When the iPad came out there were a few Apps that Apple didn't port over. For example, on the iPhone and iPod touch you have a built-in Weather App, Stocks App and a Calculator. For whatever reason Apple chose to leave those Apps off the iPad and needless to say 3rd party developers didn't waste any time filling the void. There are several weather apps for the iPad. There are also several calculator Apps available. I've gone through three so far. Keep in mind that there hasn't been much in terms of revolutionary features of a calculator in the last 20 years. They do what they do. So why go through three of them? Since they all pretty much do the same basic things, I'm looking for aesthetics and bells and whistles that go over the top. I liked my first two because they filled the screen. The second one I tried had a multiple skins that you could choose from depending your color choices and lighting conditions.
Why I like Calculator HD for iPad
I like this one because you can not only configure which calculator you get in portrait and landscape orientations (basic, scientific, basic with tape, etc.), but you also get the added benefit of a electronic "paper" tape. When adding a list of numbers an on screen tape is a handy way to know that you didn't miss anything. While there's also a Note Pad mode that puts a scratch pad to your right, I probably wont' use that feature much. It's nice to have though. The only thing I would like to see is the ability to have the calculator fill the screen. There is a lot of fluff around the edges with pretty pictures and while I do appreciate pretty pictures that space could be used to make the whole thing bigger (as the other calculators I have do) and therefore making the buttons easier to press and the display easier to read. Don't get me wrong, the current interface certainly isn't small, but it could be bigger is all I'm sayin'.
You can get Calculator HD for iPad here from the 
I think the notepad will really be useful when doing complex calculations that require many steps. I often get half way through a calculation and then forget where I was, and have to start all over again. The notebook enables you to write down various answers as you go along. I would definitely prefer the notebook on the side than a full screen calculator.
Why not a full screen calculator with a notepad on the side? My issue is not with the notepad, but with the pictures of the pen, eraser, etc. A simple zoom button for those times when I want the whole calculator (notepad and all) to fill the screen is what I’m asking for.
I don’t understand why seemingly every iOS calculator is compelled to make calculator apps look like real-world plastic calculators. The faux desktop thing is a pointless waste of screen space. The whole trend is just lazy “design”.
Check out Digits Calculator. It uses the full screen, has a tape function, which you can also email. It’s a simple, clean, elegant no nonsense design.
It’s $1.99 and has the added advantage of being a universal app!
http://itunes.apple.com/app/digits-calculator-for-ipad/id364500115?mt=8#
I’ve gone through a few calculator apps myself, and I love the paper tape feature on this one. I’d love to see one with simple fractions to help with my fourth graders homework.
No calculator on your iPad? Style it up with the Olivetti inspired Red Valentine
Olivetti designed many masterpieces in industry, but there is no question that the most iconic was the bright red colored Valentine portable typewriter released in 1969.
At that time there was no such concept as the mobile office. When the Valentine typewriter hit the streets, people rushed the stores to buy one. Suddenly it was the in thing. Light, fashionable and portable, you would see young romantics writing letters with their red Valentine’s in parks, out in the countryside, and from cafes, anywhere but in an office.
The Valentine became a star in the popular media when Richard Burton was photographed ariving at Heathrow Airport with the typewriter on one arm, and his wife Elizabeth Taylor on the other.
Mobile offices are now everywhere. Add a touch of retro 1970’s glamour to yours with this Red Valentine Calculator – a tribute to the original typewriter masterpiece by Ettore Sottsass.
View in App Store:
http://itunes.apple.com/us/app/red-valentine/id446995310?mt=8
I would recommend another one: FastCalc (or FastCalc free) because you can see the whole expression you are typing.
You can also change the colors of the display and the keyboard.