Synonyms of certain

1 Syllable

set, real, bound, calm, sure

2 Syllables

steadiest, special, steadier, conscious, bullseye, stated, steady, downright, odds-on, anier, realer, coming, cockier, for-sure, sure-thing, realest, well-set, sure-fire, factual, foolproof, risk-free, riskless, actual, cocky

3 Syllables

inferred, authentic, true-hearted, testable, provable, unerring, destined, genuine, reliable, convinced, questionless, satisfied, undoubting, emphatic, returnless, nontoxic, implicit, secure, cocksure, explicit, ironclad, incorrupt, truehearted, palpable, failsafe, undoubted, inerrant, several, sanguine, untroubled, assured, believing, approved, non-toxic, undoubtful

4 Syllables

double-checked, surefire, high-principled, predictable, infallible, unconcerned, dependable, unchallenged, demonstrable, undisturbed, unperturbed, inerrable, apodictic, axiomatic, foregone, undoubtable, peremptory, deductible, undisputed, unquestioning, positive, honest-to-god, really-truly, determining, determinant, foreseeable, univocal, assertive, decisive, unrefuted, undeniable, nonpoisonous, attestable, evincible, self-confident, confirmable, uncontested

5 Syllables

irreversible, unrepeatable, inescapable, determinate, indisputable, irrefutable, undeceivable, anticipated, irrevocable, irrebuttable, unanswerable, honest-to-goodness, unavoidable, unquestionable, categorical, unconditional, apothegmatic, unequivocal, indubitable, unhesitating, supraliminal, legitimate, unconditioned, inevasible, unescapable, uncontestable, unrepealable, unreversible

6 Syllables

incontrovertible, determinative