user9205: I want to know how I can calculate the profit of bitcoin mining in bitcoins. I am interested in the formula that is used by calculators. For example lets say I have 700mh/s hashrate, current difficulty is 908350862,437. The calculator says I will mine 0.00038755 BTC per day. What algorithm is used to calculate these figures?

user6256: The average time to find a block can be approximated by calculating:
time = difficulty * 2**32 / hashrate

Where difficulty is the current difficulty, hashrate is the number of hashes your miner calculates per second, and time is the average in seconds between the blocks you find. 

user14920: I want to give an extended answer as I found the pre-existing here or elsewhere in need of a bit more clarification, specifically for newcomers.
Formula
In order to calculate the approximated total bitcoin earnings value per month from a mining operation (not taking into consideration mining costs [electricity, hardware maintenance etc...]) the following formula can be used:
H = Hashrate (hashes / second)
D = Difficulty (Reference for values below)
B = Reward per Block (Reference for value below)
N = Number of days per month (default = 30)
S = Number of seconds per day (S = 60 * 60 * 24 = 86400)


Example
H = 21,990,232,555,520 h/s (~= 20TH/s)
D = 47,427,554,950.648
B = 25
N = 30
S = 86400


The total number of bitcoins earned per month as per the defined variables above will be ~6.995. Dismiss N from the numerator and you'll get the daily value.
Python
A quick python statement as an example implementation (you can type this directly in the terminal)
$ python -c "print (30*21990232555520*25*86400)/(47427554950.648*2**32)"
$ 6.99542703277

Some References:
Difficulty (Current Value | Wiki)
Reward per Block (Current and Future values)

9205: <p>I want to know how I can calculate the profit of bitcoin mining in bitcoins. I am interested in the formula that is used by calculators. For example lets say I have 700mh/s hashrate, current difficulty is 908350862,437. The calculator says I will mine 0.00038755 BTC per day. What algorithm is used to calculate these figures?</p>
 6256: <p>The average time to find a block can be approximated by calculating:</p>

<pre><code>time = difficulty * 2**32 / hashrate
</code></pre>

<p>Where difficulty is the current difficulty, hashrate is the number of hashes your miner calculates per second, and time is the average in seconds between the blocks you find. </p>
 14920: <p>I want to give an extended answer as I found the pre-existing here or elsewhere in need of a bit more clarification, specifically for newcomers.</p>

<h2>Formula</h2>

<p>In order to calculate the approximated total bitcoin earnings value per month from a mining operation (not taking into consideration mining costs [electricity, hardware maintenance etc...]) the following formula can be used:</p>

<pre><code>H = Hashrate (hashes / second)
D = Difficulty (Reference for values below)
B = Reward per Block (Reference for value below)
N = Number of days per month (default = 30)
S = Number of seconds per day (S = 60 * 60 * 24 = 86400)
</code></pre>

<p><img src="https://i.stack.imgur.com/aqXbM.png" alt="Mining Earnings Formula"></p>

<h2>Example</h2>

<pre><code>H = 21,990,232,555,520 h/s (~= 20TH/s)
D = 47,427,554,950.648
B = 25
N = 30
S = 86400
</code></pre>

<p><img src="https://i.stack.imgur.com/aQODb.png" alt="Mining Earnings Formula"></p>

<p>The total number of bitcoins earned <em>per month</em> as per the defined variables above will be <strong>~6.995</strong>. Dismiss <code>N</code> from the numerator and you'll get the daily value.</p>

<h2>Python</h2>

<p>A quick python statement as an example implementation (you can type this directly in the terminal)</p>

<pre><code>$ python -c "print (30*21990232555520*25*86400)/(47427554950.648*2**32)"
$ 6.99542703277
</code></pre>

<h2>Some References:</h2>

<p><strong>Difficulty</strong> (<a href="http://blockexplorer.com/q/getdifficulty" rel="noreferrer">Current Value</a> | <a href="https://en.bitcoin.it/wiki/Difficulty" rel="noreferrer">Wiki</a>)</p>

<p><strong>Reward per Block</strong> (<a href="https://en.bitcoin.it/wiki/Controlled_supply" rel="noreferrer">Current and Future values</a>)</p>
