A calculator developed by a 12-year-old girl and her Irish father has become the number one-rated app worldwide on Amazon’s Android App Store.

Calculator Plus, invented by Isabel Hughes and her father, Dublin native Aidan, has just become the highest-rated app in all categories out of a total of 83,981 apps on Amazon’s Android App Store

It is the best-selling calculator app for the Kindle Fire tablet in the US and Europe – and has been downloaded over 2.5 million times. It is also the most downloaded calculator on the Windows 8 platform.

And the father and daughter duo have a second reason to celebrate as their ground-breaking Fraction Calculator Plus app has been named as Amazon’s Free App of the Day in both the US and Europe on Monday May 20.

The Fraction Calculator app has two great features not found elsewhere: An innovative three-keypad layout to enter fractions and a large, easy to read, display that shows fractions the way you write them.

It is the number one selling Fraction Calculator on Android, Kindle Fire, iPhone, iPad and Barnes and Noble Nook tablet in the US.

Isabel came up with the idea for the chart-topping Fraction Calculator Plus while doing her mathematics homework after school with Aidan.

“We had a sheet of fraction problems to solve and it was taking forever, so I wondered if there was a good calculator for fractions available in the App Store,” said Isabel, who lives with her dad in Seattle, Washington.

“It turns out that there wasn’t so we sat down and made one that looked as realistic as possible. I never would have believed it would become a hit all over the world.

“All sorts of people have found a use for the fraction calculator – we have even been thanked by quilt makers who say that it has made their job much easier!”

Aidan, originally from Raheny in Dublin, is a former Microsoft software developer, and now heads up Digitalchemy, who have also had a major hit with Calculator Plus.

USA Today recently named Calculator Plus as one of their 25 essential Kindle Fire apps and Amazon have pre-installed it in their shop demo models across the US.

“The beauty of Calculator Plus app is that it remembers everything you calculate, which makes it perfect for running a bunch of tasks in tandem over a long period of time,” said Aidan

“Again it came from Isabel and I bemoaning the fact that you lose everything when you turn a calculator off.

“Isabel has always had an intuition for seeing what people actually want in programs.

“When she was nine she brought an idea to the developers of Microsoft’s Bing, and it was adopted into the search engine interface.”

Aidan and Isabel have launched other apps, including Calculator Free, the best-selling Calculator app for Windows 8.