How to Convert a Decimal to an Octal: Step-by-Step Instructions

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Everything you need to quickly convert base 10 into base 8
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Octal is the base 8 number system, that only uses the digits 0 through 7. Its main advantage is the ease of conversion with binary (base 2), since each digit in octal can be written as a unique three-digit binary number. [1] Converting decimal to octal is a little more difficult, but you don't need to know any math past long division. Start with the division method, which finds each digit by dividing by powers of 8. The remainder method is faster and uses similar math, but it can be a little harder to understand why it works.

Method 1
Method 1 of 2:

Converting with Division

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  1. Of the two methods on this page, this method is easier to understand. If you're already confident working in different number systems, try the faster remainder method, below.
  2. For this example, we'll convert the decimal number 98 into octal.
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  3. Remember that "decimal" is called base 10 because each digit represents a power of 10. [2] We call the first three digits 1s place, the 10s place, the 100s place — but we could also write this as the 10 0 place, the 10 1 place, and the 10 2 place. Octal, or the base 8 number system, uses powers of 8 instead of powers of 10. Write a few of these powers of 8 in a horizontal line, from largest to smallest. Note that these numbers are all written in decimal (base 10):
    • 8 2   8 1   8 0
    • Rewrite these as single numbers:
    • 64  8  1
    • You don't need any powers of 8 larger than your original number (in this case, 98). Since 8 3 = 512, and 512 is larger than 98, we can leave it off the chart.
  4. Take a look at your decimal number: 98. The nine in the 10s place tells you that there are nine 10s in this number. 10 goes into this number 9 times. Similarly, with octal, we want to know how many "64s" go into the final number. Divide 98 by 64 to find out. The easiest way to do this is to make a chart, reading top to bottom: [3]
    • 98
      ÷
    • 64    8   1
      =
    • 1 ← This is the first digit of your octal number.
  5. Calculate the remainder of the division problem, or the amount left over that doesn't go in evenly. Write your answer at the top of the second column. This is what's left of your number after the first digit is calculated. In our example, 98 ÷ 64 = 1. Since 1 x 64 = 64, the remainder is 98 - 64 = 34. Add this to your chart: [4]
    • 98    34
      ÷
    • 64   8   1
      =
    • 1
  6. To find the next digit, we move one step down to the next power of 8. Divide the remainder by this number and fill out your chart's second column:
    • 98    34
      ÷      ÷
    • 64    8    1
      =     =
    • 1     4
  7. Just as before, find the remainder of your answer and write it at the top of the next column. Keep dividing and finding the remainder until you've done this for every column, including 8 0 (the ones place). Your final row is the final decimal number converted to octal. [5] Here's our example with the full chart filled out (note that 2 is the remainder of 34÷8):
    • 98   34    2
      ÷     ÷     ÷
    • 64   8    1
      =    =     =
    • 1    4     2
    • The final answer: 98 base 10 = 142 base 8. You can write this as 98 10 = 142 8
  8. To check your work, multiply each digit in octal by the power of 8 it represents. You should end up with your original number. Let's check our answer, 142:
    • 2 x 8 0 = 2 x 1 =2
    • 4 x 8 1 = 4 x 8 = 32
    • 1 x 8 2 = 1 x 64 = 64
    • 2 + 32 + 64 = 98, the number we started with.
  9. Practice this method by converting the decimal number 327 into octal. When you think you have the answer, highlight the invisible text below to see the whole problem laid out.
    • Highlight this area:
    • 327  7   7
      ÷     ÷    ÷
    • 64   8   1
      =    =    =
    • 5    0    7
      •
    • The answer is 507.
    • (Hint: it's fine to have 0 as the answer to a division problem.)
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Method 2
Method 2 of 2:

Converting with Remainders

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  1. We'll start with the decimal number 670 .
    • This method is faster than the successive division method. Most people find it more difficult to understand why it works, and may want to start with the easier method above.
  2. Ignore decimal values for now. You'll see why this calculation is useful soon.
    • In our example: 670 ÷ 8 = 83 .
  3. Now that we've "counted by 8" as many times as we can, the remainder is the small number left over. This is the last digit of our octal number, in the ones place (8 0 ). The remainder is always smaller than 8, so it can't be represented by any of the other digits. [6]
    • In our example: 670 ÷ 8 = 83 remainder 6 .
    • Our octal number so far is ???6.
    • If your calculator has a "modulus" or "mod" button, you can find this value by entering "670 mod 8."
  4. Set aside the remainder and return to your division problem. Take your answer and divide by 8 again. Note the answer, then find the remainder. This is the second-to-last digit of your octal number, the 8 1 = 8s place.
    • In our example: The answer to our last division problem was 83.
    • 83 ÷ 8 = 10 remainder 3.
    • Our octal number so far is ??36.
  5. As before, take the answer to your last division problem. Divide it by 8 again, and find the remainder. This is the third-to-last digit of your octal number, the 8 2 = 64s place.
    • In our example: The answer to our last division problem was 10.
    • 10 ÷ 8 = 1 remainder 2.
    • Our octal number so far is ?236.
  6. When you calculate your last division problem, the answer will be 0. The remainder to this problem is the first digit in your octal number. You have now fully converted the decimal number.
    • In our example: The answer to our last division problem was 1.
    • 1 ÷ 8 = 0 remainder 1.
    • Our final answer is the octal number 1236. We can write this as 1236 8 to show that it is an octal number.
  7. If you're having trouble understanding this method, here's an explanation: [7]
    • You start with a pile of 670 units.
    • The first division problem divides these into groups, with 8 units in each group. Anything left over, the remainder, doesn't fit into the octal 8s place. It must be in the 1s place instead.
    • Now you take your pile of groups, and divide them into sections with 8 groups each. Each section now has 8 groups with 8 units each, or 64 units total. The remainder doesn't fit into these, so it can't fit into the octal 64s place. It must be in the 8s place.
    • This continues until you discover the entire number.
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Community Q&A

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  • Question
    How I convert octal to decimal?
    Community Answer
    Every number in the ones place counts as one, every number in the tens place counts as 8 and every number in the 100's place counts as 64. Use this formula to find the answer. Example: 326; 3(64)+2(8)+6(1); 192+16+6; 214.
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  • Question
    What is 40 decimal converted to octal solution?
    Purplecakecaroline
    Community Answer
    50 octal is the solution you are looking for.
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  • Question
    How can I convert the decimal number 645 into octal number?
    Community Answer
    645÷8=80 rem 5 and then 80÷8=10 rem 0, next 10÷8=1 rem 2 and finally 1÷8=0 rem 1. The remainders in ascending order are the answer so you answer is 1205.
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      Practice Problems

      • Try to convert these decimal numbers on your own, using either method above. When you think you have the answer, highlight the invisible text on the right side of the equation. (Note that 10 means decimal and 8 means octal.)
      • 99 10 = 143 8
      • 363 10 = 553 8
      • 5210 10 = 12132 8
      • 47569 10 = 134721 8

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      References

      1. ↑ https://www.mathsisfun.com/definitions/octal.html
      2. ↑ https://www.ducksters.com/kidsmath/decimals_place_value.php
      3. ↑ http://www.free-online-calculator-use.com/decimal-to-octal-converter.html
      4. ↑ https://www.rapidtables.com/convert/number/decimal-to-octal.html
      5. ↑ https://www.rapidtables.com/convert/number/decimal-to-octal.html
      6. ↑ https://www.cuemath.com/numbers/decimal-to-octal/
      7. ↑ https://www.cuemath.com/numbers/decimal-to-octal/

      About This Article

      Article Summary X

      To convert from decimal to octal using division, start by writing down the decimal number. Next, list the powers of 8 and divide the decimal number by the largest power of 8. Calculate the remainder, then divide the remainder by the next power of 8. Repeat this process until you've solved the problem! To learn how to convert decimal to octal using remainders, read on!

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      Reader Success Stories

      • Shalia Johnson

        May 11, 2016

        "I practiced both methods. The division method laid the foundation for me, but the remainder method was so much ..." more
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