QTc Calculator
milliseconds (ms) — from ECG
beats per minute (bpm)
normal cutoffs differ by sex
—
ms — QTc (Bazett Formula)
All Formula Results
Bazett (most used)—
Fridericia (preferred at high HR)—
Framingham—
Hodges—
QTc (Bazett)
—
ms
QT (measured)
—
ms
RR interval
—
seconds
QTc Normal Range & Interpretation
Bazett: QTc = QT ÷ √(RR in seconds) [most common, overestimates at high HR]
Fridericia: QTc = QT ÷ RR^(1/3) [preferred at HR >60 or <100]
Framingham: QTc = QT + 0.154×(1−RR)
Hodges: QTc = QT + 1.75×(HR−60)
RR interval = 60 ÷ Heart Rate
Fridericia: QTc = QT ÷ RR^(1/3) [preferred at HR >60 or <100]
Framingham: QTc = QT + 0.154×(1−RR)
Hodges: QTc = QT + 1.75×(HR−60)
RR interval = 60 ÷ Heart Rate
Normal QTc Values
- Normal males: QTc ≤440 ms
- Normal females: QTc ≤460 ms (women have naturally longer QTc due to hormonal effects)
- Borderline prolonged (males): 441–460 ms — monitor, identify and remove risk factors
- Borderline prolonged (females): 461–480 ms
- Prolonged: >460 ms males / >480 ms females — avoid QT-prolonging drugs, check electrolytes
- Critical / High TdP risk: >500 ms in either sex — drug withdrawal, electrolyte correction, cardiac monitoring, consider admission
Why QTc Varies by Formula
All QTc formulas attempt to remove the effect of heart rate on QT duration. Bazett's formula (QT ÷ √RR) is most widely used but overcorrects at high heart rates (>100 bpm) — making QTc falsely high in tachycardia. Fridericia (cube root correction) performs better at high and low heart rates and is recommended by the FDA and most regulatory bodies for drug-induced QT assessment. For clinical bedside use, Bazett remains the standard.
Common QT-Prolonging Drugs
Cardiac Drugs
- Amiodarone High
- Sotalol High
- Quinidine High
- Procainamide High
- Disopyramide High
- Dofetilide High
Antibiotics
- Azithromycin High
- Clarithromycin High
- Ciprofloxacin Moderate
- Moxifloxacin High
- Chloroquine High
- Fluconazole Moderate
Psychiatry
- Haloperidol High
- Quetiapine Moderate
- Olanzapine Moderate
- Citalopram ≥40mg High
- Tricyclics (TCA) High
- Lithium Moderate
Others
- Ondansetron ≥32mg IV High
- Methadone High
- Domperidone High
- Tamoxifen Moderate
- Arsenic trioxide High
Risk Factors for Drug-Induced QT Prolongation
- Female sex (naturally longer baseline QTc)
- Hypokalaemia (most common electrolyte cause — repolarisation reserve reduced)
- Hypomagnesaemia
- Hypocalcaemia
- Bradycardia — longer RR interval gives more time for delayed repolarisation
- Congenital Long QT syndrome
- Heart failure, left ventricular hypertrophy
- Combining multiple QT-prolonging drugs
Related Calculators
⚠ Medical Disclaimer: QTc calculation is a screening tool only. Accurate QT measurement from a 12-lead ECG requires trained clinical assessment — the QT interval should be measured in leads II or V5, from QRS onset to T-wave end, averaged over 3–5 beats. Machine-measured QT values may be inaccurate. Always interpret in clinical context. QTc >500 ms warrants urgent cardiology review.