Web/iOS/Android: Most mortgage calculators will tell you how much your monthly payment will be or even how much total interest you'll pay each month compared to the principal. This calculator shows you that, but it includes more detailed information on how much you'll pay over time, too.

Retirement is a long time away and it can seem like the contributions you make don't make a huge difference. However, with a little maths (or this tool) you can see exactly how much another day of freedom costs.

The hotter it is, the slower you run -- but humidity plays a role too. When the air is full of moisture, sweating doesn't cool you down like it usually would. You won't be able to run as fast, because your body is trying to prevent you from overheating.

Google's wildly extensive calculator feature is useful for all sorts of things, and blogger Phil shows us one particularly helpful use: figuring out how long that big download is going to take.

Google’s sense of humour is evident in its many Easter eggs and search tricks, but it pops up elsewhere too. For instance, Google’s online calculator can handle conversions into the never-actually-used-in-the-real-world Potrzebie Unit System, a ludicrous system of weights and measures invented for Mad Magazine. Note: including measurements in Potrzebie units in your next presentation may not guarantee a promotion.

We reckon the calculator in Windows 7 is pretty neat, but it's not to everyone's taste. If you prefer your calculations displayed as you make them, Moffsoft FreeCalc is a pretty handy option.

If you checked out the PSU calculator we shared with you and wished it had a more detailed selection process for really granular and precise control you'll definitely want to check out the eXtreme Power Supply Calculator.

WhatsTheCost is a financial calculator designed to show you how much something really costs you over time whether it's your mortgage, credit card debt, or even your pack-a-day cigarette habit.

Mac OS X only: Mac users envious of Windows 7's new souped-up built-in calculator can take solace in Magic Number Machine, a free, advanced graphic calculator for OS X. Features include 25 digits of precision, hexadecimal, binary, decimal and octal display including fractional, scientific notation and negative numbers in each, complex numbers, array functions, and a whole menu of scientific constants. It's been a long time since I've needed a calculator this advanced and complex, so mathletes and scientists, let us know how it is. The Magic Number Machine is a free download for Mac only. Thanks, enderwiggin7!

Magic Number Machine

Windows Vista only: We took a liking last week to the Windows 7 calculator and its many uses for serious mathletes, as well its templates for calculating mortgages, salaries, and other real-world numbers. Now you can get that same power on your Windows Vista system, thanks to (we presume) one intrepid hacker who pulled it out of the Windows 7 preview. You can run it as a no-install app or make it your system's default calculator, as the Daily Gyan blog explains in the via link below, but you'll want to back up your original calculator first. The files are shared through Box.net at the moment, and may get pulled down if traffic gets heavy (or a certain development firm gets antsy); we'd appreciate any alternate links in the comments. The Windows 7 Calculator is a free download for Windows Vista systems only. For a deeper look at other Windows 7 utilities, check out our Top 10 Things to Look Forward to in Windows 7.

Windows 7 Calculator

While mathletes, scientists, coders, and statisticians will appreciate Windows 7's built-in calculator's programmer, statistics, and scientific modes, everyday people will love figuring out things like hourly wages and mortgage payments without a spreadsheet. Yes, it may seem ridiculous to give more than a few lines of attention to something as minor as a calculator, but until you've seen the built-in calculator in the Windows 7 Preview, you've haven't known pure calculator bliss. This default calculator goes above and beyond the brick on your desk by including unit conversions, date calculations, and a neat new set of "templates" that let you do things like figure out gas mileage, hourly wages, mortgage payments, leases, and more. Check out some of the calculator goodness in the Windows 7 Preview.

DIYer RazorG illustrates how to make a cheap metal detector using an old AM radio, tape, and a calculator. Tune your AM radio with the volume on maximum onto the high end of the frequency but not directly on a broadcast station. Once you can hear the static from the AM radio, position the radio and the calculator (turned on) close together until you hear a loud tone. Then, tape the units together securely. You now have a working metal detector which you can experiment with by putting it near silverware or other piece of metal. This works, RazorG says, because the loud tone that comes from the radio is the calculator's electronic circuit board which produces a radio frequency signal. The radio waves from the calculator bounce off the spoon and are heard on the radio's speakers. Turn a Calculator into a Metal Detector

Mapquest has added a new feature on its Gas Prices page that pops up a small, simple calculator that can help you figure out how much it will cost to, say, visit your in-laws. Gas cost calculators are, of course, nothing all that new, but Mapquest's tool sits on the same page as its local gas price finder, eliminating at least one step in the process. Mapquest also added alternative fuel stations to its local gas listings for those rocking a diesel, bio-diesel, or other eco-conscious wheels.

Mac OS X Leopard only: Run quick calculations from Leopard's Spotlight search box. Enter your formula and the answer will appear as you type, similar to how it works in Firefox's Google search box. Google's implementation is a tad better; it can do metric conversion (try 24km in miles), but Spotlight's good for pure math.

Spotlight