Question: Wikipedia [says](https://en.wikipedia.org/wiki/Kilobyte) kB = 1000 bytes and KB = 1024 bytes.

Which one are we using for Bitcoin?

Answer: 
According to the [Bitcoin.it-wiki page on Transaction fees](https://en.bitcoin.it/wiki/Transaction_fees#Sending) are dependent on multiples of **1,000 bytes** (rounded up).

The above page cites Gavin Andresen's [post on 0.8.2 Release notes](https://bitcointalk.org/index.php?topic=219504.msg2311420#msg2311420):

> 
> The default fee for low-priority transactions is lowered from 0.0005 BTC
>  (***for each 1,000 bytes in the transaction***; an average transaction is
>  about 500 bytes) to 0.0001 BTC. [emphasis mine]
> 
> 
>
Here is the latest changed(commit) made for the fee drop: <https://github.com/bitcoin/bitcoin/commit/beabca2be092d0e2a1de26989d4e63a12cce1284#diff-279f95e952b2c1cff13bdfbcd7bc36ee>

It seems to be a multiple of 1,000 bytes as it's seen in the source code.

So yes, it's a multiple of **1000 bytes**, not **1024 bytes**.

Cheers,

-Besir