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.

Notes about Usage

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.