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Guide: How to Use Your Slide Rule


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Ever wonder what those "little numbers" on the Breitling Navitimer dial bezel were for, or if you've wondered about the utility of the Slide Rule and how it can help you in land or in air, then I have constructed this guide for you.

If you're deciding to disregard this guide because your rep Slide Rule is inaccurate, don't discredit it just yet! I will show you how you can use it for common calculations and other novelty functions that you can gain for your advantage when being asked by friends and admirers what the feature does! Don't be embarrassed by not being able to know the functions of your own watch again!

The slide rule on the Navitimer series (and other Breitlings/Sinns that use this feature [does not include Bentley lines]) is based on the E6B flight computer; a computer not in the sense that it's not electronic, but a tool for pilots to aid in calculating fuel burn, wind correction, time en route, and other items. In the air, the E6B can be used to calculate ground speed as well.

Paper version of Slide rule:


This is the little nifty card of the slide rule that comes with all Navitimers that would help the prospective user to figure out how to use it, which is accompanied by this diagram that is included in the manual:


A full-scale version can be downloaded here:

Full-size scan of paper slide-rule

Full-size scan of diagrams

As you can see, there are ten functions that accompany the slide rule, and each can be used in conjuction with everyday life; whether you are a air pilot or land junkie. Let's go step by step on their functions:

1: Multiplication


To multiply with the Navitimer, use the unit index (number "10" in red on the inner scale). Always set the multiplier (the number by which another is multiplied) opposite the unit index on th einner scale and read the answer on the outer scale opposite the multiplicand number (number to be multiplied by another) appearing on the inner scale.


To multiply 7 x 12, set 12 (the multiplier) on outer scale opposite unit index ("10") on the inner scale. Opposite 7 (the multiplicand) on the inner scale, read the answer 84 on the mobile dial.

2: Division


To divide with the Navitimer, also use the unit index (red "10). Place the dividend (the quantity to be divided by another number) on the outer scale opposite the divisor (quantity by which another is divided) on the inner scale. Opposite the unit index (numeral "10" on inner scale), read answer on outer scale.


Divide 120 by 4. Place 120 on outer scale opposite 4 on the inner scale. Read answer, 30, on outer scale opposite unit index (numeral "10" on inner scale).

3: Calculating Ground Speed


The mobile outer and fixed inner scales are used for determination of ground speed problems. Two of the following quantities are available for its solution: time, distance, ground speed.


Known: Distance and Time:

Required: Ground Speed.

A pilot finds by the use of checkpoint that he has traveled 104 miles in 35 minutes. What is the ground speed?

Solution: Move the mobile scale until 104 on the mobile scale is set opposite 35 on the fixed scale. Opposite the hour index (the arrow marked "MPH" directly over the hour 12) read 178 miles per hour on the mobile scale.

4: Calculating Ground Speed (Function 2)



Known: Distance and Speed.

Required: Time.

A pilot wants to know how long it will take to go 486 miles at a ground speed on 156 miles per hour.

Solution: On the mobile scale set 156 opposite the hour index on the fixed scale. On the inner scale opposite 486 on the mobile scale read 187 minutes (or 3 hours and 7 minutes).

5: Calculating Miles per Minute


This may be read after the speed in miles per hour has been obtained. The speed given in miles per hour on the mobile scale when set opposite the hour index on the fixed scale can be readily converted into miles per minute by reading the number on the mobile scale appearing opposite the figure "10" on the fixed scale. The figure "10" is often referred to as the "unit index".


In the previous example, the ground speed was 156 miles per hour. With the 156 on the mobile scale set opposite the hour index, what is the speed in miles per minute?

Solution: With 156 miles on the mobile scale set opposite the hour index read the speed in miles per minute, or the figure on the mobile scale which appears above the figure "10" on the fixed scale, which is 2.6 miles per minute.

There are times when a pilot may want to know the time required to travel a short distance, such as the distance from the cone of silence to the edge of an airport, or between the inner marker and range station. Since the distance is short, the time required may be less than a minute, in which case the time has more siginificance when expressed in seconds. In such cases the "second" index is used. This is the figure "36" on the inner scale (there are 3600 seconds in an hour).

6: Calculating Gasoline Consumption


Two of the following quantities are available for gasoline consumption problems: Total gallons used, time, rate of consumption.


Known: Time and rate of consumption.

Required: Total gallons used.

A pilot wishes to know how many gallons are necessary to fly 3.5 hours at an average rate of consumption of 11.5 gallons per hour.

Soultion: Opposite the "hour index", set 11.5 on the mobile scale. Then, opposite 210 minutes on inner scale on the mobile scale read 41 gallons.

7: Calculating the rate of climb or descent


Two of the following quantities are available for solution: total altitude of descent, time and rate of descent (or climb).


Known: Rate of ascent and total elevation in ascent.

Required: Time.

A pilot climbs to 7400 feet above his starting point at the average rate of 500 feet per minute. How long will this require?

Solution: Set 500 on the mobile scale opposite the "unit index" ("10" on the fixed scale). Opposite 7400 on the mobile scale, read answer, 14.8 minutes on the fixed scale.

8: Calculating the distance of climb or descent


Two of the following quantities are available: Distance, time, speed. The methos used in examples 4 and 5 should be used.


The pilot in the previous example wishes to know how fat he will have traveled when his climb is finished. His average true air speed is 120 miles per hour and he is aided by a tail wind of 20 miles per hour.

Solution: Set 140 miles per hour (120+20) on the mobile scale opposite "hour index" on fixed scale. Opposite 14.8 minutes (computed from example 7) on fixed scale read 34.5 miles on mobile scale.

9: Nautical and statute mile conversion


On the fixed scale both nautical (Naut.) and statute (Stat.) miles are shown; also shown are Kilometers (KM) (red mark just to the right of the MPH). The conversion from statute to nautical or nautical to statute miles is read directly on the mobile scale.


Known: 60 statute miles.

Required: Nautical miles.

Solution: Set 60 on mobile scale opposite "Stat." on fixed scale. Opposite "Naut." on fixed scale, read 52 nautical miles on mobile scale.

10: Nautical and statute mile conversion (Function 2)



Known: 60 Stature miles.

Required: Kilometers.

Solution: Set 60 on mobile scale opposite "Stat.". Opposite the "KM" mark (the red mark, just to the right of "MPH" on the fixed scale), read 96.5 kilometers on the mobile scale.

And you may have once thought the Slide Rule was just placed purely for cosmetic purposes. Look at the utility that it will provide you in the future!

Now that you've been armed with this information, go out there and do some old-fashioned hand calculating!

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My old school Navi is accurate, but the current version is off. Even with accurate markings it is difficult to get an accurate reading though. The markings are so small that it is hard to determine if the result of an equation is 72 or 73 for instance. But overall it can be a handy tool when in a pinch, and a fun toy to impress your friends with!

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And mods, if this information proves to be useful, you may pin it.

The Breitling and Navitimer Guides are already pinned. I will add this to the Breitling Guide as soon as possible, and I hope Andreww will link it to his Navi Guide as well.

Fantastic article, much appreciated!

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  • 9 months later...
Bump for a great thread, everyone should know this if they own a Navitimer!

If you want to learn how to use the circular slide rule on the Bentleys, and also how to use the Tach on any watch with one, check out my guide:

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  • 9 years later...
  • 3 months later...

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