spoken usage in SI

Reading numbers aloud is simplified by the written usage rule that numbers are to be broken into groups of three digits, starting from the decimal point. Read aloud, starting at the left, each group of three digits consists of:

the name of the first digit
"hundred"
the name of the number represented by the next two digits
for numbers greater than 999, the name of the order of magnitude for that group of three, such as thousand, million, and so on.

For example, the number "123 456" is read aloud as "one hundred twenty-three thousand four hundred fifty-six."

Such expressions as "fifteen hundred" and "a hundred twenty-three" should not be used.

Decimal fractions

In numbers with a decimal fraction, the digits after the decimal point are to be spoken as a series of the names of the individual digits. For example, "123.456 millimeters" would be read aloud as "one hundred twenty-three point four five six." It should not be read as "one hundred twenty-three and four hundred fifty-six thousandths."

The decimal prefixes in SI greatly simplify, and hence clarify, spoken numbers. A number that contains many trailing zeros can usually be simplified by choosing a larger unit, for example, not "one million two hundred thirty thousand centimeters" but "one point two three kilometers." Of course, this cannot be done when the trailing zeros indicate significant figures.