Watch Slide Rule: What is it, and how does it work?

Have you ever heard about an analogue computer? You might be wondering about how a computer can be an analogue. Well, I am going to tell you an interesting story about such a computer called “the slide rule”. This is the kind of ruler or scale by which you can do simple computing tasks like multiplying, dividing, measuring speed, conversing, etc. Multiplying by a simple ruler sounds interesting, right?

This interesting scale was introduced by William Oughtred in 1627. It is developed based on the theory of logarithm. But logarithm is a bit of a complex theory so we are not going after this. The slide has a body and a slider. Both the body and slider have numerical scales etched on them. There is a cursor that has hairlines and slides along the whole body of the scale.

Watch Slide Rule

Generally, a slide rule is linear-shaped. But another type of slide rule was developed which is called the circular slide rule. The circular slide rule is used in specialized watches made for astronomers, pilots, mechanical engineers and scientists. Like the linear slide rule, there is a fixed and a movable scale (bezel) that work together at a time. The scale was popular in the 1970s. Breitling is the first watch manufacturer company that produced such a type of watch. Later, Ikepod and Richard Mille produced these watches for special purposes.

If you ever saw a watch with a slide rule and don’t know how to use it, here is a little guide for you.

Prerequisite of calculation by watch slide rule:

First of all, your assumption quality has to be good.

Because lots of calculation has to be done by your assumption and previous knowledge. You are using a slide rule function just to help you find out possible answers. Remember that addition and subtraction are not possible in a slide rule.


If you want to multiply two numbers such that 9 and 8. Take 9 on the moving scale and then align with 10 on the fixed scale. Now, look at 8 on the fixed scale you will find 72 in the movable scale which is your answer.

Again, multiply 20 by 80. You have to do the same thing set 2 from the mobile scale and align with 10 on the fixed scale then look at 8 on the fixed scale you will find 16 on the movable scale which is your answer (160). You have to assume 2 as 20, 8 as 80 and 16 as 160.

Suppose, you have decimal numbers to multiply such that 5.60 and 7.89. It is complex to determine decimal numbers on a slide rule. In this case, to reduce the hassle, you should round off the decimal numbers. Rounding off 5.60 we get 6, and 7.89 will be 8. Then align a number with 10 on the fixed scale.

Then look at another number on the fixed scale to find out the possible result.

Every time while multiplying take at the 10 in the fixed scale.


As you know in division there are always two numbers. The number which will be divided (dividend) and the number by which the number will be divided (divisor).

While dividing in a slide rule, align the dividend on the movable scale with a divisor in the fixed scale. Then look at 10 on the fixed scale where the result/ quotient is aligning with it on the movable scale.

Have a look at the following simple example:

You are given to divide 15 (dividend) by 3 (divisor). As mentioned above, you take 15 on the bezel or mobile scale then align it to 3 on the fixed scale. Now look at 10 on the fixed scale, your result 5 (50) must be on the rotatable scale aligning with 10.


Now turn to a bit of complex calculation. Let, you are buying a pen with a book value of $20. There is a commission of about 10%. You are to compute the payable amount after the discount on a slide rule.

Again 10 on the fixed scale will work for you. Align 90 on the Bezel (Mobile scale/ outer scale) with the 10 on the fixed. Then have a look at 2 (20) on the fixed scale where the discounted price (18) will be at the outer scale.

Conversion Of Currencies

Before converting with currencies, you have to know the per-unit value of one currency to another. You are asked to find out the 35 Canadian Dollars (CAD) in USD. You know that 1CAD~0.75 USD. Now, take 0.75 on the Bezel and adjust with 10 on the fixed scale. Then look at 3.5 (35) on the fixed scale. You will get your answer on a rotatable scale.

Temperature Conversion:

You also can convert Fahrenheit to Celsius by this sliding scale. Well, the basics are Celcius has 100 divisions 0° to 100°. Fahrenheit has 180 divisions between 32° to 212°. So, the ratio of Fahrenheit to Celcius is 100/180 or 5/9. So, 1 of F = 5/9 of C. Then add 32 to F as freezing is 32° in Fahrenheit scale.

Now, we will calculate 50° Fahrenheit to °Celsius on a sliding scale. First of all, subtract 32 from the given 50°F you will find 18. Now, The inner scale has to be set on 9 while the Bezel or upper scale on 5. Look at 1.8(18) on the inner or fixed scale you will find 2 (20) on the upper scale is perfectly aligned with it. This is your answer 2 (20°c).

By setting 5 on the fixed scale aligned with 9 on the upper scale you can calculate °C to °F after adding 32 in mental calculation.

Other Uses

Many specialised watch manufacturer companies provide MPH, NAU functions in their watches. MPH is used for converting Mile per hour to Kilometres. NAU enables the user with a feature to convert Nautical miles to miles.

Who manufactures the best watches with slide rules?

Swiss luxury watchmaker Breitling is the best-known Slide Rule Watch manufacturer. It introduced Navitimer in 1952. These watches are still popular among pilots, airlines, and aeroplane manufacturers.

The most popular Breitling navitimer watches are Navitimer Chronograph and Navitimer Automatics.

There are plenty of models of this brand available on e-commerce sites.

Other well-known slide rule watch manufacturers are Casio, Seiko, Forsinig, Citizen, Megir etc.


The slide rule lost its appeal as lots of digital computing devices were developed after the 1970s. But, still, they bear the glory of history as the analogue computer. What an amazing invention that is!

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