the first of the woodstocks, the hp-21 replaced the hp-35 as the basic scientific model in 1975.

like all the woostocks, it has a nice shape and size. it starts up displaying two decimal places by default which can be changed up to 8+2. a switch selects between degrees or radians. notice that the shifted functions are written on the key slope rather than on the faceplate and that the four basic operators are on the left side of the keyboard. later all calculators gravitated towards the, now normal, + - * / on the right.

your hp-21 is another professional-quality pocket instrument from the hewlett-packard line of calculators - calculators whose durability, small size, and ease of operation have made them the choice for use by climbers on the frigid crags of mt. everest and astronauts orbiting the vast depths of space, as well as doctors, engineers, scientists, and other people who require instant, accurate answers to complex and highly technical problems. you're in good company with hp!

can you calculate?

the manual contains many fascinating example problems. here's one of them

having lost most of his equipment in a blinding snowstorm, ace explorer buford eugobanks is using an ordinary barometer as an altimeter. after measuring the sea level pressure (30" of mercury) he climbs until the barometer indicates 9.4" mercury. although the exact relationship of pressure and altitude is a function of many factors, eugobanks knows than an approximation is given by the formula: alt (feet) = 25,000 ln(30/pressure).
where is buford eugobanks?

it comes with a splendid owner's manual and fur lined protective zipper case.