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QTc Interval Calculator

Calculate the corrected QT interval using Bazett, Fridericia, Framingham, and Hodges formulas. Prolonged QTc (>500ms) increases risk of Torsades de Pointes and sudden cardiac death.

Concept Fundamentals
<440ms M/<460ms F
Normal QTc
>500ms
Danger Zone
4 Methods
Formulas

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Key values and context for this tool: • Normal QTc: <440ms M/<460ms F • Danger Zone: >500ms • Formulas: 4 Methods

Key values
<440ms M/<460ms F
Normal QTc
Key value
>500ms
Danger Zone
Key value
4 Methods
Formulas
Key value

Ready to run the numbers?

Why: Calculate QTc using Bazett, Fridericia, Framingham, and Hodges formulas.

How: QTc prolongation (>450ms men, >470ms women) increases risk of Torsades de Pointes and sudden cardiac death.

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QRS onset to T end
ms
Use if RR known; else use HR
ms
If RR not used
bpm

For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.

QTc corrects the QT interval for heart rate. Prolongation (>450ms men, >470ms women) increases Torsades de Pointes risk. Formulas: Bazett (QT/√RR), Fridericia (QT/∛RR), Framingham, Hodges. Sources: Bazett 1920, Fridericia 1920, AHA/ACC, ESC.

450
Prolonged (M) ms
470
Prolonged (F) ms
500
Severe (ms)
4
Formulas

Sources: Bazett 1920, Fridericia 1920, AHA/ACC, ESC

Key Takeaways

  • • Bazett overcorrects at high HR, undercorrects at low HR; Fridericia often more accurate
  • • QTc >500ms is high risk for TdP; consider holding QT-prolonging drugs
  • • Women have longer QT; use gender-specific thresholds
  • • Check electrolytes (K, Mg, Ca) when QTc is prolonged

Did You Know?

📈 Bazett formula (1920) is most widely used despite limitations at extreme HR
📊 Fridericia (1920) uses cube root; may be better at bradycardia/tachycardia
💡 Hodges: QTc = QT + 1.75(HR − 60); linear correction
🌍 FDA recommends QT assessment before drugs that prolong QT
📈 QTc >500ms or ΔQTc >60ms from baseline: high TdP risk
🎯 Congenital LQTS: genetic channelopathies; screen family if suspected

How Does QTc Work?

Bazett

QTc = QT ÷ √(RR). RR in seconds. Overcorrects at high HR.

Fridericia

QTc = QT ÷ ∛(RR). Often preferred at extreme heart rates.

Hodges

QTc = QT + 1.75(HR − 60). Linear; no RR needed.

Expert Tips

Use lead II or V5 for QT measurement; take the longest QT across leads
At HR <50 or >100, prefer Fridericia or Hodges over Bazett
Correct electrolytes before attributing prolongation to drugs
Tisdale score and other tools help stratify drug-induced TdP risk

QTc Categories

QTc (ms)Category
<440 (M) / <460 (F)Normal
440–470 (M) / 460–480 (F)Borderline
>470 (M) / >480 (F)Prolonged
>500Severely Prolonged

Frequently Asked Questions

What is QTc?

QTc is the QT interval corrected for heart rate. QT varies with RR interval; QTc normalizes it. Prolonged QTc (&gt;500ms) increases risk of Torsades de Pointes and sudden cardiac death.

How do correction formulas compare?

Bazett: QTc = QT ÷ √(RR); overcorrects at high HR. Fridericia: QTc = QT ÷ ∛(RR); often better at extreme HR. Hodges: QTc = QT + 1.75(HR − 60); linear. Framingham uses different correction. Prefer Fridericia or Hodges at HR &lt;50 or &gt;100.

What defines prolonged QTc?

Normal: &lt;440ms (M), &lt;460ms (F). Borderline: 440–470ms (M), 460–480ms (F). Prolonged: &gt;470ms (M), &gt;480ms (F). Severely prolonged: &gt;500ms. Thresholds vary by guideline.

What causes drug-induced QT prolongation?

Antiarrhythmics, antipsychotics, antibiotics, antidepressants, and others. Check FDA labels. Correct electrolytes (K, Mg, Ca) before attributing prolongation to drugs. Consider Tisdale score for risk stratification.

What is Torsades risk?

QTc &gt;500ms or ΔQTc &gt;60ms from baseline: high TdP risk. Consider holding QT-prolonging drugs. Monitor ECG. Congenital LQTS requires genetic screening and family evaluation.

When should QTc be measured?

Before starting QT-prolonging drugs, in LQTS screening, electrolyte disorders, and drug safety monitoring. ECG required; measure QT from QRS onset to T-wave end. Use lead II or V5.

Key Statistics

1920
Bazett/Fridericia
450
Prolonged M (ms)
470
Prolonged F (ms)
TdP
Prolongation risk

Official Data Sources

AHA/ACC ↗
ECG guidelines
ESC ↗
European guidelines
FDA ↗
QT drug guidance
PubMed ↗
QTc research

⚠️ Disclaimer: This calculator is for educational purposes only. It is not a substitute for professional medical advice. QTc interpretation requires physician review. Always consult a qualified healthcare provider.

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