per vs pre

Writers often mix up per (a preposition meaning "for each" or "according to") and pre- (a prefix meaning "before"). That slip can change a sentence or produce a nonword. Below: a compact rule, clear hyphenation/spacing notes, many wrong/right examples, and ready-to-use rewrites.

Quick answer

Use per when you mean "for each" or "according to" (a standalone preposition). Use pre- (no space) when you mean "before" and you attach the prefix to a base word. Hyphenate pre- only for clarity (double vowels, a capitalized root, or to improve readability).

per = for each / according to: "$10 per hour", "per person".
pre- = before (prefix) and usually attaches: "preheat", "preview", "preexisting".
Quick test: substitute "for each" and "before". Whichever fits is the right choice.

Core explanation: the basic difference

Per is a standalone preposition that needs an object and expresses rate, ratio, cost, or reference: per hour, per capita, per contract.

Pre is a prefix meaning "before." It sticks to a root word to form modifiers and verbs: pregame, preview, precalculus.

per → separate from the noun it modifies: per person, per test.
pre- → attached to a base word (usually no space): pretest, preexisting.

Spacing and hyphenation with pre-

Most style guides prefer no space: preorder, preheat, precalculus. Add a hyphen only when it improves readability: when the root starts with a capital (pre-October), when two vowels clash (pre-existing is optional), or when the compound would be hard to parse.

No space: preorder, preheat, precalculus.
Hyphenate for clarity: pre-existing (if it reads oddly), pre-October, pre-election coverage.
Never write "pre order" or "perorder" if you mean the prefix or the preposition-spacing matters.

Wrong: The pre existing policy was updated.
Right: The pre-existing policy was updated. (Or: The preexisting policy was updated.)
Wrong: I per ordered the book last week.
Right: I preordered the book last week.

Grammar test: quick algorithm to choose correctly

Ask: Does the phrase mean "for each"? → use per. Does it mean "before" or modify timing/order? → use pre-. If neither fits, rewrite the clause with "for each" or "before" to check meaning.

Step 1: Replace the suspect word with "for each". If the sentence works → per.
Step 2: Replace it with "before" or "prior to". If that works → pre- (attach or rewrite).
If both fail, rewrite the sentence to remove ambiguity.

Work - Wrong: Please per-approve the expense before submission.
Work - Right: Please pre-approve the expense before submission.
Work - Wrong: Prices are $5 per hour/person.
Work - Right: Prices are $5 per hour per person. (Or: $5 per hour for each person.)

Memory tricks to lock it in

Per = "for each" (think counting). Pre = "before" (think time or preparation).

Say it out loud: "for each" → per; "before" → pre-.
Visual cue: per stands alone; pre sticks to the next word.

Usage test: "for each person" → "per person"; "before class" → "pre-class" or "before class".

Try your own sentence

Test the whole sentence-context usually makes the correct choice obvious.

Examples you can copy (work, school, casual)

Common mistakes paired with direct corrections. Copy the corrected sentence or the short rewrite into your draft.

Work examples focus on billing, scheduling, and documents.
School examples cover classes and exams.
Casual examples cover everyday speech and purchases.

Work - Wrong: We billed the client pre hour for the consultant's time.
Work - Right: We billed the client per hour for the consultant's time.
Work - Wrong: Please per-view the slide deck before the meeting.
Work - Right: Please preview the slide deck before the meeting.
Work - Wrong: Charges are $30 pre person for the workshop.
Work - Right: Charges are $30 per person for the workshop.
Work - Wrong: We scheduled a pre meeting to align goals.
Work - Right: We scheduled a pre-meeting to align goals. (Or: We scheduled a meeting before the full session.)
Work - Wrong: Per-approval is needed before shipment.
Work - Right: Pre-approval is needed before shipment.
Work - Wrong: Please perapprove this request.
Work - Right: Please pre-approve this request.
School - Wrong: She's taking percalculus next semester.
School - Right: She's taking precalculus next semester.
School - Wrong: Turn in the per-class reading tonight.
School - Right: Turn in the pre-class reading tonight.
School - Wrong: Study per exam: review 3 chapters each.
School - Right: Study for each exam: review 3 chapters per test. (Or: Study before the exam.)
Casual - Wrong: Let's meet per dinner at 7.
Casual - Right: Let's meet before dinner at 7. (Or: Let's meet pre-dinner for drinks.)
Casual - Wrong: I per-ordered the concert tickets.
Casual - Right: I preordered the concert tickets.
Casual - Wrong: He said per-party snacks would be fine.
Casual - Right: He said pre-party snacks would be fine. (Meaning: snacks prepared before the party.)
Work - Wrong: Per holiday hours apply.
Work - Right: Pre-holiday hours apply.

How to fix your sentence (step-by-step + rewrite templates)

Run the checklist, then apply one of these templates to rewrite without guessing.

Checklist: identify the suspect word → replace with "for each" and "before" → pick per or pre- → hyphenate if needed → read aloud.
When unsure, use a two-word rewrite ("before X" or "for each X") until you confirm the correct form.

Rewrite:
Wrong: "I per-ordered the book." → Test: "I ordered the book before it's released." → Fix: "I preordered the book."
Rewrite:
Wrong: "Charges are $30 pre person." → Test: "Charges are $30 for each person." → Fix: "Charges are $30 per person."
Rewrite:
Wrong: "We have a per-class quiz every Friday." → Test: "We have a quiz before class." → Fix: "We have a pre-class quiz every Friday."
Rewrite:
Wrong: "Please perapprove this request." → Test: "Please approve this request before submission." → Fix: "Please pre-approve this request."
Rewrite:
Wrong: "Perholiday hours apply." → Test: "Hours before the holiday apply." → Fix: "Pre-holiday hours apply."

Real usage and tone: where each form belongs

Use per in technical, legal, financial, and scientific text where rates and precision matter: "per capita", "per annum", "per liter".

Use pre- in modifiers and verbs that signal timing or sequence: "pretest", "preempt", "preexisting". Hyphens change readability, not meaning.

Use per for numbers, pricing, formulas and explicit rates.
Use pre- to form words that modify timing or order.
If either option sounds odd, rewrite with "before" or "for each" to match your tone.

Formal example: "The dosage is 5 mg per kg of body weight."
Work example: "We held a pre-meeting to set the agenda."

Similar mistakes to watch for

Other short words and prefixes cause similar confusion. Focus on meaning, not just spelling.

pre vs post: pre- = before, post- = after (prewar vs postwar).
re- (again) vs pre- (before): resubmit vs presubmit-use resubmit unless you explicitly mean "before submission" and opt for "before submission" instead.
per vs para/para- in technical fields: different roots and meanings-check subject-specific usage.

School - Wrong: The team needs to posttest the members pre training.
School - Right: The team needs to pre-test the members before training. (Or: The team needs to administer a pretest.)
Wrong: He was hired per her.
Right: He was hired before her. (Use "per" only for rates: "per head".)

FAQ

Can I ever write "per" instead of "pre"?

Only when you mean "for each" or "according to." If you mean timing ("before"), use pre- attached to the base word or rewrite with "before" for clarity.

Should I write preexisting or pre-existing?

Both are acceptable. Many guides prefer preexisting; add a hyphen when readability suffers. Follow your organization's style guide for consistency.

Why do I sometimes see "pre-order" with a hyphen and "preorder" without?

Hyphenation varies by style, frequency, and clarity. Use "preorder" for common verbs; use "pre-order" when a hyphen avoids confusion (capitalized root, doubled vowels, or awkward reading).

Is "per person" always correct for pricing?

Use "per person" when you mean "for each person." If you mean "before people arrive," choose pre- or "before." Confirm whether you intend a rate (per) or timing (pre-/before).

How can I stop mixing them up?

Do the substitution test: replace the suspect word with "for each" and "before." If "for each" fits, use per; if "before" fits, use pre-. Read aloud and consider a grammar tool to flag nonwords.

Quick fix checklist - copy this into your editor

1) Identify the suspect word. 2) Replace it with "for each" and "before" to test meaning. 3) Use per (separate) for rates; pre- (attached) for "before." 4) Hyphenate pre- only for clarity. 5) If uncertain, rewrite the clause with "for each" or "before."