Today, a calculator is a total commodity, built for free into virtually every handheld digital device. In 1973 when Olivetti released the Divisumma 18, personal calculators were just at the dawn of becoming truly handheld and digital (HP launched the groundbreaking HP-35 calculator in 1972). But the standard aesthetic was very much 70's technical - black, angular, shiny plastic.

In keeping with Olivetti's ethos of humanizing technology, the Divisumma 18 had a completely different take on what a calculator should look like. It was designed by Italian designer and architect Mario Bellini, who, over the time of his relationship with Olivetti from 1963 - 1991, designed numerous office products for the firm.

After having seen it only in photos, I was surprised at how large it is in person once I acquired one. Today we think of calculators as thin, light, cheap and disposable. The 18 was anything but! See it here in comparison to a 5x9" Moleskine and a pen for scale. It's not exactly a hand-held device...more like a laptop calculator.

There was no digital display - all output was achieved with a thermal printer. Unlike most printing calculators where the paper roll hangs out back from the body like a lifeboat, the roll in the Divisumma 18 was fully integrated, keeping the overall form pure. Further adding to its length and heft was a detachable rechargeable battery pack so that the calculator could become even more portable and not be tied down by a power cord.

Battery pack hinges to detach

Divisumma with battery pack removed

The rubber membrane stopped just to the right of the printer area. The printer section contained the roll of paper exposed under an acrylic window, a slider on/off switch, and a 4-position wheel to set the number of decimal places used in calculations. The scallop cut-out around the decimal wheel is beautifully sculpted.

On/off slider switch, and decimal places selector wheel

The Divisumma 18 is famous in industrial design circles for its "volcano" keys - cylindrical keys that blend smoothly into the surface below - a detail that has been called "the most influential button design ever" and has been borrowed liberally ever since. It's something that is so simple, yet required an innovative process to create: a rubber surface that is almost like a stretched skin, uniformly covering the keys and the entire top surface (down to the part line running through the middle).

(Bellini used the same key design and rubber membrane on the desktop version, the more subdued black and gray Divisumma 28 who's angled face is similar to his Yamaha cassette deck we have featured. It proved too tricky and expensive to continue with on other products, sadly.)

Despite the sleek appearance of its keys, they clicked with a solid mechanical noise and feel. Underneath that rubber membrane were old-fashioned mechanical micro-switches (see here for many photos of the internals). The keys had a very light touch so they were easy to operate quickly (they were 9mm in diameter on 18mm center-to-center spacing - which if I recall correctly is the same spacing that AT&T decided on for its old telephones after extensive human factors testing), and the rubber membrane deformed slightly as you pushed each button down.

Due to this innovative rubber top-housing, many current samples of the 18 are in poor shape. UV rays have hardened, cracked or faded the rubber, and most you find on eBay are worse for wear. I was very lucky to find this virtually mint example, complete with all packaging and instructions, and a few extra thermal silver printing rolls. While it does turn on, it doesn't fully operate. That's too bad, but for me I mostly enjoy it for its tangible qualities, not its calculating ones.

What gives it soul?

Despite being 40+ years old at this point and by today's standards very crude for what it does, the Olivetti Divisumma 18 is a product that continues to make everyone who sees it smile and want to touch it.

Both the top and bottom surfaces have sculpting that is pleasurable for the fingers. Bellini explicitly wanted the calculator to be anthropomorphic, making the technical device feel more familiar and comfortable, less alien. The 1970's were a period of mass adoption of increasing levels of technology, and as with so many products that Olivetti produced, Bellini sought to put the person back in control by taking away the mystery of the product. As with the Valentine, both the product design and its advertising positioned the calculator as freeing you from the hum-drum of office life.

In his article on the history of Olivetti calculators, Larry Gilbert observes, "Olivetti clearly intended to market this to a new breed of consumer. Advertising images showed a young businessman with modish hair walking in the open air under a blue sky clasping one of these units in his hand, the vivid yellow contrasting strongly with the dark suit. The Divisumma 18 is a true period icon, appearing in many books that discuss landmark 20th century designs."

In 1987, the Museum of Modern Art held a retrospective of Bellini's work, and writing about the Divisumma 18 in the catalog, Cara McCarty wrote:

 
“Because the calculator is portable and intended to be hand-held, Bellini personalized it through an inventive use of form and materials. What is especially intriguing is the continuous, flexible, rubber-skin keyboard. The skin, which protects the machine from dust, is anthropomorphically suggestive. Articulated pushbuttons, covered with the soft rubber skin, are like nipples. The emphasis is not on calculating and power, but on stimulating a sense of pleasure. Emotive responses are not usually associated with adding machines, yet this is an irresistible artifact. One cannot help but want to hold it, touch it, play with it.”
— Cara McCarty
 

Packaging and Manual

We've got an extensive look at the packaging and the manual for the Divisumma 18. Take a look >

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