I just (April, 2009) purchased a Seiko SNAB67 Flight Master chronograph. It has a slide rule bezel styled after the famous Breitling Navitimer (which in turn used features from the "E-6B" flight computer). If you haven't seen one before, it has the following features:

• "C" and "D" (or fundamental) scale log rules
• A time scale matching hours to minutes
• Markers for converting from nautical to statutory miles and to kilometers and thousands of feet
• Conversions for U.S. gallons to Imperial gallons or liters or pounds of fuel or oil
• Conversions from pounds to kilograms

Breitling still makes the Navitimer, but many manufacturers currently make watches with slide rule bezels such as:

• A.I. Wajs
• Sinn
• Hacher
• TAG-Heuer
• Tissot
• Ikepod
• Ventura
• Sector
• Zeno
• Torgoen
• Chase Durer
• ESQ
• Formex
• Wenger (the Swiss Army company)
• Seiko, Pulsar, Alba, and Lorus
• Citizen and Q&Q
• Casio
• Orient
• Kentex
• Poljot
• Vostok
• Invicta
• Nautica
• National Geographic
• Aeromatic

Many but not all slide rule watches by these manufacturers have the time scale and unit conversions of the Navitimer. The prices of watches by the above manufacturers range from $50 (Casio) to thousands (many of the Swiss brands.)

My watch did not come with any instructions regarding the use of the slide rule. The instructions I found online for using slide rule watches or slide rules in general were confusing, loaded with math terminology or (in one case) simply wrong! Once I finally figured it out, I decided to write this guide as a reference for myself and hopefully a resource for others. Read on for the How-To!

Determine Your Type of Slide Rule

If both the outside and inside rules have identical scales with a full range from 10 to 90, then you have what are known as "C" and "D" log scales. These are useful for multiplication and division as well as ratios and percentages (which are really just multiplication.)

If the outside rule is the reverse of the inside rule, then you have the "C" and "CI" or inverse scales. Sadly, this page will not be helpful to you. But a quick Google search for "slide rule CI scale" should get you going.

Understand Slide Rule Basics

The explanations I found for using slide rules really confused me at first because they assumed that I already understood some basic slide rule theory. But I finally 'got it' and found that the principle is really quite simple.

The most important thing to remember when using the circular slide rule is to ignore decimal place when inputing numbers. Always select the two-digit number that is equal to the number you want multiplied or divided by a power of ten. Rather than try to make that statement any clearer, let's just see some examples:

Number to input	Select on the slide rule
5	50
3.4	34
230	23
10,000	10

Of course, you'll have to remember to shift the answer by an appropriate amount when you get it back. Here's an example: 5,000 * 6 will be input as 50 * 60 and will return 30. You have to figure out the appropriate amount of zeroes to get the correct answer of 30,000.

The nice thing is that using the slide rule for a while improves your ability to estimate correct answers.

Perform Multiplication

Multiplying numbers on the slide rule is easy. Basically, you set a multiplier on one scale that creates a lookup table on the other scale. Here's how to do it. (My instructions will refer to the inner and outer rules. Since the scales are identical, you can replace one for the other in my instructions.)

Let's do 5 * 6:

1. Align the 10 of the inner rule with the 50 of the outer rule. This sets the multiplier to 5.
2. Now look at the 60 on the inner rule. It should line up with 30 on the outer rule. 60 here represents the 6 from our equation. 30 is our answer.
3. Now we double check the decimal place of our answer. As it happens, 5 * 6 does equal 30, so there's no need to adjust.

What's handy is that we have now set up the slide rule to multiply any number by 5 without making any more adjustments. We just need to look at a number on the inner rule and see the answer in the outer rule (for example, 24 is currently aligned with 12 because 5 * 24 = 120.) As you have probably figured out already, you have also set the rule to multiply by 50, 500, 50,000 and 0.005...

Let's try another one. 3.5 * 4,000:

1. Align the 10 of the inner rule with the 35 of the outer rule.
2. Now look at the 40 on the inner rule. It should line up with 14 on the outer rule.
3. Knowing that our answer should be in the thousands, let's adjust 14 so that it fits: 14,000. And that's correct!

Try a few others and check your answers with a calculator. You'll quickly get a hang of it.

Perform Division

Dividing is just the reverse of multiplication. This time we align the two numbers of our problem, and the answer will appear next to the 10.

We'll start with 60/3:

1. Align 60 of the outer rule with 30 of the inner rule. I like doing it this way because it lets me visualize "60 over 30."
2. Now look at the 10 on the inner rule. It aligns with 20 on the outer rule.
3. 20 is correct, so there's no need to adjust the number.

Now let's try something a little less simple, 5/7:

1. Align 50 of the outer rule with 70 of the inner rule.
2. Now look at the 10 on the inner rule. It aligns just a little past 71 on the outer rule.
3. Adjusting the decimal point correctly gives us roughly 0.71. The correct answer is actually 0.714286... The slide rules on watches are so small that it is hard to get more than two digits of accuracy. Still, it's a wonderful estimation tool.

Distance and Volume Conversions

If your watch has them, you can use the unit conversion marks to convert amounts of one unit to another. All of the conversions work the same way, so once you've figured one out, you'll know how to do them all. It's very easy.

As an example, let's convert nautical miles to kilometers. We'll convert 22 nautical miles. (My instructions assume that the unit indicators are on the outer rule. If they are on the inner rule, simply reverse the instructions.)

1. Align 22 of the inner rule with the nautical miles (Naut) indicator on the outer rule.
2. Now look at the kilometer (Km) indicator of the outer rule. It aligns just slightly past 40 on the outer rule.
3. There is no need to adjust the decimal. 40 is right. The precise answer is 40.744.

Use this same method to convert any of the following (if indicated on your watch):

• Nautical miles, statutory miles, kilometers, and 1000s of feet
• Pounds of fuel, pounds of oil, imperial gallons, U.S. gallons, and liters
• Kilograms and pounds

Time calculations

Any slide rule watch will allow you to do these calculations. Some make it simpler by including a time scale on one of the rules which translates from minutes to the familiar HH:MM format. One of the many original uses of flight computers was to estimate time of arrival. Here's how you might calculate that.

Let's say you have a ground speed of 43 miles per hour and you know that your destination is 50 miles away. Let's figure out your ETA. (For this I will assume that the time scale, if any, is on the inner rule. Reverse if needed.)

1. Align 60 (for 60 minutes per hour) on the inner rule with 43 (your speed) on the outer rule.
2. Now look at your distance, 50, on the outer rule. It aligns roughly with 70 and with 1:10. The answer is that you'll arrive in about 70 minutes (or 1 hour, ten minutes).

Of course, the time scale only works if the two values of the equation are of similar sizes. If we had 500 miles to go instead of 50, the answer would have been approximately 700 minutes, which is obviously NOT 1:10!

You can figure out how far you'll go in a given amount of time at a given speed. Say we're going 70mph and we'll be doing it for 1:30 (or 90 minutes):

1. Align 60 on the inner rule with 70 (your speed) on the outer rule.
2. Now look at the time of 1:30 or 90 on your inner rule. It aligns roughly with 10.5 on the outer rule. The answer is that you'll travel about 105 miles.

Lastly, if you know how far you went and how long it took, you can figure out how fast you were going. Say we went 250 kilometers and it took two hours:

1. Align 2:00 (or 12 for 120 minutes) on the inner rule with 25 (for your distance of 250) on the outer rule.
2. Now look at 60 on the inner rule (to solve for minutes). It aligns with about 12.5 on the outer rule. That's the answer. You would have gone 12.5 kilometers per hour

Ratios and Percentages

Every other calculation you'll encounter is a variation of what you've seen above. Not too bad, huh? Like the unit conversions, calculating percentages is really handy in stores and restaurants. You can see what a sale item marked 20% off would cost by aligning 10 on the inner rule with 80 (if 20% is off, you'll pay 80%) on the outer rule. A $70 item on the inner rule shows a sale price of $56 on the outer rule and so forth. A 17% tip in a restaurant is easily calculated by aligning 10 on the inner rule with 17 on the outer rule. A $25 meal shows 42.5 on the outer tip rule. The tip would be $4.25.

The trick is to always apply the common-sense shift of the decimal point as needed. Once you get proficient, it's fast and easy.