user5068: Wikipedia says kB = 1000 bytes and KB = 1024 bytes.
Which one are we using for Bitcoin?

user12809: 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

user5406: According to the Bitcoin.it-wiki page on Transaction fees are dependent on multiples of 1,000 bytes  (rounded up).
The above page cites Gavin Andresen's post on 0.8.2 Release notes:

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]


5068: <p>Wikipedia <a href="https://en.wikipedia.org/wiki/Kilobyte" rel="noreferrer">says</a> kB = 1000 bytes and KB = 1024 bytes.</p>

<p>Which one are we using for Bitcoin?</p>
 12809: <p>Here is the latest changed(commit) made for the fee drop: <a href="https://github.com/bitcoin/bitcoin/commit/beabca2be092d0e2a1de26989d4e63a12cce1284#diff-279f95e952b2c1cff13bdfbcd7bc36ee" rel="nofollow">https://github.com/bitcoin/bitcoin/commit/beabca2be092d0e2a1de26989d4e63a12cce1284#diff-279f95e952b2c1cff13bdfbcd7bc36ee</a></p>

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

<p>So yes, it's a multiple of <strong>1000 bytes</strong>, not <strong>1024 bytes</strong>.</p>

<p>Cheers,</p>

<p>-Besir</p>
 5406: <p>According to the <a href="https://en.bitcoin.it/wiki/Transaction_fees#Sending" rel="nofollow">Bitcoin.it-wiki page on Transaction fees</a> are dependent on multiples of <strong>1,000 bytes</strong>  (rounded up).</p>

<p>The above page cites Gavin Andresen's <a href="https://bitcointalk.org/index.php?topic=219504.msg2311420#msg2311420" rel="nofollow">post on 0.8.2 Release notes</a>:</p>

<blockquote>
  <p>The default fee for low-priority transactions is lowered from 0.0005 BTC
  (<strong><em>for each 1,000 bytes in the transaction</em></strong>; an average transaction is
  about 500 bytes) to 0.0001 BTC. [emphasis mine]</p>
</blockquote>
