One of the original selling points of Amazon's Kindle was that the device weighed no more than the average paperback. In the brave new world of the e-reader, bibliophiles could load their gadgets with the complete works of Proust, Tolstoy and Dickens without fear of spraining their wrists on their way to work.

So imagine the consternation among gadget fans when it emerged this week that the Kindle actually weighs more when it is fully loaded with books.

John Kubiatowicz, a computer scientist at the University of California, Berkeley, tackled this vital question for the New York Times. He explained that e-readers store data by trapping electrons, and while the number of electrons in the gadget's memory does not change, it takes more energy to hold them in place than to leave them roaming free. How much more energy? Around a billionth of a microjoule for each bit of data stored.

Working from Einstein's famous equation, which states that energy and mass are equivalent, Kubiatowicz worked out how much the weight of a Kindle might change as the books built up. He compared an empty four-gigabyte Kindle with a full one, in which half the electrons were trapped, requiring an extra 17 microjoules of energy.

Popped into Einstein's formula, this gives an answer of around one attogram, meaning the weight of a full Kindle was a billionth of a billionth of a gram more than a factory-fresh one. Which isn't so bad, considering that 10,000 books – a fraction a Kindle can hold – might weigh five tonnes. An attogram, very roughly, is one tenth the weight of a small virus.

From the world of academia come calming words for anyone alarmed at the finding. "If you left your Kindle on a sunlit windowsill you'd probably absorb more energy, and hence gain weight," says Richard Jones, professor of computer systems at the University of Kent.

"If Professor Kubiatowicz is really struggling with the extra weight, he's welcome to come to Edinburgh where it's cooler, and the lack of thermal energy in his Kindle will more than compensate," adds Graeme Ackland, professor of computer simulation at the University of Edinburgh.