I don't own a smartphone, and while I do have a calculator somewhere, I almost never use it. What I do have, and use, is a slide rule. Actually, a couple of them.

The web is full of pages about the history of slide rules, and there are all kinds of complicated instructions for using them to find cotangents and so on. But... I never have to do that kind of stuff. Sine? Cosine? Literally never needed to solve one since graduating high school.

But what I do sometimes need to do is square or cube stuff, or find the square or cube roots of things. And slide rules are very useful for unit conversions, especially when you're doing multiple conversions between the same two units, like liters and gallons.

A lot of nicer slide rules came with cheat sheets for this kind of thing on their back sides, but most didn't. So here's a list of some of the things I need to refer to regularly, taken from a couple different rules I own.

To multiply: put index on C scale over first factor on D scale; read product *on D* under second factor on C. Example, 3.2x2.5 - set left index on C to 3.2 on D; read answer (8) on D scale under 2.5 on C.

To divide: put divisor on C scale over dividend on D scale; read quotient *on D* under index on C. Example, 3.2/2.5 - set 2.5 on C over 3.2 on D, read answer (1.28) on D scale under left index of C.

(from a Pickett N902-ES, a very nice metal slide rule)

Squares: with scales zero'd, number at cursor on D is squared at A. (Note that A is 1-100). Square root is of course the inverse.

Cubes: with scales zero'd, number at cursor on D is cubed at K.

Cube roots: the inverse of cubes, except that K has three scales, for numbers of one, two, or three digits. For larger numbers, calculate length modulo 3.

Reciprocals (i.e. 1/n): n on CI (note it counts right-to-left); reciprocal is at C.

Conversions:

(these all use C/D scales, interchangeably)

1 inch = 2.54 cm, setting 1 opposite 2.54

1 foot = 3.048 m, setting 292 opposite 89

1 yard = 0.9144 m, setting 187 opposite 171

1 meter = 39.37 in, setting 95 opposite 374

1 mile = 1.609 km, setting 87 opposite 140

1 square inch = 6.452 square cm, setting 31 opposite 200

1 square foot = .0929 square m, setting 155 opposite 144

1 square yard = 0.8361 square m, setting 61 opposite 51

1 square mile = 2.590 square km, setting 27.8 opposite 72

1 us gallon = 3.785 liters, setting 14 opposite 53

1 kilogram = 2.205 lbs, setting 176 opposite 388

(from a WWII Dietzgen 1769P)

Diameter of a circle to circumference = 226 opposite 710

Side of a square to diagonal of the square = 70 opposite 99

1 horsepower = 0.746 kilowatt, setting 1 opposite 746

(from a Post 1447, my favorite slide rule because they're common, cheap, high quality, and bamboo)

*back up to the main page*

...and if you for some strange reason have an interest in pre-calculator math, someone else has written an excellent guide to log tables, which, no, are not a type of furniture.