If you're like me and do not have your binary, hex, and decimal conversions committed to memory yet, you will find yourself using this trick often.  I usually use Google to evaluate expressions like "sqrt(457)" or "934*pi-3.678" quickly, but I've found that it is just not smart enough to evaluate things like "10100101 from binary to decimal" or "10100101 from base two to base ten" on its own.

For this I use Wolfram Alpha.  Wolfram Alpha is a web-based tool developed by the same people who made Mathematica that does a great job of figuring out what information you're searching for and delivering the answer.  Try out the following:

For conversions from binary:
type the binary number followed by "from binary to decimal," "from base 2 to base 10," "from binary to hex," or "from base 2 to base 16" (figs 1 and 2)

Both Google and Wolfram Alpha support the following searches:

From decimal:
type a number in decimal followed by "to binary," "to base 2," "to hex," or "to base 16" (figs 3 and 4)

By using the prefix "0x" in front of your hex number, Wolfram Alpha and Google will understand that it is in hexadecimal.  Follow it with "to decimal" or "to binary"  (figs 5 and 6)

I'll also add that Wolfram Alpha is much better than Google for evaluating complex numerical expressions because it reformats your search term in a way that is very easy to read, this way you can check to see if you've made a mistake.  Google only returns your search as a long string with a ton of parentheses. (figs 7 and 8)

The problem is that if you are using this all the time, you will never learn how to do it for yourself. If you will need to know how to do this, learn how to do it first, and then use the calculator to save time. (And yes, Windows Calc does great as well). Personally I designed my own binary convertor so that I knew that I could do the conversions on my own, and then I started using that to save time. But these conversions really are not that hard to do... so I would strongly suggest learning how to do them by hand before using this tool.
yeah but even if you know how to do the conversions, it's nice to use this to double check. it's really easy to make mistakes when you're dealing with a lot of digits.
Now that I will agree with! If you just want to check yourself then the converters are great! :)
Windows calculator does the same thing with much less typing
That's the great tool !! :D