I'm sure you've thought to yourself, if you've ever thought about mining on older machines, especially ones without a dedicated graphics card, can I make a profit with the old PCs and laptops (possibly even tablets and phones) lying around my house?
Short answer: no, unless you live in a country with ridiculously cheap power prices and don't care about your hardware deteriorating.
Long answer: I've researched this topic extensively and here are my findings. If you plan to mine crypto currency on a CPU (which is the only thing these devices would be able to mine with), you will always be in the red (e.g. losing money), on any site that calculates profits based on your hashrate and power consumption.
But why is that?
Problem 1: A huge problem is the amount of power the device would need in order to run vs. the price you pay for electricity. I used the price of 0.3$ per kilowatt hour (which is slightly below average for the German consumer). Germany is notorious for having very high electricity prices even in Europe, so feel free to research your own power price (for example Canada has one of the lowest with 0.1$ per hour, third-world countries have power prices so low you will probably make a profit if you mine with a potato connected to the internet.) The easiest way to do this is just find your last power bill and look up the price there (if you pay your electricity yourself).
Problem 2: Coins you can mine with a CPU are very limited and not very profitable to mine. You can look up "mine cryptocurrency CPU" with your favourite search engine yourself, but these days, the most "profitable" (if you can call it that) coin is Monero (XMR), which you can mine merged with Fantomcoin (FCN) for the highest profit (the architecture of these coins allow you to mine two coins at once, although FCN has tiny profits and barely anyone uses it). Monero is the 8th highest rated coin on Coin Gecko, but it would take years for you to mine even 1 unit of the currency, most of the time you will be looking at profits of fractions of a fraction (e.g. 0.0033951 XMR for a USD). Fanctomcoin is 442nd on Coin Gecko. For a dollar you get 4.32 Fantomcoins. I don't think I have to say anything more about that.
Problem 3: The absolute biggest problem with CPU mining: hash rates, hash rates, hash rates. Simply put, your CPU sucks big time at mining compared to even the worst performing graphics cards (from now on GPUs) today. My NVIDIA Geforce 1060 3GB has a hash rate of 22 Mh/s (Megahashes, 1 MH/s is 1,000,000 hashes). The CPU of my Lenovo T510 I could use to mine on the side has a hash rate of measly 35 hashes per second. That means my GPU is six-hundred-twenty-eight-thousand times more effective than my CPU at mining (22 million divided by 35).
If you're not good at maths (I'm not) that is a whole lot. Imagine you, a single person, hitting the wall in a mine with your little pick versus the entire country of Montenegro (population: 622,387) slaving away in a coal-mine. We're talking that big of a difference here.
Here's some evidence: I used the program Miner Gate (I'm not linking you to their site because this is supposed to make you want to not mine with your CPU) to test this on the before mentioned Lenovo T510 (Intel® Core™ i5 CPU M 560 @ 2.67GHz × 4) on Ubuntu 16.04 LTS (since it has a built in power monitor that can tell me the wattage I'm using). These are my results (take into consideration that Firefox and the screenshot software were running alongside, but those only impact a few hashes. I'm not getting over 40 H/s on this thing even if I turn everything else off).
Conclusion: for the average person living in the Western Hemnisphere mining with your CPU simply isn't worth it. With my equipment I would lose 5$ per month, and even if you believe that the value of the coins mined will go up, that is simply not worth it for the time, energy and money invested (not to talk about what it will do to your CPU if you run it at 100% 24/7 for a month).
Either buy a GPU and mine with your desktop or just invest into cryptos and hope the price will go up. Mining with laptops, tablets or phones (the latter two I have tested before and gotten even worse hashrate to wattage ratio, so I thought they were not worth mentioning). is demonstrably not worth it and will just ruin your hardware while losing you money.
I am not affiliated with any service mentioned above and am not part of any agenda, before people accuse me of dragging CPU mining through the mud for my own personal gain or anything like that. These are simply observations and findings of a hobby crypto enthusiast.