Synonyms for sure

These adjectives mean feeling or showing no doubt. Sure and certain are frequently used interchangeably; sure, however, is the more subjective term, whereas certain may imply belief based on experience or evidence: “Never teach a child anything of which you are not yourself sure” (John Ruskin). “In this world nothing is certain but death and taxes” (Benjamin Franklin).
Confident suggests assurance founded on faith or reliance in oneself or in others: The senator is confident of reelection.
Positive suggests full, emphatic certainty: The prosecutor had positive proof of the defendant's guilt.