<strong>Assure</strong> means to remove someone's doubt, and it takes a person as its object: <em>I assured her the parcel had been sent</em>. <strong>Ensure</strong> means to make certain something happens, and it takes an event or outcome: <em>please ensure the back door is locked</em>. You assure people; you ensure results.
The core difference
The quickest way to separate them is by what comes next in the sentence. Assure is followed by a human being. Ensure is followed by a thing that needs to happen. Only one of these two verbs can have somebody's name straight after it.
Using assure
- The surgeon assured him the procedure was routine.
- I can assure you nothing was deleted.
- She assured the tenants that the boiler would be replaced.
It always implies an audience with doubts to be settled. That is why rest assured works as a fixed phrase: it is telling the reader to stop worrying.
Using ensure
- Double-check the figures to ensure nothing is missed.
- Good drainage ensures the foundations stay dry.
- We must ensure that every applicant is treated the same way.
The test
Look at the word immediately after the verb. If it names a person, use assure. If it names something that must be made true, use ensure. A second check: try swapping in reassure — if it fits, you want assure; try swapping in make sure — if that fits, you want ensure.
And then there's insure
The third member of the family causes half the trouble. Insure is the word for financial protection everywhere — the van is insured for business use. British English stops there, and rarely uses insure for anything else.
American usage is looser, and this is the actual source of the difficulty: insure is very commonly used where a British writer would put ensure (steps taken to insure a fair hearing). Several American style guides prefer the ensure/insure split, but others still allow the older overlap, so it is a live preference rather than a settled rule — and meeting it in print is not evidence that the writer was confused.
The memory trick
Read the first letters as prompts: assure calms a person, ensure guarantees an event, insure protects against indemnity. Once the person/event split is fixed in your head, the rest follows.
The edge case
In British insurance jargon, assurance historically means cover for something certain to happen eventually — life assurance — while insurance covers events that may never occur, such as a fire. The distinction survives in company names and policy documents, and it is a genuine technical usage rather than an error.
Frequently Asked Questions
Is it "ensure that" or "assure that"?
Ensure that, when you mean make certain something happens: ensure that the form is signed. Assure needs a person in between: assure the client that the form is signed.
Which one do I use for insurance?
Insure. The van is insured, the policy insures against theft. Ensure means make certain; assure means reassure a person. British insurers also use "assurance" for life cover specifically.
Is "I want to ensure you" correct?
No — that should be "I want to assure you". Ensure cannot take a person as its object. If a human follows the verb, the verb is assure.