STOCKHOLM, Sweden — There's no question the Swedish Treskilling Yellow is the world's most valuable stamp. The last time it was on the market it sold for a healthy $1.9 million US ($2.6 million Cdn).

But Swedish researchers said Monday the stamp is worth even more if one goes by weight.

The Swedish National Testing and Research Institute measured for the first time the stamp's exact mass, volume and density and worked out its value per kilogram.

Since the stamp itself weighs just 0.03 grams (or 0.0009 ounces), and using the purchase price as the measure, then the stamp is worth a whopping 562 billion kronor ($102.1 billion Cdn) per kilogram.

But that's not to say the stamp is actually going to be sold for that sort of money. Philatelists base a stamp's value on its rarity, colour and condition, not its weight, real or projected.

But Zurich-based Stamp Collection AG commissioned the research as part of a book it is publishing about the stamp's history and it different owners through the years.

The book, The Three-skilling Yellow, is expected to be released in Europe and North America in December.

"The stamp is a true legend. The book will help us bring an important part of the Swedish national heritage to audiences all over the world," Hans Lernestaal, the company's chief executive said Monday.

To calculate the stamp's exact area, an optical microscope examined an eight-skilling stamp that is the same size as the Treskilling Yellow.

"It would have been too risky to use the original. Instead we used another stamp, an eight-skilling banco from the same printing series, which has the same physical properties as the original," said Mats Lidbeck, who oversaw the measuring at the Swedish institute.

The three-skilling denomination series was Sweden's first stamp issue and was normally printed in a greenish colour. A skilling was the hundredth of the daler, Sweden's former monetary unit.

The stamp, the same size as other modern day stamps, has been tested chemically to determine that a printing error caused the different colouring.

Printed in 1855, it was used on an envelope two years later.

A Swedish boy discovered the stamp in 1885 in a pile of letters left by his grandparents. Over the years, it has since changed hands several times with its value increasing steadily.

In 1886, a stamp dealer in the Swedish capital Stockholm, bought it for seven kronor. In November 1996, it was sold at an auction in Switzerland for 2.5 million Swiss francs ($2.7 million Cdn). Two years later it was sold to an anonymous buyer for $1.9 million US ($2.6 million Cdn).

The stamp was last displayed in public in 2001 in Arizona, but the owner hasn't made it available for public viewing since.