Matthew Rothman bought an HP 12c financial calculator for his first job out of college in 1989.

Years later, he still has the same calculator. And he still uses it constantly, just like thousands of other 12c enthusiasts.

"Whenever I switch jobs, I just peel the old business card that is on the back and tape my newest one on," says Mr. Rothman, head of quantitative equity strategies at Barclays Capital in New York.

Sales of the device, which debuted in 1981, haven't slipped even after its manufacturer, Hewlett-Packard Co. , introduced more-advanced devices or even, two years ago, a 12c iPhone application, which replicates all the calculator's functions, the company says.

"Once you learned it on the 12c, there was no need to change," says David Carter, chief investment officer of New York wealth-management firm Lenox Advisors, who has owned his 12c for 22 years and still keeps it on his desk. "It's not like the math was changing."