How to use it
- Type the number in Number. Use a point for the decimal mark, or a letter e for scientific notation.
- Read Significant figures of the number, then Rounded to the chosen figures. Change Round to this many significant figures to see other roundings.
- For a calculation, choose the Calculation with a second number and type the Second number.
- Go through How it is worked out to see which rule applies to each step.
Counting the significant figures
Every nonzero digit is significant. Zeros between nonzero digits are significant. Leading zeros are not: in 0.00450 the first three zeros only show the position of the decimal point, so the number has 3 significant figures, 4, 5 and 0.
Trailing zeros are significant only when there is a decimal point: 1200.50 has 6 figures, while 1200 has 2. In scientific notation every digit of the mantissa counts, so 1.20 × 10³ has 3 figures.
Rounding and calculations
Rounding to n significant figures keeps the first n digits from the first nonzero digit. A digit 5 or more rounds the last kept digit up, so 1200.50 to 3 figures is 1.20 × 10³.
A product or quotient has as many significant figures as the input with the fewest. A sum or difference has as many decimal places as the input with the fewest decimal places. The result is rounded to that precision.
x × y or x ÷ y: round to the fewer significant figures of x and y
x + y or x − y: round to the fewer decimal places of x and yWhat the calculator does not cover
- Numbers with more than 15 significant figures in the rounding, and numbers written with words or fractions.
- Uncertainty analysis, error propagation with standard deviations, and rounding rules other than half up.
- More than two numbers in one calculation: combine them one pair at a time.
Worked example: 1200.50 × 3.0
1200.50 has 6 significant figures and 3.0 has 2, so the product keeps 2. The exact product is 3601.5, which rounds to 3600, written as 3.6 × 10³ to show the 2 figures.
The sum 1200.50 + 3.0 = 1203.5 is different: 3.0 has one decimal place, so the sum is rounded to one decimal place.
| Result | |
|---|---|
| Significant figures of 1200.50 | 6 |
| 1200.50 rounded to 3 figures | 1.20 × 10³ |
| 1200.50 × 3.0 | 3.6 × 10³ |
| 1200.50 + 3.0 | 1203.5 |
Frequently asked questions
Why does 1200 have only two significant figures but 1200.50 has six?
Without a decimal point, the trailing zeros of 1200 are placeholders and are not counted. With a decimal point, as in 1200.50, every written digit is taken to be measured, including the zeros at the end.
How do I write a number with significant figures in scientific notation?
Write the mantissa with one digit before the point and the power of ten after it, such as 1.20e3 for 1.20 × 10³. Every digit of the mantissa is counted, so 1.20e3 has 3 significant figures.
Why is the answer rounded to the fewest figures?
A calculation cannot be more precise than its least precise input. Keeping the extra figures would suggest a precision that the measured numbers do not have.
What is the difference between significant figures and decimal places?
Decimal places count the digits after the point. Significant figures count the digits from the first nonzero digit. Products and quotients follow the figures rule, and sums and differences follow the decimal places rule.
What does the calculator do with zero?
Zero is counted as one significant figure, by convention, and the tool says so in the steps. Other conventions for written zeros exist, so check the rule your course or lab uses.
Are the numbers I type sent to a server?
No. The counting and rounding run in your browser, and nothing you type is sent to us.
Sources
Figures and text last checked on Tuesday, November 10, 2026.