user32399: Let's say I have four computers, each mining bitcoins at 5 Mhash/s. Is this the same as mining bitcoins on one computer with a speed of 20 Mhash/s?

user1611: Yes. More generally though:
x mining machines with an average hashrate of y hashes per second will have the same chance of finding an arbitrarily low blockhash in a period of time as one mining machine with a hashrate of xy hashes per second in the same length of time.

user208: Yes, and that'll net you around 0.01 USD worth of BTC per year (as of January 2016), assuming your power is free.

user141: Yes, the hashrate adds up.
But GPU mining ended in 2013. The tiny bitdust you will earn will never reach a mining pool's minimum cash out limit. So you will never see any bitcoin. This will be a pure loss consisting of spent electricity and worn-out hardware.
You could buy a USB stick for mining, for 10 USD. It would mine at a loss due to electricity costs. Even if you have free electricity it would never earn back the 10 USD. So this would also be a loss.
The only way to mine now is with modern stand-alone miners. They have a much higher hashrate and efficiency and they use a tiny built-in computer instead of connecting to your PC. You'll want to get one that runs 1 TH/s (1000 GH/s) or faster. You will not want to pay more than 0.30 USD per GH/s for a miner with power supply. You will need cheap electricity.

32399: <p>Let's say I have four computers, each mining bitcoins at 5 Mhash/s. Is this the same as mining bitcoins on one computer with a speed of 20 Mhash/s?</p>
 1611: <p>Yes. More generally though:</p>

<p><em>x</em> mining machines with an average hashrate of <em>y</em> hashes per second will have the same chance of finding an arbitrarily low blockhash in a period of time as one mining machine with a hashrate of <em>xy</em> hashes per second in the same length of time.</p>
 208: <p>Yes, and that'll net you around 0.01 USD worth of BTC per year (as of January 2016), assuming your power is free.</p>
 141: <p>Yes, the hashrate adds up.</p>

<p>But GPU mining ended in 2013. The tiny bitdust you will earn will never reach a mining pool's minimum cash out limit. So you will never see any bitcoin. This will be a pure loss consisting of spent electricity and worn-out hardware.</p>

<p>You could buy a USB stick for mining, for 10 USD. It would mine at a loss due to electricity costs. Even if you have free electricity it would never earn back the 10 USD. So this would also be a loss.</p>

<p>The only way to mine now is with modern stand-alone miners. They have a much higher hashrate and efficiency and they use a tiny built-in computer instead of connecting to your PC. You'll want to get one that runs 1 TH/s (1000 GH/s) or faster. You will not want to pay more than 0.30 USD per GH/s for a miner with power supply. You will need cheap electricity.</p>
