Past Tense of “cost”
The past tense of cost is cost; the past participle is cost.
Why “cost” behaves this way
"Cost" was borrowed into English from Old French "couster" (modern French "coûter"), ultimately from Latin "constare," meaning "to stand together" or, by extension, "to stand firm at a fixed price." As a relatively late borrowing, "cost" never adopted the regular English "-ed" past-tense ending in its core financial sense, settling instead into the same zero-change pattern as several other short verbs on this list, though a separate, fully regular verb "cost" (meaning to calculate or estimate the cost of something, as in accounting) does take "-ed" ("costed") in that specialized professional sense.
This split between an irregular, zero-change "cost" (the price of something) and a regular "costed" (the accounting process of calculating that price) is a genuine, if narrow, grammatical distinction that many native speakers aren't consciously aware of, since the two senses overlap closely enough in everyday speech that the difference rarely surfaces outside specifically financial or project-management writing.
The common confusion
The real confusion is knowing when "costed" is actually correct: everyday sentences about price always use unchanged "cost" ("it cost ten dollars"), while the specialized business sense of formally calculating or estimating expenses takes the regular "-ed" form ("the project was costed at two million dollars"), a distinction easy to miss since both describe money in some sense.
How it's actually used
"Cost" extends widely beyond money into any kind of consequence or sacrifice — "it cost him his reputation," "the decision cost lives" — treating an abstract loss as though it were a literal price paid. Project management and accounting specifically distinguish "costing" (the formal process of estimating expenses) from ordinary "cost" (the resulting price itself), a technical distinction with real professional consequences in budgeting and reporting. Insurance and risk management specifically calculate the "cost" of potential future events ("the cost of a claim"), a precise actuarial use of the word distinct from the everyday sense of a price already paid. "Cost of living" is a standard economic term measuring how much income is needed to maintain a given standard of living in a specific place, a specialized compound noun built directly on this verb's core sense.
Across the tense system
"Cost" in the passive-feeling construction "at all costs" (regardless of consequence, as in "win at all costs") functions as a fixed idiomatic phrase rather than an ordinary conjugated verb use, a common rhetorical intensifier in both business and everyday speech. "Costly" as a related adjective describes something expensive either financially or in terms of consequence ("a costly mistake"), extending the noun sense of cost into a broader descriptive judgment about a decision's price.
Common real mistakes
The most frequent error is using "costed" in the everyday financial sense where unchanged "cost" is required ("it costed ten dollars" instead of the correct "it cost ten dollars"), a mistake that likely stems from over-applying the narrower, specialized accounting sense of the regular form. Learners occasionally also confuse "cost" (a specific sum) with "expensive" (a general quality), producing awkward sentences that mix the two grammatically incompatible parts of speech. Learners also sometimes blur "cost" (an objective price) together with "worth" or "value" (a subjective assessment), since something can cost very little while still being worth a great deal to a particular person, two genuinely different concepts easily conflated in translation. A final subtlety worth noting: "cost-effective" describes a solution offering good value relative to its price, a specific business and engineering term distinct from simply "cheap," since something can be cost-effective while still being relatively expensive in absolute terms.
Examples
- The repair cost more than we expected.
- It has cost the company millions in lost revenue.
- The delay cost them the contract entirely.
- We had costed the entire renovation before starting work. (business/accounting sense)
- The trip costs less if you book early.
- Her mistake cost her the promotion.
- It's costing more every year to maintain this building.
- The war cost countless lives on both sides.
Memory trick
Default to unchanged "cost" for everyday sentences about price or consequence, and reserve regular "costed" strictly for the specialized business sense of formally calculating or estimating an expense — remembering the two are grammatically distinct despite both involving money.