HP-21

{{short description|Scientific calculator by Hewlett-Packard}}

{{For|the HP-20S variant|HP-21S}}

{{for|the planned glider|Schreder HP-21}}

{{Use dmy dates|date=April 2019|cs1-dates=y}}

File:Taschenrechner HP-21.jpg

The HP-21 was a scientific calculator produced by Hewlett-Packard between 1975 and 1978.{{cite web|url=http://www.hpmuseum.org/hp21.htm|title=HP-21|first=David G.|last=Hicks|publisher=Museum of HP Calculators|accessdate=2014-07-12}} It was designed as a replacement for the HP-35, and was one of a set of three calculators, the others being the HP-22 and HP-25, which were similarly built but aimed at different markets.{{cite web|url=http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-11.pdf|title=Hewlett-Packard Journal, November 1975|publisher=Hewlett-Packard|date=November 1975|accessdate=2014-07-12|archive-date=2017-06-10 |archive-url=https://web.archive.org/web/20170610140406/http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-11.pdf|url-status=dead}}

As with most HP calculators then and now, the HP-21 used RPN entry logic, with a four-level stack. It also had a single user-accessible memory register. As was normal at the time, memory was not preserved on power-down. A physical slider switch toggled between degrees and radians modes, which was an unusual feature. It had a 12-digit LED display, which was less than the 15 digits of the HP-35. Because of these fewer digits the HP-21 (and similar calculators such as the HP-25) could display 10-digit floating point numbers but only an 8-digit mantissa with a 2-digit exponent when scientific notation was used. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.

Its HP development codename was Pumpkin,{{cite web|url=http://www.keesvandersanden.nl/calculators/hp21.php|title=The HP-21 (Pumpkin)|first=Kees|last=van der Sanden|accessdate=2014-07-12}} and it was a member of the Woodstock series. Its US price was $125 in 1975, $80 in 1978.{{cite web|url=http://www.thimet.de/CalcCollection/Calculators/HP-21/Contents.htm|title=Hewlett Packard HP-21|first=Tony|last=Thimet|accessdate=2014-07-12}}

{{anchor|21 GD}}A version adapted to support an additional backward-facing display manufactured by Educational Calculator Devices named EduCALC 21 GD existed as well.{{cite magazine |title=The EduCALC |magazine=65 Notes |date=February–March 1976 |volume=3 |number=2 |page=8 |url=https://www.hpcalc.org/hp48/docs/columns/edu-calc.html |access-date=2022-07-01 |url-status=live |archive-url=https://web.archive.org/web/20170704064503/http://www.hpcalc.org/hp48/docs/columns/edu-calc.html |archive-date=2017-07-04 |via=HP Museum}}