# Is a slide rule bezel on an aviation watch worthless?

Mostly, but there are some uses for it.

I did not buy the aviation style watch I did for the slide rule bezel function. But I decided anyway to *try* and use it anyway.

The main problem is the size of the inner ring numbers. Tiny-sized. Can I read them? Actually, yes I can. But in all honesty, if I really wanted to use the slide rule bezel regularly, getting something bigger would definitely be better.

I did try to see if there were any decent real world uses of a slide rule bezel on a watch. I found two.

### Real world use of a slide rule bezel

These are a few examples of a slide rule bezel usage that are actually useful for some.

**Restaurant tip**

Rotate the bezel so the amount of the total bill on the outer ring is over the inner ring 10.

Look at the inner ring and find positions 11.5 and 12. 11.5 represents 15% and 12 represents 20%. Reading the outer ring number at those positions will tell you the bill + tip.

Example: You have a $45 restaurant bill. Rotate outer ring 45 over inner ring 10. Look at inner ring 11.5 and read the outer ring number at that position. You'll see it's hovering between 51 and 52. This is correct because the actual bill + tip is $51.75. Now look at the inner ring 12 for a 20% example and read the outer ring there. It's at 54. The is correct because 45 + 20% = 54.

More often than not, you will have to estimate the total.

**Average gas mileage**

You need to know how many miles have been driven since your last fill-up for this one. The next time you fill the tank, reset the trip odometer in your car.

At your next fill-up, read the trip odometer in your car. We'll say it's at 325 miles since last fill-up.

Fill the tank. We'll say you put in 9.5 gallons.

Now we need to divide 325 by 9.5.

My watch doesn't have a 325 on the bezel, so instead I will use 32.5 (roughly).

I line up 32.5 on the outer ring with 9.5 on the inner ring.

I then look at the 10 position on the inner ring. It's at about 34.

This is correct because 325 divided by 9.5 is 34.21.

### Useful?

For me personally, not terribly. I bought my watch more for appearance than bezel function.

It is cool that I can actually perform real math functions on it, but will I actually *do* that? Probably not. But it's still cool it's there.