Significant Figures Calculator
Significant figures tell you how precisely a number is known, and this calculator does both jobs you need when working with them: it counts how many significant figures a number already contains, and it rounds any number to a target count of significant figures. Understanding and applying the rules correctly is essential in physics, chemistry, biology, and engineering, where the precision of a result must match the precision of the measurements that produced it. The rules are consistent once you know them: every non-zero digit is always significant; zeros sandwiched between non-zero digits are significant; trailing zeros after a decimal point are significant; and leading zeros, such as those in 0.00314, are never significant because they only mark the decimal position. This calculator applies those rules automatically. You enter your number and the number of significant figures you want to round to, and it returns the correctly rounded result, the count of significant figures in the original number you typed, and the result expressed in scientific notation. The rounded value keeps the correct magnitude, so a number like 31,400 rounded to two significant figures appears as 31,000 rather than as a decimal. Use it to check answers in chemistry labs, to report measured values with the right precision in assignments, or simply to verify your manual sig fig counting. Enter a number and a target to see all three outputs update as you type.
How it works
To count significant figures, the calculator strips leading zeros and the decimal point, then counts the remaining digits, treating trailing zeros as significant only when a decimal point is present in the original input. To round to the target, it uses JavaScript's toPrecision method, which keeps the requested number of significant digits starting from the first non-zero digit and preserves the correct order of magnitude. The scientific notation output is derived from the same rounded value converted using toExponential.
Worked example
Enter 3.14159 with a target of 3 significant figures. The number 3.14159 contains six significant figures (all digits are non-zero). Rounded to three significant figures, the result is 3.14, because the fourth digit is 1 and rounds down. In scientific notation that is 3.14 times 10 to the power of 0.
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