offers hundreds of GRE video lessons and practice questions. Go there now.
Sign up or log in to Magoosh GRE Prep.

GRE Math: Rounding

What does it mean to round a number?  What do you have to know about rounding for the GRE? 

Not many GRE questions will say, “Here’s a number: round it to the nearest such-and-such.”  By contrast, many questions, in the course of asking something else, could ask you to round your answer to the nearest such-and-such.  In this way, rounding is one math skill you need to know for the GRE.  There are a few tricky issues, which I will address here.

 

Rounding to the nearest integer

The most common type of rounding is round decimals to the nearest integer.  The rule for rounding is simple: look at the digits in the tenth’s place, the first digit to the right of the decimal point, that digit and that digit only.  If the digit in the tenths place is {0, 1, 2, 3, 4}, then round down, which means the units digit remains the same; if the digit in the tenths place is {5, 6, 7, 8, 9}, then round up, which means the unit digit increases by one.  Here are a couple things to notice:

Observation #1: under most circumstances, rounding changes the decimal to whatever integer is closer.  For example, 4.3 is rounded to 4, and 4.9 is rounded to 5.  The exception is when the decimal is smack dab between two integers: 4.5 is exactly equidistant to both 4 and 5, but because of the “tie-breaker” rule of rounding, anything with a 5 in the tenths digit is rounded up.  This is the only case in which the “go to the closer integer” interpretation will fail.

Observation #2: Do NOT double-round.  Some people look at a number like, say, 7.49, and they erroneously think — well, that 9 would round the 4 up to 5, and then a 5 gets round up, to this number would round to 8.  WRONG!  Never round a number “in stages.”  Rounding is a one-shot deal, a one-step process.   When the number we need to round is 7.49, we only need notice that the tenth’s digit is a 4, which means the number is rounded down to 7.  One step, case closed.  In fact, all of the following numbers get rounded to 7:

7.499

7.499999

7.49999999999999999999999999999999999999999999999999999999

Here’s the truly mind-boggling part: how many numbers would there be larger than this last number, but still lower than 7.5?  INFINITY!  No matter how many additional 9′s we slap on to the end of that number, there’s still a continuous infinity of decimals larger than that number and below 7.5  No matter how finely we chop up the real number line, each tiny fragment of the line, no matter how small, still contains a continuous infinite of numbers.

Observation #3: the “tie-breaker” rule can be tricky with negative values.  For example, +2.5 gets rounded up to 3, but –2.5 gets rounded down … to –3.  As with positive numbers, the negative number ending in .5 is rounded to the higher absolute value integer, but with negatives, that’s rounding down.  (This is not the only way to formulate this rule, but this is the convention that ETS follows.)

 

Rounding to any other decimal place

Rounding to the nearest integer is really rounding to the nearest units place. Sometimes, you will be asked to round to the nearest hundreds, or to the nearest hundredths — to some decimal place other than the units place.  The rule is just a more generalized version of the previous rounding rule.

Suppose we are asked to round to some specific decimal place — call this the “target place.”  You always look at only one digit, the digit immediately to the right of the target place.  If this digit immediately to the right is {0, 1, 2, 3, 4}, then you “round down”, and the digit in the target place remains unchanged. If this digit immediately to the right is {5, 6, 7, 8, 9}, then you “round up”, and the digit in the target place increases by 1.

 

Other cases of rounding

Very occasionally, a GRE question may ask you not to round to a particular decimal place, but rather to the nearest multiple of something.  For example, suppose you are asked to round, say, to the nearest 0.05 — how do you do that?

Well, let’s think about the results first.  The result of rounding to the nearest 0.05 would be something divisible by 0.05 — that is to say, a decimal with either a 0 or a 5 in the hundredth place, no digits to the right of that, and any digits to the left of that.  The following are examples of numbers which could be the result of rounding to the nearest 0.05:

  1. 0.35
  2. 1.40
  3. 3.15
  4. 5.2
  5. 8

Notice: the second, (b) is the square root of 2 (sqrt{2} = 1.414213562….) rounded to the nearest 0.05, and the third, (c), is pi rounded to the nearest 0.05.

Let’s demonstrate the rounding by means of an example.  What numbers, when rounded to the nearest 0.05, would be rounded to 2.35?  Well, for starters, 2.35 and other “tenths” around it would be rounded to 2.35

2.32 — rounded down to 2.30

2.33 — rounded up to 2.35

2.34 — rounded up to 2.35

2.35 — stays at value

2.36 — rounded down to 2.35

2.37 — rounded down to 2.35

2.38 — rounded up to 2.40

Now, the tricky regions are those between the values that are rounded in different directions.  For example, 2.32 is rounded down and 2.33 is rounded up, so something fishy is happening between those two.  Let’s think about the hundredths between 2.32 and 2.33 — exactly between them is 2.325, the midpoint between 2.30 and 2.35, and like all midpoints, according to the “tiebreaker” rule, it gets rounded up.  Thus:

2.320 —- rounded down to 2.30

2.321 —- rounded down to 2.30

2.322 —- rounded down to 2.30

2.323 —- rounded down to 2.30

2.324 —- rounded down to 2.30

2.325 —- rounded up to 2.35 (the “tie-breaker” rule)

2.326 —- rounded up to 2.35

2.327 —- rounded up to 2.35

2.328 —- rounded up to 2.35

2.329 —- rounded up to 2.35

2.330 —- rounded up to 2.35

This is all probably far more detail than you will need to know for the GRE, but this does demonstrate the steps you would take to round any decimal to the nearest 0.05.  By analogy, you could round any decimal to any specified multiple.

 

About the Author

Mike McGarry is a Content Developer for Magoosh with over 20 years of teaching experience and a BS in Physics and an MA in Religion, both from Harvard. He enjoys hitting foosballs into orbit, and despite having no obvious cranial deficiency, he insists on rooting for the NY Mets. Follow him on Google+!

28 Responses to GRE Math: Rounding

  1. Manish A. January 12, 2014 at 9:27 pm #

    Hi Mike,

    Great article…So here’s my question, could you please give an example of a question where you round a number to the nearest multiple of 0.25 …just a question and and answer to it…

    Regards,
    Manish A.

    • Mike
      Mike January 13, 2014 at 9:15 am #

      Dear Manish,
      I’m glad you liked the article. Here are a couple of examples,
      3.14 rounded to the nearest 0.25 is 3.25.
      2.56 rounded to the nearest 0.25 is 2.50
      4.87 rounded to the nearest 0.25 is 4.75
      4.88 rounded to the nearest 0.25 is 5.00
      The GRE is quite unlikely to ask you about this.
      Mike :-)

      • Bushra January 29, 2014 at 11:27 am #

        I don’t understand how 4.87 rounded to the nearest 0.25 is 4.75.
        Would you please explain in more detail?

        • Mike
          Mike January 29, 2014 at 1:30 pm #

          Dear Bushra,
          Think about it this way. The 0.25′s are the set:
          …3.75, 4.00, 4.25, 4.50. 4.75, 5.00, 5.25, …
          These are all multiples of 0.25. Anything rounded to the nearest 0.25 will be rounded to something in that set. The number 4.87 is between 4.75 and 5.00 — the question is: to which is it closest?
          |4.87 – 5.00| = 0.13
          |4.87 – 4.75| = 0.12
          Thus, 4.87 is closer to 4.75 than it is to 5.00, so 4.75 is nearest number in the set, the set of all possible multiples of 0.25.
          Does this make sense?
          Mike :-)

  2. Shekhar December 20, 2013 at 6:22 am #

    Can some one reply that when 4.447 is rounded to nearest tenths then which will be answer.
    I. 4.4
    I. 4.5

    how about 4.4455 is rounded to nearest tenth
    I. 4.4
    I. 4.5
    Could you please reply.

    • Mike
      Mike December 20, 2013 at 8:42 am #

      Dear Shekhar,
      I’m happy to respond. :-) For both of those numbers, 4.447 and 4.4455, the number in the hundredths place, the place right after the tenths place, is a 4 — nothing beyond that matters. The hundredths place is a 4, so the tenths place gets “rounded down” to a 4, and both of those numbers, rounded to the nearest tenth, are 4.4.
      Think about it visually. Suppose make the scale really big — let’s say there’s a full meter between 4.4 and 4.5. The 4.4 is at the left side, and one meter away is 4.5. Well, on that scale, the 4.447 would be 47 cm from the left side. It would be very close to the middle, but that’s not the question. That point at 47 cm, is closer to the left side — that’s precisely why the left side, 4.4, is the “nearest” tenth.
      Does all this make sense?
      Mike :-)

  3. Kavitha October 22, 2013 at 10:50 am #

    Hi Mike,

    How to round 10.8 or 10.2 to nearest 0.1 ?

    • Mike
      Mike October 22, 2013 at 11:29 am #

      Dear Kavitha,
      The numbers 10.8 and 10.2 already ARE rounded to the nearest 0.1. What you are asking is like asking: what is 3 rounded to the nearest integer? 3 already is an integer! If a number is given to the hundredth place or more, 3.76 or 2.7182818284, then we can round to the nearest tenth (3.8 and 2.7 respectively). Does all this make sense?
      Mike :-)

      • Kavitha October 22, 2013 at 11:38 am #

        Hi Mike,

        thanks for ur reply. I got this doubt because in one of the practice test question it was told to round the answer to nearest 0.1. What exactly it means? Sorry if this sounds silly.

        • Mike
          Mike October 22, 2013 at 11:48 am #

          Dear Kavitha,
          Numbers to the nearest 0.1, that is, to the nearest tenth are numbers such as 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, etc. That’s an infinite list, in both the positive & negative directions: imagine a number line marked off in tenths that is infinite in both directions.
          When you are given a number with more than one place after the decimal, such as 1.25992105 (the cube root of 2), you could be asked to “round it to the nearest tenth”, which means, round it to the number on the infinite list above, the number line by tenths, to which it is closest. We would look at the decimal in the second decimal place, the hundredths place, to determine which way to round. Here, the hundredths place is a 5, so we round up to 1.3. That is this number (the cube root of 2) rounded to the nearest tenth. Does all this make sense?
          Mike :-)

          • Kavitha October 22, 2013 at 12:07 pm #

            Thanks Mike !!! Now I understood !!

            • Mike
              Mike October 22, 2013 at 12:41 pm #

              Kavitha,
              You’re quite welcome. Best of luck to you.
              Mike :-)

          • Neelam November 18, 2013 at 4:10 am #

            Hi Mike,
            Number 1.2599 rounded to nearest tenth is 1.3? Isn’t it? U mentioned the answer as 1.26

            • Mike
              Mike November 18, 2013 at 10:08 am #

              Neelam,
              Good eye! Yes, that was a mistake, and I corrected it. The number 1.2599 rounded to the nearest hundredth is 1.26, and to the nearest tenth is 1.3.
              Mike :-)

  4. Dave Perriam September 17, 2013 at 8:22 am #

    Hi Mike
    The negative rule I would like to teach to my aspiring L6s from Year 6. Would it be ok to teach that with negative numbers you are moving away from zero therefore you are looking at the nearest whole number moving in that direction. So -3.5 would round to minus 4. Also as a cheat take away the minus sign round it and replace the minus sign. Coming from a non academic background I try to make more challenging maths more accessible, but I am wary that this could confuse later.
    Cheers Dave

    • Mike
      Mike September 17, 2013 at 1:08 pm #

      Dave,
      I must say, I am not familiar with what an “L6″ is, and what level these students might be. The important thing to realize is — different authorities will follow different conventions as regards the “rounding with negatives” rule. The standard I explain here is consistent with the standards of the GRE. The standards to which your students will be accountable may or may not follow another, slightly different rule (e.g always round up at the 0.5, whether positive or negative). I would hesitate to say that one way is THE WAY unless you know all the conventional choices your system has made.
      Does this make sense?
      Mike :-)

  5. Anki June 27, 2013 at 7:51 am #

    Mike Sir,

    The last part seems little confusing! Will we encounter such sums on the Gre?
    Also, can you give me some other example apart from this?

    • Mike
      Mike June 27, 2013 at 1:41 pm #

      Anki,
      This last example gets into some of the hardest rounding ideas you will possibly see on the GRE. If you find it confusing, I would say — just file away for now that you found this information here, and if you happen to come across a rounding problem like this in your other practice, then come back & re-read this and it may make more sense at that point.
      Does this make sense?
      Mike :-)

  6. Chiraag March 19, 2013 at 11:05 am #

    great !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Thanks a lot MIKE!

    • Mike
      Mike March 19, 2013 at 12:04 pm #

      Dear Chiraag,
      You are quite welcome.
      Mike :-)

  7. Azfar November 21, 2012 at 10:04 am #

    so that means when ever we are asked to round to the nearest multiple , we just have to see what numbers in the options are divisible by that number.

    e.g if we have to find to the nearest multiple of .09 . we have 2 options
    a) 81
    b)85

    HERE 81 is divisible by 9 , so it is the required multiple.

    is it right?

    • Mike
      Mike November 21, 2012 at 12:17 pm #

      First of all, NO ONE on the planet is ever going to ask you to round to the nearest 0.09. It’s rare that you are asked to round to anything other than a power of ten, and when it’s not, the number is something very closed related to powers of ten —- 0.05 is halfway between two multiples of 0.01. The whole point of rounding is to get something to a “nicer” number — if it’s close to an integer for example, rounding it to the integer. Rounding things to the nearest 0.09 would round things AWAY FROM integers, which fundamentally contradicts the idea of rounding.
      Furthermore, we are talking about rounding to decimals, but then you propose two positive integers, 81 & 85. It doesn’t make sense to talk about rounding to the nearest decimal and then talk about positive integers. The whole goal of rounding decimals is getting to, or closer to, integers.
      Does all this make sense?
      Mike :-)

  8. Christine November 19, 2012 at 1:40 pm #

    hi there Mike!

    In the last part, don’t you mean 2.320 –> 2.32 instead of 3.20 and so on?

    • Mike
      Mike November 19, 2012 at 5:12 pm #

      YES! Fantastic catch! That’s a total goof on my part. I just corrected the mistake in the post. Thank you, thank you, thank you!
      Mike :-)

      • Debabrata Saha December 9, 2012 at 5:47 am #

        If 2.5 rounded to 3 but -2.5 is rounded to -2 then how – 2.4 and – 2.7 is rounded if we consider a closest nearest integer?

        • Mike
          Mike December 9, 2012 at 2:03 pm #

          Dear Debadrata,
          By ETS convention, -2.5 is rounded down to -3. The number -2.4 is rounded to the closest integer: -2.4 is closer to -2 than to -3, so it is rounded to -2. Similarly, the number -2.7 is closer to -3, so it is rounded to -3. If you draw a number line in the negative region and mark the tenths between -2 and -3, so that you can see on the number line where -2.4 and -2.7 fall, then I think you will see visually why this has to be the case.
          Does all this make sense?
          Mike :-)

          • Abirami March 20, 2014 at 9:22 am #

            Hi Mike,

            Firstly, Great Post !!! Really helped me..

            I have one doubt though. In your post you have mentioned that “+2.5 gets rounded up to 3, but –2.5 gets rounded down … to –3 ” . But here you have ,mentioned that “By convention, -2.5 is rounded “up” to -2″ . Now, I’m a little confused as to follow which rule .

            • Mike
              Mike March 20, 2014 at 10:19 am #

              Dear Abirami,
              Oops! That was a typo. i just corrected it. Everything should be consistent now. Thanks for pointing this out.
              Mike :-)


Magoosh blog comment policy: To create the best experience for our readers, we will approve and respond to comments that are relevant to the article, general enough to be helpful to other students, concise, and well-written! :) If your comment was not approved, it likely did not adhere to these guidelines. If you are a Premium Magoosh student and would like more personalized service, you can use the Help tab on the Magoosh dashboard. Thanks!

Leave a Reply