dividing?
The answer has as many significant figures as the measurement with the fewest. 12.11 × 3.2 = 38.752, but 3.2 has only two significant figures, so the answer is 39.
What is the rule for adding and subtracting?
The answer is rounded to the least precise decimal place among the numbers, regardless of how many significant figures they have. 12.11 + 3.2 = 15.31, rounded to the tenths place: 15.3.
Do exact numbers limit significant figures?
No. Counted quantities (12 eggs) and defined conversions (1 inch = 2.54 cm exactly) have unlimited significant figures and never limit the precision of an answer.
Should I round at each step of a multi-step calculation?
No. Keep extra digits through intermediate steps and round only the final answer, otherwise rounding errors accumulate.