How to use the 1970s-era GM metric converter

A printable version of these instructions is also available.

Here's a brief explanation of how to use the GM metric converter, which consists of two concentric discs: an outer disc with a logarithmic number scale and an inner disc with pointers corresponding to various unit conversions:

Converter

The same basic instructions apply to other converters that resemble a circular slide rule. You set one pointer to the value to convert, then read the converted value from another pointer. We'll do three examples.

Convert 250 pounds to kilograms

To convert 250 pounds to kilograms, we first locate the appropriate pointers on the converter. Each orange or white arc on the inner disc represents one particular conversion, shown by the units indicated; each end of the arc is the “pointer” to the numbers on the outer disc. So, in this case, we'll use the orange arc that shows pounds on one end and kilograms on the other:

Converter

Now we simply twist the discs so the pointer at the “pounds” end of the arc points to 2.5 on the outer disc:

Converter

Like any slide rule, we're responsible for the decimal point; we would position the discs the same way whether we were converting 0.25 lb, 2.5 lb, 25 lb, 250 lb, etc.

Note: If you're not familiar with setting and reading numbers from a slide rule scale, and you don't have a slide rule instruction guide handy, there are many Web sites devoted to the topic. See, for example, The basics of slide rule use from Ron Manley's Slide Rule Site, which also has various other instruction resources. For our purposes, you need learn only how to set and read numbers; the scales on a real slide rule let you do arithmetic, but this is only a unit converter, not a full-fledged slide rule. The numbers on this converter work the same way as those on regular slide rules except that ours is circular instead of linear, and regular slide rules duplicate the numbers — in effect, regular slide rules run from 1 to 10 and then continue on to 100, a necessity for calculations but not for conversions.

Now that we've positioned the discs, we can read off the converted value from the other end of the arc — the line without an arrowhead:

Converter

The value at the pointer is approximately 1.14, so 250 pounds is approximately 114 kilograms.

Recall that we're responsible for figuring out where the decimal point goes. If you think of the first digit as the units digit, then we positioned the discs for the value 2.5 and got a result of 1.14. This means 2.5 lb is 1.14 kg, or 25 lb is 11.4 kg, or 250 lb is 114 kg, or 2500 lb is 1140 kg, etc.

Convert 5 kilometers to miles

Although the arrows on the converter scales are designed to make it easiest to convert to metric units, the converter can, of course, go either way, depending on which end of the arc we start with.

So, how long is a 5K race in miles? We use the arc that shows miles on one end and kilometers on the other, and position the “kilometers” end to the value 5:

Converter

Now we read the value from the pointer at the other end of the arc:

Converter

So 5 km is approximately 3.11 miles. (It is, of course, perfectly reasonable to read the result as 3.1 mi, without bothering to try to more finely read the scale.)

Convert 5 gallons to liters

A few of the converter's arcs show a multiplier, so we'll do one final example with one such scale. To convert gallons to liters, we'll use this arc:

Converter

Positioning the “gallons” end at 5:

Converter

we read the result:

Converter

The result is 1.89, but note that the arc says “×10” so we should multiply by 10: thus, 5 gallons is about 18.9 liters. (Although the converter doesn't explicitly say, it converts to and from U.S. gallons, not Imperial gallons.)

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