How a slide rule works

A slide rule is a hand-operated mechanical analog computer that performs multiplication, division, roots, logarithms and trigonometry by aligning adjacent logarithmic scales. Invented in the 1620s, it was the everyday calculating tool of engineers and scientists until pocket electronic calculators replaced it in the 1970s.

Maker: Keuffel & Esser Co., N.Y.
Model: 4081-3 Log Log Duplex Decitrig
Serial: 157122
Made: 1939 to 1947
Owner: Milt Berg

Hairline reading

on the bodyon the slideType a reading to move the hairline to it.

Work your own equation

Try

What you can type

× and ÷ as * and /, chained as long as you like: 2*3/4
Powers with ^: 4^3, 1.05^20, 2^-3, e^1.5
Roots: sqrt(50), cbrt(125)
Angles in degrees: sin(30), cos(60), tan(35)
Logarithms: log(250) for base 10, ln(5) for base e, and exp(1.5)
Constants pi and e, and brackets to group: 2*pi*sqrt(3/8)
No + or −. A slide rule cannot add or subtract. Engineers did that part on paper.

Scales

Scale	Where	Function
C	Slide, side 1	The main multiplying scale, used against D. The letter is just its position: slide rules once had four scales, lettered A to D from top to bottom.
D	Body, both sides	The main fixed scale, where most answers are read. The fourth of the original A to D.
CI	Slide, side 1	C Inverted: 1 ÷ C. Reads right to left.
CF	Slide, side 1	C Folded: C shifted along by π. Avoids resetting when an answer runs off the end.
DF	Body, side 1	D Folded: D shifted along by π. Reads D × π directly.
CIF	Slide, side 1	C Inverted Folded: 1 ÷ CF.
L	Body, side 1	Logarithm: the common logarithm of D, on an evenly spaced scale.
LL1	Body, side 1	Log Log, first section: powers of numbers from 1.01 to 1.105.
LL2	Body, side 1	Log Log, second section: powers of numbers from 1.105 to e (2.718).
LL3	Body, side 1	Log Log, third section: powers of numbers from e to about 22,000.
A	Body, side 2	D squared. The first of the original A to D.
B	Slide, side 2	C squared, used against A. The second of the original A to D.
K	Body, side 2	D cubed. The letter is usually traced to Kubus, German for cube.
DI	Body, side 2	D Inverted: 1 ÷ D.
S	Slide, side 2	Sine: sines of 5.7° to 90°. Red numbers give cosines.
T	Slide, side 2	Tangent: tangents of 5.7° to 45°. Red numbers give cotangents.
ST	Slide, side 2	Sine and Tangent: sines and tangents of 0.57° to 5.7°.
LL0	Body, side 2	Log Log, just below 1: powers of numbers from 0.999 down to 0.905. Used with A.
LL00	Body, side 2	Log Log, further below 1: powers of numbers from 0.905 down to 0.00005. Used with A.