Significant Figures Calculator
Round numbers to a set number of significant figures, or count how many significant figures a number has. Useful for science, engineering, and reporting measurements with the right precision.
Round to significant figures
Quick examples
Significant figures rules
- Non-zero digits are always significant.
- Zeros between non-zero digits are significant (e.g. 405 has 3).
- Leading zeros are not significant (e.g. 0.0045 has 2).
- Trailing zeros after a decimal point are significant (e.g. 4.50 has 3).
- Trailing zeros in a whole number with no decimal are not significant (e.g. 450 has 2; write 450. for 3).
Table of Contents
What are significant figures?
Significant figures (sig figs) are the digits in a number that carry meaningful information about its precision. They tell you how precisely a value is known. In science and engineering, you report measurements and calculated results using a number of significant figures that matches your uncertainty — not more (which overstates precision) and not less (which throws away useful information).
Unlike decimal places, which only count digits after the decimal point, significant figures count all meaningful digits from the first non-zero digit. For example, 0.00450 has three significant figures (4, 5, and the trailing 0), while 450 with no decimal point has two (4 and 5; the trailing zero is ambiguous unless you write 450.).
How it works
This calculator has two modes:
- Round to sig figs: Enter any number and choose how many significant figures (1–20) you want. The result is rounded using standard rules and formatted so the number of sig figs is clear, switching to scientific notation when needed for very large or very small values.
- Count sig figs: Enter the number exactly as written (e.g. 450, 450., or 0.00450). The tool counts how many significant figures it has and explains the rules that apply — including whether trailing zeros count (they do after a decimal; they don’t in a whole number without a decimal).
All calculations run in your browser as you type; no data is sent to a server.
Rules for counting sig figs
- Non-zero digits are always significant (e.g. 123 has 3).
- Zeros between non-zero digits are significant (405 has 3).
- Leading zeros are not significant (0.0045 has 2).
- Trailing zeros after a decimal are significant (4.50 has 3).
- Trailing zeros in a whole number with no decimal are not significant (450 has 2; write 450. if you mean 3).
When in doubt, enter the number as you would write it in a report. The count mode uses the exact string you type to decide how many sig figs to report.
Common use cases
- Lab reports: Round final values to the number of significant figures that matches your instrument’s precision.
- Homework and exams: Quickly round or count sig figs without doing it by hand.
- Engineering and science: Keep units and precision consistent when documenting or sharing numbers.
- Multiplication and division: After a calculation, round the result to the same number of sig figs as the input with the fewest.
- Addition and subtraction: Round the result to the least precise decimal place among the inputs; the count tool helps you see how many digits are meaningful.
Examples
Example 1: Round to 3 significant figures
Question: Round 123.456 to 3 significant figures.
Answer: 123. The first three significant digits are 1, 2, and 3; the 4 is the first digit dropped and is less than 5, so we round down.
Example 2: Round a small number
Question: Round 0.00456 to 2 significant figures.
Answer: 0.0046. The two significant digits are 4 and 6 (the 5 rounds up the 4 to 5, then we keep one more digit for the second sig fig — or equivalently 4.6×10⁻³).
Example 3: Count — 450 vs 450.
Question: How many significant figures does 450 have? What about 450.?
Answer: 450 (no decimal) has 2 significant figures; the trailing zero is not counted. 450. (with a decimal) has 3 significant figures; the decimal indicates that the trailing zero is significant.
Example 4: Count — 0.00450
Question: How many significant figures does 0.00450 have?
Answer: 3. The leading zeros are not significant. The first significant digit is 4, then 5, then the trailing zero after the decimal — so 4, 5, and 0 are the three significant figures.
Frequently asked questions
Decimal places count only the digits after the decimal point (e.g. 123.456 has 3 decimal places). Significant figures count all meaningful digits in the number from the first non-zero digit (e.g. 123.456 has 6 significant figures). Sig figs are preferred in science and engineering because they reflect the precision of a measurement regardless of scale — 0.00450 and 450. both have 3 sig figs.
Without a decimal point, trailing zeros are ambiguous: 450 could mean “about 450” (2 sig figs) or “exactly 450” (3 sig figs). By convention, we assume trailing zeros in a whole number are not significant unless a decimal point is shown. Writing 450. (or 450.0) indicates that the zero(s) are measured and significant.
When rounding to significant figures, the result is formatted with JavaScript’s toPrecision(), which switches to scientific notation for very large or very small numbers so that the correct number of significant digits is obvious. For example, 9,876,543 rounded to 3 sig figs may appear as 9.88e+6 so it’s clear there are three significant figures.
For multiplication and division, the result should have the same number of significant figures as the factor with the fewest. For example, 2.5 × 3.14 = 7.85; 2.5 has 2 sig figs, so round the result to 7.9. Use this calculator to round the final answer to that number of sig figs.
Yes. In “Round to sig figs” mode, negative numbers are rounded the same way as positive ones (by magnitude); the sign is preserved. In “Count sig figs” mode, a leading minus sign is ignored when counting — e.g. -450 has 2 significant figures, same as 450.
You can choose any number from 1 to 20 significant figures. For most lab and engineering work, 2–4 sig figs are common. Very precise values or intermediate steps might use more.