# How do I perform MB and KB arithmetic?

Discussion in 'Homework Help' started by xEnOnn, Mar 31, 2011.

1. ### xEnOnn Thread Starter New Member

Feb 2, 2011
25
0
I am not sure how I should perform a division between megabytes and kilobytes.
Say I want to divide 256MB with 1KB. Apparently, the answer is 256KB.
But I don't know how I should get that. What I tried was this:
First, I convert both to a similar unit. So in this case, I convert 256MB to become 262144KB.
Then, I divide 262144KB ÷ 1KB = 262144KB. But this isn't right.
So I try another way to convert both to MB instead.
Then 1KB = (1 ÷ 1024)MB and then 256MB ÷ (1 ÷ 1024)MB = 0.25MB = 256KB. <-- Now, this is correct but how come I have to convert to MB before performing the division? Why having the both operands be in KB and then divide them gives me a different answer?

Then in another case, for 16KB divided by 16B,
when I convert both to KB, 16B=(16 ÷ 1024)KB and then have 16KB ÷ (16 ÷ 1024)KB = (1÷1024)KB = 1B
But if I convert both to B, then 16KB=16*1024=16384B. And then 16384B ÷ 16B=1024B=1KB.
Again in this case, both give me a different answers when divided with the same unit!

What's wrong?
Why doesn't KB ÷ KB give me an answer in KB?

Say for the first case, 256MB. I convert to 262144KB. When divided by 1KB, I get back 262144KB, which is wrong. Unless I assume that 262144 is in Bytes, then I can convert it to the correct answer by having 262144B ÷ 1024 = 256KB. BUT since I had KB ÷ KB, why is my answer in Bytes?

2. ### Papabravo Expert

Feb 24, 2006
11,076
2,152
If you knew anything about dimensional analysis you would know that dividing two quantities with the same units gives a dimensionless number.

Code ( (Unknown Language)):
1.
2. 256KB / 1kB = 256, with no units whatsoever
3.
4. also
5.
6. 262,144 Bytes / 1024 Bytes = 256, again with no units whatsoever
7.
Got it??!!

3. ### xEnOnn Thread Starter New Member

Feb 2, 2011
25
0
Why is the 262,144 in bytes and not in KB?
From 256MB, I multiply by $2^{10}$, which is 1024, and I should convert 256MB to 262,144KB isn't it?

4. ### someonesdad Senior Member

Jul 7, 2009
1,585
141
You need to learn the difference between SI decimal prefixes (e.g., "k" and "M") and the IEC binary prefixes established about a decade ago. A megabyte is always 1e6 bytes; use MiB if you want to indicate mebibyte. Then there won't be any confusion.

When you're in doubt, in either system, substitute what the symbol means (e.g., 10^6 or 2^20); then the arithmetic is trivial.