They backup (back up)

Use "back up" (two words) for the action: to move backward, to support someone, or to copy data. Use "backup" (one word) for the thing: a copy, a plan, or a spare.

Quick answer

"Back up" = verb (two words). "Backup" = noun or adjective (one word).

Action? Write two words: "back up the files."
Thing or descriptor? Write one word: "daily backups" or "backup plan."
Avoid "back-up" unless a specific style guide asks for the hyphen.

Core rule

If the phrase names an action someone does, use the verb form: back up, backing up, backed up (two words). If it names a thing or describes a noun, use the single-word form: backup, backups (one word).

Verb: "Back up your files before updating."
Noun: "We store daily backups off-site."

Spacing and hyphenation

Modern usage keeps the verb as two words and the noun/adjective as one. Hyphenation ("back-up") is rare and normally only used when a publisher's style requires it.

'back up' = verb (two words)
'backup' = noun or adjective (one word)
Use 'back-up' only if a style guide explicitly requires it

Real usage: work, school, and casual examples

Short, copy-ready examples that show whether the phrase is a verb or a noun/adjective.

Work (verb): "Can you back up the presentation before the meeting?"
Work (noun): "We keep encrypted backups of client data."
Work (adj): "Start the backup process at 10 PM."
School (verb): "Back up your research files to two locations."
School (noun): "Bring a backup copy of your slides on a USB."
School (adj): "The lab uses a backup drive for experiments."
Casual (verb - support): "I'll back you up if they question your idea."
Casual (noun): "Do you have a backup charger?"
Casual (verb - move): "Back up the car a bit so I can get out."

Common mistakes - wrong / right pairs you can copy

Decide whether the phrase is an action or a thing, then switch spacing accordingly.

Wrong: "They backup the files every night." →
Right: "They back up the files every night."
Wrong: "Please backup your work to the cloud." →
Right: "Please back up your work to the cloud."
Wrong: "I have a back up of my files on an external drive." →
Right: "I have a backup of my files on an external drive."
Wrong: "She wants to back-up her phone before the update." →
Right: "She wants to back up her phone before the update."
Wrong: "If you don't backup, you'll lose everything." →
Right: "If you don't back up, you'll lose everything."
Wrong: "We need a back up plan for the launch." →
Right: "We need a backup plan for the launch."
Wrong: "I bought a back up battery for my laptop." →
Right: "I bought a backup battery for my laptop."
Wrong: "Can you backup the folder on the server?" →
Right: "Can you back up the folder on the server?"

Try your own sentence

Test the sentence as a whole: context usually makes the correct form clear. If an auxiliary or imperative appears (can, will, should), it's likely a verb.

Rewrite help - step-by-step fixes and ready rewrites

Three quick checks and practical rewrites you can paste into emails, reports, or papers.

Step 1: Identify function - action or thing?
Step 2: If action → use "back up" and match tense (backed up, backing up).
Step 3: If thing/adjective → use "backup" (plural: backups).

Rewrite:
Original: "He backed-up his phone." → "He backed up his phone."
Rewrite:
Original: "We need a back up plan if the demo fails." → "We need a backup plan if the demo fails."
Rewrite:
Original: "Can you backup the dataset before analysis?" → "Can you back up the dataset before analysis?"
Rewrite:
Original: "The backup copy of the thesis is on my drive." → "The backup copy of the thesis is on my drive." (correct as-is)
Rewrite:
Original: "They backup me in meetings." → "They back me up in meetings."
Rewrite:
Original: "I keep a back up charger in my bag." → "I keep a backup charger in my bag."

Grammar checklist for proofreading

Use this quick checklist on your final pass to catch the most common slip-ups.

Role: Is it an action (verb) or a thing (noun/adjective)?
Tense: Verbs change form (backed up, backing up). Nouns do not (backup, backups).
Nearby words: Auxiliaries (can, will, should) or imperatives usually signal a verb.
Style: Drop the hyphen unless a style guide requires it.

Tip: If the sentence reads "Will you ___ the files?" it's almost always "back up."

Memory tricks and quick tests

Short tricks that help decide fast.

Rhyme: "Action = two; Thing = one."
Substitute with "copy" or "reserve." If it fits, use "backup" (one word).
Substitute with "move back" or "support." If it fits, use "back up" (two words).
Compare: login (noun) vs log in (verb); setup (noun) vs set up (verb).

Similar mistakes to watch for

Many phrasal verbs form single-word nouns or adjectives. The same spacing logic applies.

set up (verb) → setup (noun/adj): "Set up the device." vs "Follow the setup guide."
log in (verb) → login (noun/adj): "Log in now." vs "Enter your login details."
check in (verb) → check-in (noun): "Please check in." vs "The check-in desk is here."

Wrong: "I need to setup my account." →
Right: "I need to set up my account."
Wrong: "Please log-in to the portal." →
Right: "Please log in to the portal."

FAQ

Is "backup" one word or two?

"Backup" is one word when it's a noun or adjective ("a backup," "backup plan"). Use two words - "back up" - when it's a verb ("to back up files").

When should I hyphenate "back-up"?

Hyphenation is uncommon. Only use "back-up" if a specific style guide requires it. Otherwise prefer "back up" (verb) and "backup" (noun/adjective).

How do I remember which form to use?

Remember: "Action = two; Thing = one." If "copy" can replace it, use "backup." If "move back" or "support" fits, use "back up."

Will grammar checkers catch this mistake?

Most modern grammar tools spot incorrect spacing for these pairs and suggest the correct form based on context. Still, apply the simple tests above if a suggestion seems uncertain.

Is "backups" correct for plural?

Yes. When referring to multiple copies or reserves, use "backups" (one word): "We keep daily backups."

Need a quick check?

When unsure, paste the full sentence into a context-aware grammar checker or run the substitutions above. Use the examples and rewrites here as quick fixes for emails, reports, assignments, and messages.