How it works
To round, look at the digit just to the right of the place you are keeping. If it is 5 or more, round up; if it is less than 5, leave the kept digit as is and drop the rest. Negative numbers are rounded the same way by size, so −2.5 rounds to −3.
Significant-figure rounding keeps a set number of meaningful digits regardless of where the decimal point is: 0.004567 to 2 significant figures is 0.0046, and 123,456 is 120,000.
The calculator rounds using the decimal digits you typed, so 1.005 to two places gives 1.01 rather than the 1.00 some software produces from binary rounding error.
Formula
rounded = round(x × 10^d) ÷ 10^d, where d is the number of decimal places kept (negative for tens, hundreds…)
Questions people ask
What is 2.5 rounded to the nearest whole number?
Using the standard round-half-up rule it is 3. Some fields use "banker's rounding" (round half to even), which gives 2; this calculator uses the common school rule.
What is the difference between decimal places and significant figures?
Decimal places count digits after the decimal point. Significant figures count meaningful digits from the first non-zero digit. 0.0520 has 4 decimal places but 3 significant figures.
How do I round to the nearest hundred?
Look at the tens digit. 1,249 rounds to 1,200 because the tens digit is 4; 1,250 rounds to 1,300.