A one hundred is the standard form because “one” starts with a /w/ sound, so it takes “a,” not “an.”
You’ll see this slip in essays, captions, invoices, and on signs: an one hundred or a one hundred. It looks tempting because “one” starts with the vowel letter O. English doesn’t pick a or an by letters. It picks by sound. Once you lock that in, the choice stops being guesswork.
This article gives you a clean rule you can apply in seconds, plus a set of quick patterns you can copy into your own writing. If you’re writing for school, work, or tests, you’ll be able to choose the right form without second-guessing.
An one hundred or a one hundred for exams and formal writing
Write a one hundred. Say it out loud and you’ll hear why: “one” begins with the same opening sound as “won,” a consonant /w/. Since the next sound is a consonant sound, the correct article is a.
| Phrase you want | Correct article | Sound check |
|---|---|---|
| ___ one hundred | a | Starts with /w/ (“wun”) |
| ___ one-time deal | a | Starts with /w/ (“wun”) |
| ___ university | a | Starts with /y/ (“you”) |
| ___ hour | an | Starts with a vowel sound (silent h) |
| ___ 8-page report | an | Starts with a vowel sound (“eight”) |
| ___ FBI file | an | Starts with vowel sound (“eff”) |
| ___ UN meeting | a | Starts with consonant sound (“you”) |
| ___ MBA program | an | Starts with vowel sound (“em”) |
| ___ European city | a | Starts with /y/ (“you”) |
| ___ honest answer | an | Starts with a vowel sound (silent h) |
Why “a” wins before “one”
The rule is sound-based: use a before a consonant sound, and an before a vowel sound. Purdue OWL states the choice depends on the “phonetic (sound) quality” of the next word, not the written letter. The Cambridge Dictionary grammar notes that words written with a vowel letter can still begin with a consonant sound, including “a one …”.
Here’s the practical takeaway: if the next word begins with the /w/ sound, you’ll use a. That includes a one, a once-in-a-lifetime, and a one-off. Your eyes might want to match “an” to the letter O. Your ear is the better judge.
If you’re stuck on an one hundred or a one hundred, say “one” aloud; that /w/ sound points to “a.”
If you want to see the rule on an authority page, check Purdue OWL’s “A versus An” article rule and the Cambridge Dictionary “a/an and the” grammar note. Both put the sound rule front and center.
What your mouth does at the start of “one”
Try saying “one” slowly. Your lips round and push air the same way they do for “we,” “win,” and “wonder.” That is the /w/ sound. Since /w/ is a consonant sound, the article that fits is a.
This is why you’ll hear people say “a one-way ticket,” “a one-bedroom,” and “a one-off mistake.” All of those phrases begin with the same opening sound.
When “one hundred” and “a hundred” mean the same thing
Most of the time, one hundred and a hundred point to the same number: 100. The difference is tone and context, not math. In daily writing, “a hundred” feels natural. In forms, math work, and places where clarity matters, “one hundred” can feel steadier.
Here are a few settings where “one hundred” tends to show up:
- Scores and grading: “She got one hundred on the quiz.”
- Checks and invoices: “Amount: one hundred dollars.”
- Measurements and labeling: “One hundred milliliters.”
- Counting with contrast: “Not ninety-nine, one hundred.”
Even in those settings, the article choice stays the same. If you put an indefinite article in front, you’ll still write a one hundred, not an one hundred.
Choosing a or an before numerals like 100
Writers sometimes swap one hundred for the numeral 100 and then hesitate again: should it be “a 100” or “an 100”? The sound rule still applies. Most readers say “one hundred” when they see “100,” so the article stays a: “a 100-mile drive,” “a 100-page book,” “a 100% score.”
If you know your audience will read the numeral differently, let sound lead. A good check is to read the full phrase aloud the way a reader would. If the first sound after the article is /w/, use a. If it starts with a vowel sound, use an.
A or an before one hundred in English writing
If your sentence starts with the number phrase, you can often skip the article and sound cleaner: “One hundred students attended.” In the middle of a sentence, writers tend to choose between “a hundred” and “one hundred.” Both are fine. Pick the one that matches your tone, then apply the same sound rule if you add an article.
A quick habit that helps: when you see 100, decide what you would say out loud. If you would say “one hundred,” write “a 100-…” as your default. If you would say “a hundred,” write “a hundred-…” and you never face the an question at all.
When the noun is plural
Phrases like “a one hundred dollars” can sound off because the article is pointing at a single unit while the noun is plural. In most sentences, “a hundred dollars” or “one hundred dollars” reads smoother. If you’re writing a check line or a receipt note, dropping the article is often the clean choice: “One hundred dollars only.”
Hyphens and number phrases with one hundred
When a number phrase acts like an adjective right before a noun, hyphens can keep it easy to read. You’ll see this in things like “a one-hundred-page book” or “a hundred-dollar bill.” Style guides differ on the details, so follow your school or workplace rules if you have them.
In day-to-day writing, these patterns tend to look natural:
- a one-hundred-point scale (clear when the full phrase is needed)
- a hundred-point scale (shorter, still clear)
- a $100 bill (common in money contexts)
Hyphens do not change the article choice. “One-hundred” still starts with the /w/ sound from “one,” so it stays a.
Common traps that make “an” feel tempting
Trap 1: judging by the first letter
English spelling and English sound do not always match. “One” is the classic case. The first letter is O, but the first sound is /w/. That mismatch is what trips people.
Trap 2: copying a pattern from “an eight”
Numbers can pull you into the wrong habit. “Eight” starts with a vowel sound, so “an eight-year plan” sounds right. “One” does not work the same way. It begins with a consonant sound, so it stays “a one-year plan.”
Trap 3: mixing up “one” with “once” and “only”
These words feel like they belong together, but they still start with /w/ at the beginning when spoken: “once” often starts like “wuns,” and “only” often starts like “woh-.” That keeps the article as a in common speech: “a once-in-a-lifetime chance,” “a only child” is less common, yet the sound still points to a.
Quick edits that fix the sentence without sounding stiff
Sometimes the cleanest fix is not just switching an to a. You can also reshape the sentence so it reads like something a person would actually say. Here are a few options you can swap in when your draft feels clunky.
- Use “a hundred” when the context is casual: “He scored a hundred.”
- Use “a perfect score” when the number is a grade: “She earned a perfect score.”
- Use “one hundred percent” when you mean certainty or a full mark: “The report is one hundred percent complete.”
- Use the numeral when space is tight: “A 100-word limit.”
All of these keep you away from the awkward spot where an and one collide. They’re handy in headlines, subtitles, and short captions.
What to do with “an” in older or stylized writing
You may spot “an one” in old books, transcriptions, or stylized writing. English articles shifted over centuries, and older usage sometimes kept an in places modern English would not. In modern editing, treat it as a period flavor, not a model for standard writing.
If you’re writing for school, business, or general web readers, stick with the modern sound rule. It’s the form most teachers, editors, and style checkers expect.
Proofread checklist for “a/an” around numbers
When you’re proofreading fast, you don’t need to recite grammar terms. Use a quick spoken test.
- Read the phrase out loud at normal speed.
- Listen to the first sound after the article, not the first letter.
- If it starts with a consonant sound, use a.
- If it starts with a vowel sound, use an.
- When in doubt, swap in a synonym like “a hundred” or “a perfect score.”
This same check fixes “a university” and “an hour,” and it settles “a 100” versus “an 8” without drama.
An One Hundred Or A One Hundred in real sentences
Here are sentence patterns you can copy. Each one keeps the phrasing natural and keeps the article choice correct.
| If you wrote | Edit it to | Why it reads better |
|---|---|---|
| an one hundred score | a one hundred score | Matches the /w/ sound in “one” |
| an one hundred percent | a one hundred percent | Keeps the same sound rule |
| an one hundred dollars | a one hundred dollars | Works when the noun stays plural |
| an 100-page essay | a 100-page essay | Most readers say “one hundred” |
| an one-year contract | a one-year contract | “one” starts with /w/ |
| an once-in-a-lifetime deal | a once-in-a-lifetime deal | Most speakers start “once” with /w/ |
| an university course | a university course | “u” can sound like “you” |
| a eight-hour shift | an eight-hour shift | “eight” starts with a vowel sound |
Common score wording that stays clean
Scores are where this shows up the most. Teachers and students write fast, then an autocorrect or a quick glance lets the error slip through. Here are three clean formats that work in most contexts:
- a perfect score (good for headlines and reflections)
- a score of 100 (good for reports and rubrics)
- one hundred out of one hundred (good when you want full clarity)
If you still want the “one hundred” wording with an article, keep it as a one hundred. The sound rule does not change with context.
One last check before you publish
Search for the exact string an one in your draft. If you spot it, swap it to a one, or rewrite the phrase as “a hundred,” “a score of 100,” or “a perfect score.” That single pass catches the problem fast, and the rest of your copy can stay as it is.
You came here to settle the article before one hundred. The answer is steady: write a one hundred in standard English, because the next word starts with a consonant sound.