Purpose　-　Owing　to　the　cable　flexibility,　it　is　practically　a　lot　easier　to　measure　the　high-vibration　frequencies　of　the　cable　than　the　fundamental　vibration　frequency.　The　objective　of　this　study　is　to　present　a　method　to　determine　the　cable　tension　based　on　frequency　differences　so　that　the　effects　of　cable　sag　and　bending　stiffness　can　be　included.　Design/methodology/approach　-　The　paper　includes　theoretical　derivation,　laboratory　study　to　verify　the　method　and　practical　application　in　a　real　bridge.　Findings　-　It　is　suggested　to　measure　the　high-vibration　frequencies,　and　to　use　the　vibration　frequency　difference　to　determine　the　fundamental　vibration　frequency　of　the　cable　and　then　to　estimate　the　cable　tension.　The　reliability　of　the　method　is　verified　by　laboratory　tests　and　the　method　is　then　applied　to　determine　cable　tensions　in　a　real　bridge.　Originality/value　-　This　paper　provides　theoretical　derivations　to　demonstrate　that　under　certain　conditions,　the　frequency　difference　of　a　cable　with　sag　and　bending　is　almost　equal　to　the　natural　frequency　of　the　same　cable　when　it　is　taut.　This　unique　characteristic　of　cable　vibration　is　used　to　determine　the　cable　tension　similar　to　the　fundamental　frequency-based　taut-string　formula　that　is　commonly　used　in　practice.