Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math: FAQ

   Rounding Numbers  

_____________________________________________
Dr. Math FAQ || Classic Problems || Formulas || Search Dr. Math || Dr. Math Home
_____________________________________________

How do you round a number to the nearest tenth (10th), hundredth (100th), thousandth (1000th), etc.?

Here are two explanations from the Elementary Fractions and Decimals area of our archives, followed by links to more information:



Rounding Decimals
  The first thing to know about rounding decimals is that it works 
  exactly like rounding whole numbers. To round to the nearest tenth, you 
  do the same thing you do to round to the nearest ten, except that you 
  are working with a different digit in the number. I can use the same 
  picture to explain both:

      |                        |              X-------->|
      +----+----+----+----+----+----+----+----+----+----+
     140  141  142  143  144  145  146  147  148  149  150

  This shows that 148 is between 140 and 150, and is closer to 150 
  because it's bigger than 145. 
  
  The next picture shows that 1.48 is between 1.40 and 1.50, and is closer 
  to 1.50 because it is bigger than 1.45:

      |                        |              X-------->|
      +----+----+----+----+----+----+----+----+----+----+
     1.40 1.41 1.42 1.43 1.44 1.45 1.46 1.47 1.48 1.49 1.50


  To round to the nearest ten, you drop all digits to the right of the 
  tens place, replacing them with zero, so that you are left with a 
  multiple of 10. Then you check to see whether your original number is 
  actually nearer to the next multiple of 10, by checking whether the 
  next digit (the leftmost one you dropped) was 5 or more:

      148 rounded to nearest 10:
        |\
        | \ replace 8 with 0
        V  \
      140   8 is greater than 5, so add 10:
       |
       |
       V
      150

  What all this means is that 148 is between 140 and 150, but is closer 
  to 150 so we round it to that.

  Now, to round to the nearest tenth, you do the same thing, but using 
  the tenths digit rather than the tens:

      1.48 rounded to nearest 0.1:
    |\
    | \ replace 8 with 0
    V  \
      1.40   8 is greater than 5, so add 0.1:
        |
        |
        V
      1.50

  Again, this means that 1.48 is between 1.4 and 1.5, but is closer to 
  1.5. If the number had been 1.45, exactly between the two, you would 
  still round up to 1.5. If it were 1.43, you would have rounded down, 
  leaving it at 1.40, because 3 is less than 5.

  The only real difference is that when you put zeroes to the right of a 
  decimal, you can ignore them completely because they don't affect the 
  number. So you can write the answer as 1.5 rather than 1.50.

  - Doctor Peterson, The Math Forum


Rounding to the Nearest...
  We always want to keep two things in mind when we round any number. 
  The first is what we are rounding to. In your example we are rounding 
  to tenths, so we should end up with one digit to the right of the 
  decimal point, because that is the tenths place. Now immediately you 
  can see that 451 is not an acceptable answer, but 451.7 or 451.8 
  could be.

  The second thing to keep in mind is the method we are using to round
  by. In your problem that is the nearest method. In the nearest method
  we look at the digit to the immediate right of the digit we will round
  to (so for this example we will look at the hundredths digit), and if
  it is a five or greater (5 through 9) we round up (add 1 to the digit 
  we are rounding and drop all the digits to the right). If the digit 
  to the right of our rounding digit is less than 5 (0 through 4) we 
  round down (drop all digits to the right of digit we are rounding).

  So for our example:

    number to be rounded to nearest tenth      451.7576  
    number to the right of the tenths digit    5
    since the number is a 5 we round up   451.8

    so our answer is 451.8

  What if we had 451.7476 instead?

    number to be rounded to nearest tenth      451.7476  
    number to the right of the tenths digit    4
    since the number is a 4 we round down      451.7

    so our answer is 451.7

  How about rounding 451.7576 to the nearest hundredth?

    number to be rounded to nearest hundredth      451.7576  
    number to the right of the hundredths digit    7
    since the number is a 7 we round up       451.76

    so our answer is 451.76

  Now you try: (solutions below)

    a) round 72.3478 to the nearest tenth
    b) round 15.4201 to the nearest tenth
    c) round 5.74999 to the nearest tenth
    d) round 87.5555 to the nearest hundredth

  I hope this is a bit clearer now. If anything was not clear in my 
  explanation, just write back noting what was confusing, and I will try 
  to make it clearer. To check your work, here are the answers to the 
  questions above: 

    a) 72.3 
    b) 15.4
    c) 5.7
    d) 87.56 .  

  I hope that you have more fun in class!  Good luck,

  -Doctor Keith,  The Math Forum


More from the Dr. Math archives:



On the Web:

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. Math ®
© 1994-2014 Drexel University. All rights reserved.
http://mathforum.org/
The Math Forum is a research and educational enterprise of the Drexel University School of Education.The Math Forum is a research and educational enterprise of the Drexel University School of Education.