When writing numbers, insert a comma after every three digits, starting from the right.

If the number has a decimal point, start from the first digit to the left of the decimal point.

Don’t use commas with years or addresses.

When writing decimals that are smaller than one, use a zero before the decimal point.

Use en dashes (–) to separate numbers in ranges.

If you’re using an en dash, you don’t need the words from or between.

Spell out fractions.

Use numerals for whole numbers and fractions.

Spelled-out fractions are almost always hyphenated. Omit the hyphen only when you’re referring to the fraction as a separate entity, and not as part of something else.

SparkNotes uses numerals for numbers in the thousands and spells out simple numbers in the millions and billions.

When fractions of large numbers are involved, we use numerals and spell out.

Here’s the rule for writing about money: numerals go with signs, and spelled-out words go with spelled-out words.

We at SparkNotes spell out ordinals under 100 and use numerals for ordinals over 100.

Use numerals when referring to a percent.

Make numbers plural as you would any other noun.

There is no need to include an apostrophe before the s when making numerals plural. Simply add an s.

Use roman numerals as little as possible. Arabic numbers (1, 2, 3, and so on) or their spelled-out versions are easier to understand quickly.

Adding a smaller roman numeral to the left of a numeral makes the number smaller. For example, 4 is written IV. Adding a smaller roman numeral to the right of a numeral makes the number larger. For example, 15 is written XV.

Our suggestions for when to spell out numbers and when to use numerals vary depending on the text in question. If you’re working on a very technical paper, you may want to use numerals for all numbers. If you’re working on a more chatty text, you may want to spell out numbers from one to ten. SparkNotes spells out numbers from zero to one hundred. Whichever method you choose, be sure to stay consistent. And we also spell out any number that starts a sentence.

We at SparkNotes use numerals for numbers over one hundred.

Spell out numbers mentioned in dialogue.

Always use numerals in charts and tables.

Odd: One resident turned eighty, another turned ninety-one, and another turned 101.
Better: One resident turned eighty, another turned ninety-one, and another turned one hundred and one.

It’s also okay to use numerals if your instincts are crying out for them—if you’re talking about a shoe size, for instance. There are no hard-and-fast rules about when to make exceptions; you just have to go with your gut.

There are several ways to write telephone numbers. One option is to separate the numbers with hyphens.

Another option is to enclose the area code in parentheses and separate the other two sets of numbers with a hyphen.

If you like, you can include the numeral 1 before area codes. This is often done with toll-free numbers.

Many companies spell out words in their phone numbers by replacing numerals with corresponding letters from the number pad. The idea is that consumers will remember words more easily than they will remember numbers. In writing, these words should be spelled out in capital letters.

If you use o’clock, you must spell out the time.

If you use a.m. or p.m., there’s no need to write in the morning or at night.

Use numerals if you want to stress a precise time.

See Military Terms for questions on the twenty-four-hour clock.