How to use a slide rule bezel

Fergus Nash

We can all pretty much agree that pilot’s watches are awesome, whether or not it’s a historical legend or something more contemporary. There’s something about their utilitarian approach which is inspires our day-to-day lives, letting our imaginations soar whenever we check the time. That said, most pilot’s watch owners don’t actually fly planes. But, if you’ve got a watch with a slide rule bezel, you don’t have to let it go unused. They can be used for plenty of useful calculations and conversions, and some that can’t be easily done with a phone calculator app. For everyone who scorned their maths teachers telling them they wouldn’t have a calculator with them in real life, here’s some easy ways to use a slide rule bezel.

Multiplication

It’s important to remember that slide rule bezels aren’t intended to be precision instruments, and that there is going to be some margin for error when you’re only going off minuscule printed markers. But, if you’re not so quick at mental maths, getting a ballpark figure is extremely helpful. With a slide rule bezel, you can use the number 10 on the inner scale for multiplication. Let’s say you’re trying to work out the potential payout on a roulette bet, and you’ve put 7 chips directly on lucky number 21. The odds for a straight bet are 35:1, which makes the question 35 x 7. Rotate the bezel so that 35 on the outer scale is aligned with 10 on the inner scale, and the number aligned with 7 on the inner scale will then be your answer. You can see it’s directly between 24 and 25, therefore your answer is 245.

Another thing to remember is that these two sliding scales can repeat infinitely, so you do need to be roughly aware of what factor of 10 you’re going to end up with. For example, you know that 35 x 7 is not going to be 24.5 or 2,450, so the logical answer has to be 245. As a general rule, you can pretty much ignore the actual number on the slide rule as long as the first two digits are the same. For example, the number 75 could represent 7.5, 75, or 75,000 if you ever needed it to.

Percentages as discounts

The most classic example of using a slide rule bezel in real life is calculating tips at a restaurant, but percentages can crop up in plenty of ways. For this example, I’m going to imagine a fictional paradise where Rolex are having a 40% off sale (alternatively, pick any waitlist-heavy brand). The current retail price of a Submariner Date in black is A$15,250. If we’re trying to find the discounted price, we’re going to focus on the 60% that we will be paying rather than the 40% we won’t be paying. Percentages are just another kind of division, and division is just multiplication in reverse, so we’ll begin by lining up 60 on the outer scale with 10 on the inner scale. That’s easy enough, as they’re both round numbers and usually highlighted in red.

Now I can look for 15.25 on the inner scale, and see that it aligns with 91.5 on the outer scale. Voila! Knowing roughly where the decimal point should be, we now know the sale price to be A$9,150. If a sale is storewide, you can leave this calculation set up and happily browse to calculate anything.

Percentages as tips

A discount is paying less, and a tip is paying more. That’s obvious enough, so all it requires is figuring out the percentage then adding it back onto your total. Now that we’ve gotten the hang of it, let’s try with some more annoying numbers. Pretend that you’ve had a phenomenal burger with some crazy, stacked milkshake monstrosity, and your bill comes to $36.65. Your server made you chuckle heartily with a gag about recurring decimals, so you want to tip 33.33%.

Remembering that slide rules can’t always be totally precise, you estimate where 33.33 is on the outer scale and line it up with 10 on the inner scale. Looking over to roughly where 36.65 is on the inner scale, it’s lined up to approximately 12.4 on the outer scale. Checking with a calculator will reveal the actual tip would be $12.21, but it’s close enough. Your waiter will appreciate the generous tip and the joke.

Unit conversions

Conversions probably aren’t as common to come up day-to-day like multiplication and division, but every now and again you might need to figure out Celsius to Fahrenheit, pounds to kilograms, miles to kilometres, and so on. Most slide rule bezels will include markings to make this easier for you, for example, the Breitling Navitimer GMT I’m using for demonstrative purposes includes markings for converting miles or nautical miles into kilometres. It is still possible to do other conversions, but you need to know (or at least quickly Google) the factor. For example, let’s convert imperial gallons into metric litres.

1 gallon is equal to 3.785 litres, so we’ll begin by lining up the 10 of the inner scale with an approximation of 37.8 on the outer scale. Now we have effectively set up a multiplication table for 3.78. 8 gallons is roughly 30 litres, 15 gallons is roughly 56 litres, and so on.

Conclusion

As you can see, pretty much all regular-world applications of the slide rule bezel rely on some form of multiplication and division, which are effectively the same things in reverse. Speaking as someone with dyscalculia, this kind of practicality is far more useful than any other kind of watch complication, including something as essential as a date window. Once you’ve figured out how to perform one operation, you’ll be showing off your mathematical ability in no time.