This significant figures calculator does the two jobs you need when working with sig figs: it counts how many significant figures a number already has, and it rounds any number to a chosen number of significant figures. Significant figures are the digits in a value that actually carry information about how precisely it was measured, and following the rules keeps you from claiming more accuracy than your data supports. The rules are straightforward once learned: every non-zero digit is significant, any zeros trapped between non-zero digits are significant, and trailing zeros are significant only when a decimal point is present. Leading zeros, like those in 0.0042, are never significant because they only set the decimal place. You enter a number to see its significant figure count, and separately enter a number and a target to round it correctly while preserving the right place value, so a result is shown as 4500 rather than 4.5 when that is what the rounding calls for. This is essential for physics, chemistry and engineering work, where reported results are expected to match the precision of the instruments used, and it is a common requirement in school and university assessment. Enter your values to see both the count and the rounded result update instantly.
To count, the calculator ignores leading zeros and the decimal point, then counts the remaining digits, keeping trailing zeros only when a decimal point is present. To round, it keeps the requested number of significant digits from the first non-zero digit and adjusts the place value so the size of the number stays correct.
The number 0.0042100 has five significant figures: the 4, 2, 1 and the two trailing zeros, since a decimal point is present. Rounded to two significant figures it becomes 0.0042.
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