Per Diems vs Per Diem: Which Form to Use at Work

Quick Answer: The usual English plural of per diem is per diems when the phrase is a countable noun meaning daily allowances or daily fees. Keep per diem unchanged when it modifies another noun or works as an adverb meaning "by the day."

Per diem is a Latin-derived English term that means "by the day" and can function as a noun for a daily allowance, an adjective for a daily rate, or an adverb meaning "for each day." Major dictionaries list the noun plural as per diems. The form depends on the job the phrase does in the sentence.

Professionals in Singapore meet the term in expense claims, contracts, and HR emails. Adding -s in the wrong place makes a message look careless. Precise use depends on whether the phrase is naming allowances, describing a rate, or saying "by the day."

What Per Diem Means as a Noun, Adjective, and Adverb

English borrowed per diem from Latin, where it means "by the day" or "for each day." In modern business English it does three jobs. The spelling stays two lowercase words with a space: per diem, not perdiem and not an acronym in capitals.

The noun sense is now the most common one in workplace writing. The adjective and adverb senses are older, and they still appear in contracts, payroll notes, and job ads. Identifying the job first is what tells you whether a plural -s is possible.

As a noun: a daily allowance or daily fee

As a noun, a per diem is a daily allowance or a daily fee. In travel and finance writing, it is usually money given for expenses such as meals while someone is working away from the usual workplace. In some contracts, it is a daily fee for a service or for equipment, not only meal money.

This noun can be counted. One traveller may receive one per diem for each day on a trip. A finance team may process several of those payments in a week. That countable use is the only everyday case in which per diems is the natural plural.

As an adjective: a daily rate or daily arrangement

As an adjective, per diem describes something calculated, paid, or arranged by the day. Dictionaries record senses such as "based on use or service by the day" and "paid by the day." The phrase comes before another noun and does not take a plural ending.

Typical collocations include a per diem rate, per diem expenses, per diem interest, and per diem staff. The plural, if any, belongs to the noun that follows: per diem rates, not per diems rates.

As an adverb: by the day

As an adverb, per diem means "by the day" or "for each day." It tells how someone is paid or how a charge is calculated. This is the sense closest to the original Latin phrase, and it does not change in the plural.

You can write that a consultant is paid per diem, that a hospital hires therapists per diem as needed, or that a fee is charged $80 per diem. The -s would be wrong there because the phrase is not naming several allowances. It is describing the method of payment.

When to Write Per Diems and When to Keep Per Diem

Per diems is usually the better plural when you are counting daily allowances or daily fees. Per diem is the better form when the phrase describes another noun or tells how someone is paid. The choice should depend on the grammatical job, not on how formal the document is.

English did not import a Latin plural for the whole phrase. In Latin, diem is already the accusative singular of dies ("day") after the preposition per. English treats the borrowed unit as one noun and adds -s: per diems. Forms such as per dies or per dia are not the English plural.

Merriam-Webster, Britannica Dictionary, and Dictionary.com all list the noun plural as per diems. That is the form to use in expense English when the word is a thing you can count.

Use per diems for countable daily allowances

Write per diems when the phrase stands alone as a plural noun. The test is simple: could you replace it with daily allowances or daily fees and still have a grammatical sentence?

Natural examples include "Please submit your per diems by Friday," "Finance processed three per diems this week," and "Staff per diems vary by destination." In each sentence, the phrase is the thing being submitted, processed, or compared.

The same plural is acceptable in speech among colleagues who already use the term. It is not slang. It is ordinary American and international business English, and Singapore workplaces that copy global travel templates use it in the same way.

Keep per diem unchanged before another noun

Keep per diem unchanged when it modifies another noun. The first word group is an adjective. The second word carries number.

Write per diem rates, per diem allowances, per diem payments, and per diem employees. Do not write per diems rates or per diems staff. If you need a plural idea, pluralise the head noun: one per diem rate, two per diem rates.

This is the same pattern as other invariable modifiers. English does not add -s to full-time in full-time staff, and it does not add -s to per diem in per diem staff.

Keep per diem unchanged as an adverb

Keep per diem unchanged when it answers "how" rather than "what." If the phrase can be replaced by by the day or for each day, it is an adverb and takes no -s.

Write "The interpreter is paid per diem," not "paid per diems." Write "Meals are reimbursed $50 per diem," not "$50 per diems." The number, if there is one, belongs to the money or the days, not to a plural noun per diems.

The table below summarises the three jobs. Use it as a scanning guide before you send an expense email or edit a contract clause.

The trade-off is precision versus readability. Per diem is precise in finance templates that already use the term. Daily allowance or paid by the day is clearer for mixed-language teams who may not know the Latin phrase. Consistency inside one document matters more than mixing both styles for variety.

1. Ask whether you can count it. If you can say "one" or "three," you are looking at the noun, and per diems is available.
2. Look at the next word. If a noun such as rate, staff, or interest follows, keep per diem unchanged and pluralise that noun if needed.
3. Test the meaning "by the day." If the phrase tells how payment works, it is an adverb and stays per diem.
4. Rewrite if the reader may not know the term. Daily allowances, daily rates, or paid by the day will often be safer in a first email to a mixed team.

Formal Business-Writing Examples

Formal writing does not require a different plural. It requires the right function. The same three tests apply in expense claims, travel emails, HR notes, and contracts. The samples below are wording models, not a company's policy amounts.

Expense claims and travel emails

Noun, singular: "Each traveller receives a per diem for meals on overnight trips." The phrase names the allowance itself.

Noun, plural: "Please submit your per diems for 12 to 14 August together with the trip report." Finance is collecting several daily allowances, so the plural noun is natural.

Adjective: "Please confirm the per diem rate for Jakarta before we issue the travel request." Rate is the noun. Per diem only describes it, so there is no -s on per diem.

Adjective with a plural head noun: "Updated per diem rates take effect on 1 April." The plural is rates.

Adverb: "Meals are covered $60 per diem; keep taxi receipts for reimbursement." Here per diem means "for each day." A plural per diems would make the sentence harder to parse, because the reader might think several allowances are being named.

Contracts, HR, and day-rate work

A second workplace meaning appears in staffing. People employed per diem work on the days they are called in, rather than on a regular salary. The adverb describes the hiring method: "The hospital employs some respiratory therapists per diem as needed."

The adjective follows the same rule: per diem staff, per diem nurses, a per diem contract. Do not write per diems staff. If you mean the daily fees those workers earn, the noun plural can appear later in the sentence: "Submit per diem timesheets on Monday so we can process the per diems."

In Singapore job ads and internal HR notes, this day-rate sense is easy to confuse with travel money. Nearby words settle the meaning. Meals, lodging, trip, and claim point to an allowance. Shift, cover, locum, and called in point to day-rate work. If either reading is possible, replace the Latin phrase with daily allowance or paid by the day.

Common Errors With Per Diem

Most mistakes come from adding -s to the wrong job, or from treating the Latin phrase as if it were an acronym. The corrections below are usage judgements for workplace English, not claims about any one company's house style.

Two other slips are common in email. The first is writing PER DIEM as if the letters stood for an English acronym. They do not. Keep lower case unless the phrase starts a sentence. The second is using per diem as a synonym for salary. A per diem is typically a daily allowance or a daily fee, not a person's ordinary monthly pay.

Hyphenation is a style choice, not a grammar test. The dictionary form is unhyphenated per diem. Some writers hyphenate the adjective before a noun (a per-diem basis), but Latin phrases in English are usually left with a space. Do not hyphenate the noun or the adverb unless a house style sheet requires it.

Italics are optional in teaching notes and unnecessary in ordinary business documents. In a finance email, write the phrase in regular type. Save italics for when you are explaining the term itself, as this page does.

Practical Usage for Singapore Professionals

The problem usually appears at the moment of sending. A professional in Singapore writes to finance after a regional trip, or an HR coordinator drafts a note about weekend cover. The Latin phrase is already in the template, so the writer copies it and then has to decide whether to add -s.

The cost of a wrong form is not only a grammar mark. A plural on the adjective can make a claim look sloppy. A missing plural on the noun can make it unclear whether one day's allowance or several days' allowances are due. In mixed-language teams, an unexplained per diem can also be read as day-rate hiring when the writer only meant meal money.

Assess the sentence on three points: what is being counted, whether a noun follows, and whether the reader already uses the term. If the email may be forwarded to a client, a vendor, or a new colleague, daily allowance or paid by the day often prevents a second email asking what the Latin means.

That register choice is a workplace English skill, not only a vocabulary fact. Adult English courses in Singapore often practise this kind of decision: when a Latin business phrase is precise, and when a plain synonym is safer for mixed teams.

iWorld Learning is a Singapore-based English language school that helps adults improve practical English for work through small classes, CEFR-based learning paths, and internationally certified TESOL/TEFL teachers. If you want guided practice with expense emails, contract wording, and other workplace terms, look at iWorld Learning's business English courses in Singapore. Adult courses include a free trial class, and you can contact the team to ask about class times at the Orchard campus, 111 Somerset Road, #10-19, near Somerset MRT on the Red Line.

FAQ

Should I write per-diem with a hyphen?

The standard dictionary form is two words with a space: per diem. You do not need a hyphen for the noun or the adverb. Some house styles hyphenate the adjective before a noun, as in a per-diem basis. If your company template already uses the spaced form, keep that. Do not write perdiem as one word.

What is the difference between a per diem and a reimbursement?

A per diem is usually a set daily amount for expenses such as meals. A reimbursement is money paid back against actual costs, often with receipts. Company policy decides which system applies. In writing, do not treat the two as synonyms unless your policy uses them that way. If you are unsure, name the policy term your HR team uses.

Can I use per diem in a formal email in Singapore?

Yes, if the reader already uses the term in travel or HR documents. It is established business English, not slang. For mixed-language teams, daily allowance or daily rate can be clearer. Match the wording in the policy you are quoting. If the email may be forwarded outside the team, daily allowance often reduces confusion.

Should I italicise or capitalise per diem?

In ordinary business writing, write per diem in regular type, not italics, and in lower case unless it starts a sentence. It is a borrowed phrase, not an acronym, so PER DIEM looks mistaken. Italics are optional in teaching notes to mark the term. Follow your organisation's style sheet if it already specifies a form.

How do you pronounce per diem?

In American English, common pronunciations are puhr DEE-uhm and puhr DYE-uhm. British dictionaries often show a form closer to pur DEE-em. In Singapore workplaces, either clear pronunciation is acceptable if colleagues recognise the term. If the listener looks unsure, say daily allowance instead of repeating the Latin phrase more loudly.

What does per diem staff mean?

Per diem staff are workers paid by the day rather than on a regular salary. The phrase is an adjective plus a noun, so it stays per diem staff, not per diems staff. This sense is common in healthcare and project work. It is different from a travel per diem, which is expense money. Check nearby words before you decide which meaning is intended.

Summary

Per diems is the usual plural when you are counting daily allowances or daily fees. Per diem stays unchanged when it modifies another noun or works as an adverb meaning "by the day." English added -s to the whole borrowed phrase. It did not create a Latin-looking plural such as per dies.

Choose the form by the grammatical job, then stay consistent with your organisation's policy wording. If the reader may not know the term, daily allowance, daily rate, or paid by the day will often be the clearer workplace choice.