Type any number to see how many significant figures it has, with the trailing-zero and decimal-point rules applied, then round it to the number of sig figs you need.
Type the number exactly as written — trailing zeros and the decimal point matter. For example “120” has 2 significant figures, but “120.” has 3 and “120.0” has 4.
Rules used: all non-zero digits count; zeros between significant digits count; leading zeros don’t; trailing zeros count only when a decimal point is present.
How to Count Significant Figures in a Number
Start at the first non-zero digit and count every digit from there, including zeros sitting between other digits. Trailing zeros count only when the number has a decimal point. So 0.004520 has 4 significant figures, while 120 has 2 and “120.” has 3.
- Skip the leading zeros in 0.004520; counting starts at the 4.
- Count the non-zero digits 4, 5 and 2.
- Add the trailing 0, since there is a decimal point: 4 in total.
| Number | Significant figures | Why |
|---|---|---|
| 0.00450 | 3 | Trailing zero after the decimal counts |
| 100 | 1 | No decimal, so trailing zeros are skipped |
| 100.0 | 4 | Decimal point, so all zeros count |
| 1002 | 4 | Zeros between digits count |
Formula: All non-zero digits are significant. Zeros sandwiched between significant digits are significant. Leading zeros (before the first non-zero digit) are never significant. Trailing zeros only count as significant when the number contains a decimal point. To round to N sig figs, the calculator keeps the first N significant digits (per these same rules) and rounds the next digit up or down accordingly, adjusting the decimal position or exponent as needed.
- Number: the value to analyze, typed exactly as written (trailing zeros and the decimal point both matter)
- Round to (optional): the number of significant figures to round the value to
How to Round to 3 Significant Figures (and Other Counts)
Keep the first N significant digits and round using the digit right after them, so 0.004520 to 3 gives 0.00452.
| Number | Round To | Result |
|---|---|---|
| 0.004520 | 3 | 4 sig figs; rounds to 0.00452 |
| 120 | 1 | 2 sig figs; rounds to 100 |
| 120. | 2 | 3 sig figs; rounds to 120 |
| 1002 | 2 | 4 sig figs; rounds to 1000 |
| 3.1400 | 3 | 5 sig figs; rounds to 3.14 |
| 0.0700 | 1 | 3 sig figs; rounds to 0.07 |
Sig Figs in Lab Work
Record every digit your instrument shows, plus one estimated digit on an analogue scale, then type the number exactly as recorded.
- Enter numbers as recorded. Typing 2.50 instead of 2.5 tells the calculator the zero was measured, which changes the count from two to three.
- Watch ambiguous whole numbers. Are trailing zeros significant without a decimal? Not by these rules, so 1,500 reads as two significant figures even if you meant three or four; writing it as 1.50 × 103 makes your intent clear.
- Exact numbers, such as counted items or defined conversions, have unlimited significant figures and never limit your answer.
The significant figures rules for multiplication and division are simple: round the answer to the fewest significant figures among the inputs. Addition and subtraction follow a decimal-places rule instead, so a sig fig count is not the whole story there. Keep extra digits through intermediate steps and round only the final result. The significant figures article on Wikipedia summarises these conventions, though your course or lab manual has the final say.
Common questions about significant figures
How do you count significant figures in a number?
Start at the first non-zero digit and count every digit after it, including zeros between digits. Trailing zeros count only if the number has a decimal point, so 0.004520 has 4 significant figures and 120 has 2.Are trailing zeros significant without a decimal?
No. In a whole number like 120 or 1,500, trailing zeros with no decimal point are treated as placeholders and not counted. Writing “120.” or 1.50 × 103 shows the zeros were measured.How many significant figures are in 0.00450?
It has 3. The three leading zeros only place the decimal point, while 4, 5 and the trailing zero after the decimal all count.Do leading zeros ever count as significant figures?
No. Zeros before the first non-zero digit, like the two zeros in 0.0045, never count. They set the size of the number, not its precision.How do you round to 3 significant figures?
Find the third significant digit and look at the digit after it. If that digit is 5 or more, round up; otherwise leave it. For example, 0.004520 becomes 0.00452 and 3.1400 becomes 3.14.Why do decimal places differ from significant figures?
Decimal places count only the digits after the decimal point, while significant figures count every precise digit, including those before it. 1002 has zero decimal places but 4 significant figures.Related Calculators
Once you’ve counted your sig figs, the rounding calculator covers other rounding rules like decimal places or nearest multiples, while the scientific notation converter is useful for expressing very large or small precise values compactly. For general arithmetic on the numbers involved, the scientific calculator is a good companion tool.