Metric units are becoming more common in the heat-treatment industry. As such, it is important to better understand the metric system and conversions to English units. The International System of Units (SI) assigns a specific unit to each physical quantity (e.g., length, mass, volume) . Larger and smaller multiples of this unit are called out by adding so-called SI prefixes (Table 1).

The most commonly used SI prefixes (Table 2) are in use daily.

Although unit names are ordinary words, note that the unit symbols:

• Are case-sensitive, so uppercase and lowercase letters have different meanings. For example, mm is the millimeter (one-thousandth of a meter), but Mm is the megameter (one million meters).
• Don't have singular and plural forms. It's 1 mL and 2 mL (no “s” at the end).
• Aren't abbreviations, so there's no period after a unit symbol like there is for English units (unless the unit symbol happens to fall at the end of a sentence).

## Examples and Relationships among Units

A few examples can be helpful to put metric units in context.

• One metric ton: 1 liter of water weighs 1 kilogram, so 1 cubic meter (1,000 liters of water) weighs 1,000 kilograms or 1 metric ton.
• Common U.S. coins: A U.S. cent weighs exactly 2.5 g, while the nickel weighs exactly 5 g.
• Household doorknob: A household doorknob is typically about 1 meter high.
• CD or DVD: The diameter of a CD or DVD is 12 cm.

We'll continue to explore the metric system next time.