Unless you've got a Frieden calculator or a super-brain, if you need to do some complex math, you need a slide rule.  The "slip-stick" has been the inveterate friend of mathematicians, physicists, and engineers since the late 19th Century.  Essentially an analog computer, the slide rule can help you do basic multiplication and division, solve trigonometric functions, determine logarithms and powers, and compute square roots.

My personal slide rule is perhaps the most robust of them all, with 27 scales (31 if you count the scales with two sets of markings).

Roots, powers, and multiplication

And here's the other side:

Logarithms!

Note Pickett's distinctive "eye saver" yellow coloring. This thing is a brick, and because the leather has already eroded some, I've gotten a custom belt holster.  I feel like Marshal Dillon carrying this thing around.

I'll confess — I don't know how to use half of the functions on this thing. I'm more apt to consult a book of logs than use a slide rule (I joke that slide rules are great if you want to multiply 4 x 3 and get 11.9.)

If you want to learn how to use the slip-stick, there are books:

Good luck!