Why Slide Rules?

Since the electronics revolution of the late 1970’s quick numerical calculations have been performed with pocket calculators or, in today’s 21st century environment, perhaps through “calculator” apps on one’s laptop, phone, or mobile device. A basic modern calculator will have keys for entering numbers and for performing functions such as addition and subtraction, multiplication and division, and perhaps squares and square roots and percentages. Some of the more sophisticated calculators and applications have functions such as cubes and cube roots, reciprocals, exponentials and numbers raised to arbitrary powers or arbitrary roots, logarithms, trigonometric functions such as sine, cosine, and tangent, and maybe more. With the exception of basic addition and subtraction, which the user was assumed to be able to readily perform, all of these other functions just mentioned were accessible using slide rules. And, like the electronic calculators, there were basic slide rules for the basic functions above, and more sophisticated slide rules for performing the more advanced functions.

Basic Advanced
Calculator App
Slide Rule

For a computational example, let’s calculate 5 times 4.7 using the A and B scales on a slide rule, such as the top two scales on the basic slide rule in the bottom left image above. Using the rule, we slip the indicator, or cursor, so that its hairline is over the 5 on the A scale. We then move the left-hand side of the slide (center section) to line up the 1 on the B scale with the hairline. We can then move the cursor’s hairline to 4.7 on the the B scale. The result is found beneath the hairline on the original A scale: 23.5. Many other standard operations and techniques for using the slide rule can be found in Chapter 3, and the mathematics behind their guiding principles can be found in Chapters 1 and 2.


The earliest pocket calculators that became affordable to the general public were sometimes named “slide rule calculators” to emphasize that they performed most of the same functions. Below is an image of my pocket calculator from college.

Slide Rule Calculator

Early electronic calculators of the 1970’s were capable of performing numerical calculations with multi-digit (usually 10-13) precision; but for many computations, only 2-3 digit accuracy – or 4 at most – is sufficient for the project at hand. Note that an error in the last digit in a three-digit number is on the scale of 1 part out of a few hundred, say 1 out of 500 as an example; this amounts to an error of about 0.2% or so for a typical calculation. Computations necessary for the building of buildings and sidewalks, bridges and automobiles, even airplanes and rocket ships could be performed through the use of slide rules for many of the relevant calculations. Mechanical computing machines and other techniques could be used sparingly in special cases where more accuracy was critical.

For calculations that need to be repeated many times, such as in design work or operations, computer programs and applications are very often used today. Prior to computers, a wide variety of specialty slide rules permitted designers and other professionals to perform expeditiously such repetitive tasks. The image below compares a small computer code that evaluates an engineering equation on a personal computer, and a slide rule from the 1950s that was designed to perform the same calculation. While such specialty slide rules had little practical application outside of their intended use, this also makes many of them rather rare and thus often highly collectible.

A spring design calculation slide rule.


As mentioned at the beginning of our introduction, it was the invention of the concept of a logarithm that paved the way for the invention of a logarithmic scale, with which one could perform the multiplication of arbitrary numbers by adding or subtracting lengths along a physical rule. Following this invention in the early 17th century, additional scales were gradually created over time and added to the slide rules produced over the centuries so that more complex calculations could take place on these relatively simple and easy to carry devices.

Our first two chapters will introduce the concept of the logarithm and its use in performing multiplication and division calculations, and then will present a modern description for how logarithms of numbers can be obtained. With this information at hand, our third chapter will discuss how logarithmic scales on slide rules allow for a wide variety of calculations, and will show some of the diverse mathematical functionality found on many of the popular slide rule models from the last two centuries of their development. Before diving right in, our final introductory section will preview the various chapters, special sections, and other content that can be found on this web site.