How do I convert from megabytes to “metric” megabytes?

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What is the mathematical formula to convert binary bytes into metric bytes, and vice versa?

The reason I ask this is because I can’t seem to develop the equation myself. I know that 1 metric kilobyte=1000 bytes, and 1 binary kilobyte=1024 bytes, but even knowing this, my equations don’t seem to be correct, according to online conversion sites. I wish to know this equation as unfortunately I can’t find any conversion app for Windows Phone 8 that will convert the two, and wish to have it handy whenever I need to calculate how much storage there is. For instance, all advertized sizes for hard drives and other devices are in metric bytes, for some reason. However, the actual disk space you have to store files on is less then that, since computers use binary bytes. Therefore, it’s good to have a mathematical equation in order to convert metric storage over to binary storage, and see how much space you actually have to use. So if there are any of you that are good at math and computer storage, I’d appreciate the help!