Per the 18th edition of the Chicago Manual of Style, section 9.20:
- For nontechnical contexts, write out the word percent.
- ex) With 90 percent of the work finished, we can take a break.
- In scientific or statistical writing, you'll typically use the % sign.
- ex) Only 20% of the ants were observed to react to the stimulus.
- In a column of data which is all percentages, don't include the % in each cell. Just note it in the column header. (CMOS 18, 3.85)
Related: Writing Out Numbers
- Percentages are usually given as numerals, unless at the start of a sentence. (CMOS 18, 9.20)
- ex) Five percent of sentences have typos.
- ex) As many as 5 percent of sentences have typos.
Other Rules for Numbers
There are many noted exceptions and other examples provided by Chicago, 9.2 onwards, regarding large numbers, currencies, quantities, fractions, sizes, etc. This is just to highlight a few of the more common numerical situations:
- For nontechnical contexts, you'll generally write out all numbers between zero and one hundred. (CMOS 18, 9.2)
- ex) The property has a ninety-nine year lease.
- ex) My house is 105 years old.
- Larger numbers: nontechnical contexts, write out whole/rounded numbers. (CMOS 18, 9.4, 9.9)
- ex) About forty-seven thousand people attended the event. The event was attended by 47,128 people.
- ex) The country's population has surpassed three hundred million people. There are 385 million people in the country.
- For units of measure expressed as a symbol or abbreviation, the number is given as a numeral. (CMOS 18, 9.18)
- ex) 50 km, 4.5 L of water, 3.5 yds of fabric
- Years are nearly always written as numerals, though it's recommended you avoid beginning a sentence with a year.
- ex) The Great Depression began in 1929.
