Significant Figures Calculator
This significant figures calculator does two things. It rounds any number to the sig figs you choose, and it counts how many significant figures a number already has, highlighting exactly which digits count. Type a value and you'll see the answer, the scientific notation form, and the rule behind it, so it's a sig fig calculator and a quick refresher in one.
- Round to N sig figs
- Count the sig figs
- Highlights the digits
- Scientific notation
- Rules explained
Last updated June 16, 2026 Count from the first non-zero digit Reviewed by the Calcowa math team
Enter a valid number above.
Negatives and decimals are fine. Keep trailing zeros to mark them as significant.
What are significant figures?
Significant figures, often shortened to sig figs, are the digits in a number that carry real meaning about its precision. They're a quick read on how exact a measurement is. Reporting a length as 4.50 m claims more precision than 4.5 m, because that trailing zero says you measured to the hundredth. Counting and rounding to the right number keeps your answers honest, so you're not claiming more accuracy than you've got, which is why every science and engineering class leans on them.
Significant figures rules
Deciding which digits count comes down to four rules, and you'll see the calculator follow every one:
- 1
Non-zero digits always countEvery digit from 1 to 9 is significant, so 4.5 and 372 have two and three significant figures.
- 2
Zeros between digits countA zero trapped between non-zero digits is significant, so 1002 has four significant figures.
- 3
Leading zeros never countZeros before the first non-zero digit only place the decimal, so 0.0045 has two significant figures.
- 4
Trailing zeros count with a decimalA trailing zero is significant only when a decimal point is present, so 1200 has two but 1.200 has four.
How do you round to significant figures?
Find the first significant digit, count off as many as you need, then look at the very next digit to decide the rounding. If it's 5 or more you round up, and if it's less you round down. To take 12345 to 3 significant figures, you keep 1, 2, 3; the next digit is 4, so it rounds down and the answer is 12300. That's the whole method. The zeros that fill the rest hold the place value but don't add precision. Switch the mode above and you'll see this worked out for any number you type.
Significant figures and scientific notation
Scientific notation and significant figures are a natural fit, because writing a number as a single digit before the point times a power of ten shows the sig figs without any placeholder zeros. So 12300 to 3 significant figures is 1.23 × 10⁴, and 0.0045 is 4.5 × 10⁻³. The result above gives the scientific notation form too, so you've got both at once. For the full conversion, the scientific notation calculator handles every direction, and the rounding calculator rounds by decimal place instead.
Frequently asked questions
How many significant figures does 100 have?
Written plainly, 100 has just one significant figure, since the trailing zeros don't count without a decimal point. Write it as 100. or 1.00 × 10² and it has three. That's exactly why scientific notation is handy here.
Count significant digits from the first non-zero digit, keep as many as you need, then look at the next digit to decide whether to round up. To round 12345 to 3 significant figures, you keep 1, 2, 3; the next digit is 4, so it rounds down to 12300. The trailing zeros hold the place value but don't count as significant. This sig fig calculator does the rounding and shows the step.
Four rules cover it. All non-zero digits count. Zeros between non-zero digits count. Leading zeros never count; they only set the decimal place. Trailing zeros count only when there's a decimal point, so 1200 has two significant figures but 1200. and 1.200 each have four. The counter above applies all four for you.
0.00120 has three significant figures: the 1, the 2, and the final 0. The three leading zeros aren't significant because they only place the decimal, while the trailing zero counts since it sits after a decimal point. Drop the leading zeros and you're left with 120, which is the three significant digits.
No. Leading zeros, like the zeros in 0.0045, never count. They're there only to position the decimal point, so 0.0045 has two significant figures, the 4 and the 5. Writing it as 4.5 × 10⁻³ makes that clear, since scientific notation strips the placeholder zeros away.
Three significant figures means you keep the first three meaningful digits and round the rest. So 3.14159 to 3 significant figures is 3.14, 45678 becomes 45700, and 0.0089234 becomes 0.00892. It's a common precision in science and engineering, tight enough to be useful without pretending to more accuracy than a measurement holds.
No, they count different things. Decimal places count digits after the point; significant figures count meaningful digits wherever they sit. 0.0045 has four decimal places but two significant figures. When a problem asks for significant figures, you'll want this tool; for plain decimal-place rounding, the rounding calculator handles that instead.
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