When you have to round a number, you are usually told how to round it. It's simplest when you're told how many "places" to round to, but you should also know how to round to a named "place", such as "to the nearest thousand" or "to the ten-thousandths place". You may also need to know how to round to a certain number of significant digits; we'll get to that later.
In general, you round to a given place by looking at the digit one place to the right of the "target" place. If the digit is a five or greater, you round the target digit up by one. Otherwise, you leave the target as it is. Then you replace any digits to the right with zeroes (if they are to the left of the decimal point) or else you delete the digits (if they are past the decimal point).
I'll use the first few digits of the decimal expansion of pi: 3.14159265... in the examples below.
- Round pi to five places.
"To five places" means "to five decimal places". First, I count out the five decimal places, and then I look at the sixth place:
3.14159 | 265...I've drawn a little line separating the fifth place from the sixth place. This can be a handy way of "keeping your place", especially if you are dealing with lots of digits.
The fifth place has a 9 in it. Looking at the sixth place, I see that it has a 2 in it. Since 2 is less than five, I won't round the 9 up; that is, I'll leave the 9 as it is. In addition, I will delete the digits after the 9. Then pi, rounded to five places, is:
3.14159
- Round pi to four places.
First, I go back to the original number (not the one I just rounded in the previous example). I count off four places, and look at the number in the fifth place:
3.1415 | 9265...
The number in the fifth place is a 9, which is greater than 5, so I'll round up in the fourth place, truncating the expansion at four decimal places. That is, the 5 becomes a 6, the 9265... part disappears, and pi, rounded to four decimal places, is:
3.1416
- Round pi to three places.
First, I go back to the original number (not the one I just rounded in the previous example). I count off three decimal places, and look at the digit in the fourth place:
3.141 | 59265...
The number in the fourth place is a 5, which is the cut-off for rounding: if the number in the next place (after the one you're rounding to) is 5 or greater, you round up. In this case, the 1 becomes a 2, the 59265... part disappears, and pi, rounded to three decimal places, is:
3.142
This rounding works the same way when they tell you to round to a certain named place, such as "the hundredths place". The only difference is that you have to be a bit more careful in counting off the places you need. Just remember that the decimal places count off to the right in the same order as the counting numbers count off to the left. That is, for regular numbers, you have the place values:
...(ten-thousands) (thousands) (hundreds) (tens) (ones)
For decimal places, you don't have a "oneths", but you do have the other fractions:
(decimal point) (tenths) (hundredths) (thousandths) (ten-thousandths)...
For instance:
- Round pi to the nearest thousandth.
"The nearest thousandth" means that I need to count off three decimal places (tenths, hundredths, thousandths), and then round:
3.141 | 59265...
Then pi, rounded to the nearest thousandth, is 3.142.
Round 2.796 to the hundredths place.
The hundredths place is two decimal places, so I'll count off two decimal places, and round according to the third decimal place:
2.79 | 6
Since the third decimal place contains a 6, which is greater than 5, I have to round up. But rounding up a 9 gives a 10. In this case, I round the 79 up to an 80:
2.80
Another consideration in rounding is when you have to round to "an appropriate number of significant digits". What are significant digits? Well, they're sort of the "interesting" or "important" digits. For example,
- 3.14159 has six significant digits (all the numbers give you useful information)
- 1000 has one significant digit (only the 1 is interesting; you don't know anything for sure about the hundreds, tens, or units places; the zeroes may just be placeholders; they may have rounded something off to get this value)
- 1000.0 has five significant digits (the ".0" tells us something interesting about the presumed accuracy of the measurement being made: that the measurement is accurate to the tenths place, but that there happen to be zero tenths)
- 0.00035 has two significant digits (only the 3 and 5 tell us something; the other zeroes are placeholders, only providing information about relative size)
- 0.000350 has three significant digits (that last zero tells us that the measurement was made accurate to that last digit, which just happened to have a value of zero)
- 1006 has four significant digits (the 1 and 6 are interesting, and we have to count the zeroes, because they're between the two interesting numbers)
- 560 has two significant digits (the last zero is just a placeholder)
- 560. (notice the "point" after the zero) has three significant digits (the decimal point tells us that the measurement was made to the nearest unit, so the zero is not just a placeholder)
- 560.0 has four significant digits (the zero in the tenths place means that the measurement was made accurate to the tenths place, and that there just happen to be zero tenths; the 5 and 6 give useful information, and the other zero is between significant digits, and must therefore also be counted)
If you need to express your answer as being "accurate to" a certain place, here's how the language works with the above examples: Copyright © Elizabeth Stapel 2006-2008 All Rights Reserved
- 3.14159 is accurate to the hundred-thousandths place
- 1000 is accurate to the thousands place
- 1000.0 is accurate to the tenths place
- 0.00035 is accurate to the hundred-thousandths place
- 0.000350 is accurate to the millionths place (note the extra zero)
- 1006 is accurate to the units place
- 560 is accurate to the tens place
- 560. is accurate to the units place (note the decimal point)
- 560.0 is accurate to the tenths place
1) All nonzero digits are significant.
2) All zeroes between significant digits are significant.
3) All zeroes which are both to the right of the decimal point and to the right of all non-zero significant digits are themselves significant.
Here are some rounding examples; each number is rounded to four, three, and two significant digits.
Round 742,396 to four, three, and two significant digits:
- 742,400 (four significant digits)
- 742,000 (three significant digits)
- 740,000 (two significant digits)
- 0.07284 (four significant digits)
- 0.0728 (three significant digits)
- 0.073 (two significant digits)
Round 231.45 to four, three, and two significant digits:
231.5 (four significant digits)
231 (three significant digits)
230 (two significant digits)
Rounding Addition
How do you round when they give you a bunch of numbers to add? You would add (or subtract) the numbers as usual, but then you would round the answer to the same decimal place as the least-accurate number.
Round to the appropriate number of significant digits:
13.214 + 234.6 + 7.0350 + 6.38
Looking at the numbers, I see that the second number, 234.6, is only accurate to the tenths place; all the other numbers are accurate to a greater number of decimal places. So my answer will have to be rounded to the tenths place:
13.214 + 234.6 + 7.0350 + 6.38 = 261.2290
Rounding to the tenths place, I get:
13.214 + 234.6 + 7.0350 + 6.38 = 261.2
Rounding Multiplication
How do you round, when they give you numbers to multiply (or divide)? You would multiply (or divide) the numbers as usual, but then you would round the answer to the same number of significant digits as the least-accurate number.
Simplify, and round to the appropriate number of significant digits:
16.235 × 0.217 × 5
First, I note that 5 has only one significant digit, so I will have to round my final answer to one significant digit. The product is:
16.235 × 0.217 × 5 = 17.614975
...but since I can only claim one accurate significant digit, I will need to round 17.614975 to 20, which is accurate to one significant digit.
16.235 × 0.217 × 5 = 20
How to Round Numbers
- Rule One. Determine what your rounding digit is and look to the right side of it. If the digit is 0, 1, 2, 3, or 4 do not change the rounding digit. All digits that are on the right hand side of the requested rounding digit will become 0.
- Rule Two. Determine what your rounding digit is and look to the right of it. If the digit is 5, 6, 7, 8, or 9, your rounding digit rounds up by one number. All digits that are on the right hand side of the requested rounding digit will become 0.
- Rounding with decimals: When rounding numbers involving decimals, there are 2 rules to remember:
Rule One Determine what your rounding digit is and look to the right side of it. If that digit is 4, 3, 2, or 1, simply drop all digits to the right of it.
Rule Two Determine what your rounding digit is and look to the right side of it. If that digit is 5, 6, 7, 8, or 9 add one to the rounding digit and drop all digits to the right of it.
Rule Three: Some teachers prefer this method:
This rule provides more accuracy and is sometimes referred to as the 'Banker's Rule'. When the first digit dropped is 5 and there are no digits following or the digits following are zeros, make the preceding digit even (i.e. round off to the nearest even digit). E.g., 2.315 and 2.325 are both 2.32 when rounded off to the nearest hundredth. Note: The rationale for the third rule is that approximately half of the time the number will be rounded up and the other half of the time it will be rounded down.
Reference:
http://www.purplemath.com/modules/rounding3.htm
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