When a writer wants to compare the size of something, it's easy to know what to say when it's greater: Just use "more." That is, I want more mashed potatoes, or my friend has 10 more pencils than I do. However, it gets a little more complicated when we take it back the other way, because we have two options: fewer or less.
So let's look at the most appropriate time to use fewer and less.
Fewer vs. Less
For the most part, fewer is used to indicate a smaller number of people or things. In this example, I might have 7 fewer friends than my brother or 5 fewer books than my friend. Fewer is used with countable nouns, like crayons, sticks, or words.
Less, on the other hand, is mostly used for indicating a smaller amount of uncountable nouns, like milk, glue, or time. As such, I might say that I have less milk than my friend or less glue than my classmates. Milk and glue are not countable.
That said, ounces and bottles are countable. So I could use similar examples to say something like "give me six fewer ounces of milk" or "our class has five fewer bottles of glue."
Fewer is never used for uncountable nouns, but less is sometimes used with countable nouns, especially when used as part of the phrases "less than" and "or less." This tends to crop up most frequently with increments of time, money, weight, and percentages.
When in doubt, be safe and use "fewer" for countable nouns and "less" for uncountable nouns.
Here are a few examples:
Correct: Joey has fewer marbles than Julia.
Incorrect: Joey has less marbles than Julia.
Correct: Could you take up less space for your workout?
Incorrect: Could you take up fewer space for your workout?
Correct: Spend $15 or less on the present.
Incorrect: Spend $15 or fewer on the present.
While there can be exceptions for time, money, weight, and percentages, you'll likely be on the right path using fewer whenever you're using it with countable nouns and less with uncountable nouns.
No matter what type of writing you do, mastering the fundamentals of grammar and mechanics is an important first step to having a successful writing career.