# How to Use a Circular Slide Rule

Our next project will include a circular slide rule in the design of the watch. Using a slide rule is a bit of a lost art but with a bit of practice, it can be a useful every-day tool.  With a bit of practice, it becomes easy to use.

In this post, we’ll show you how to:

• Convert currency
• Multiply
• Divide
• Convert units

### Example: Multiply 35 times 6

To multiply two numbers, align one of the numbers to the arrow at 1. Find the other number on the same ring as the arrow and read across to find the result.

### Example: Multiply 6 times 35

The order does not matter. To multiply two numbers, align one of the numbers to the arrow at 1. Find the other number on the same ring as the arrow and read across to find the result.

As expected, 6 x 35 gives the same result as 35 x 6.

### Example: Divide 450 by 6

To divide a number by another number, align the two numbers on each ring. Read the result at the arrow on the ring with the number you are dividing by – the ring with the denominator.

### Example: Divide 6 by 450

To divide a number by another number, align the two numbers on each ring. Read the result at the arrow on the ring with the number you are dividing by – the ring with the denominator.

Since we are now dividing by 450, the result will be across from the arrow on the same ring as the 4.5 number.

### Example: Currency Conversion

Set up the slide rule to convert between two currencies. In this example, 1 Canadian dollar = \$0.75 US Dollars. Set the cursor on the inner ring to point to 7.5 on the outer ring. Read from the inner ring to the outer ring to convert \$CAD to \$USD. Read from the outer ring to the inner ring to convert \$USD to \$CAD.

### Example: Unit Conversion

Set up the slide rule to convert meters to feet. Set the cursor on the inner ring to point to 3.28 on the outer ring. Read from the inner ring to the outer ring to convert m to feet. Read from the outer ring to the inner ring to convert feet to m