Round any number to a chosen precision: decimal places, significant figures, or the nearest whole number, 10, 100, or 1000. The rounding rule applied is explained below the result.
Standard rounding: round half up. Example: 3.14159 to 2 decimal places = 3.14.
Rounding trades exactness for a number that is easier to read or report, and the rule that decides it is simple: look at the first digit being removed. If it is 5 or more, increase the last kept digit by 1. If it is 4 or less, leave the last kept digit unchanged. Which precision to pick depends on context: a price rounds to the cent, a population estimate might round to the nearest thousand, and a lab measurement rounds to the number of significant figures the instrument can actually support.
Decimal-place rounding fixes a position after the decimal point. Significant-figure rounding instead counts meaningful digits starting from the first non-zero one, which matters for very small or very large numbers where leading zeros are not meaningful. Rounding to the nearest 10, 100, or 1000 applies the same half-up rule to the digit just past the target place, then zeroes out everything below it.
When you need exact integer factors before rounding, use the factoring calculator. To reduce fractions, find the GCF first with the GCF and LCM calculator and then round the result here.
For more on rounding conventions, see Wolfram MathWorld's page on rounding.
Rounding often comes right after finding an exact factor or a common denominator.
The standard rule is round half up: if the digit to be dropped is exactly 5, round up. So 2.5 rounds to 3, and 3.45 rounded to one decimal place becomes 3.5. Some technical fields use round half to even (banker's rounding) to reduce statistical bias, but round half up is the convention in most schools and everyday calculations.
Look at the units digit. If it is 5 or more, round up to the next ten. If it is 4 or less, round down. For example, 47 rounds to 50 and 43 rounds to 40.
Significant figures (sig figs) are the meaningful digits in a number, starting from the first non-zero digit. To round 0.004567 to 2 sig figs, the first two meaningful digits are 4 and 5, so the result is 0.0046. To round 1234 to 3 sig figs, look at the fourth digit (4): it is less than 5, so the result is 1230.
Round the absolute value using the usual rule, then reattach the negative sign. So -2.6 rounded to the nearest whole number is -3 (because 2.6 rounds up to 3, and the sign is reattached).
Rounding adjusts a number to the nearest value at the chosen precision, going up when the dropped digit is 5 or more. Truncating simply removes the extra digits without checking their value. For 3.78 rounded to one decimal place the answer is 3.8, but truncated it is 3.7.
Marcus Vance spent a decade building data pipelines and testing calculation logic before he started writing about the math behind the tools. On MathCalcTools he checks every formula against the source math, then writes out the steps so a reader can follow the same path a calculator takes.