sliderule. Also, abacus.
You might also be able to rent an idiot savant to do calculations.
I got my first TI calc back in the 70's. Still have it and it's still very pretty. Of course, it's not an HP 12C. I just did a search and it seems HP still makes those
I still have my Casio fx-78 10 DIGIT Scientific Calculator...
It always sits on my night stand. It's the smallest calculator I remember seeing. 2.1" x 3.6" x 0.2"
Bought it about 1980 at ASUC store in Berkeley. I asked the guy how long the battery would last. He said "2 years." I have never changed the battery. The LCD display is a bit weak but still readable. It has a side slider on--off switch, but auto-off too.
I had a relatively bulky calculator before getting the Casio, got it around 1975, it was before they got the miniaturization going. Of course it was a wonder. I took a numerical analysis math course at Cal about that time, so I understood how this stuff was done using polynomial series. Interestingly, I didn't recall during the course that mention was made of the newly developing market for hand calculators, but I realized after the course concluded that what I had studied was the key to their functionality, which was kinda neat.
This section explains how calculators and computers work out values of sine, cosine, tangent, etc.
www.intmath.com