QTc Calculator (Cálculo QT)
Enter the measured QT interval and heart rate to estimate corrected QT (QTc) using common formulas: Bazett, Fridericia, Framingham, and Hodges.
What Is “Calculo QT”?
“Calculo QT” usually refers to calculating the corrected QT interval (QTc) from an ECG. The raw QT interval changes with heart rate: it gets shorter when the heart beats faster and longer when it beats slower. Because of that, clinicians use a correction formula to normalize QT and compare values more consistently.
The QT interval reflects electrical depolarization and repolarization of the ventricles. A significantly prolonged QTc can increase the risk of dangerous arrhythmias, especially Torsades de Pointes. That is why a quick, accurate calculation can be helpful in medical review, medication checks, and emergency evaluation.
How This QT Calculator Works
This tool uses your measured QT interval (in milliseconds) and heart rate (in bpm) to estimate the RR interval and then apply four common correction formulas:
- Bazett: QTc = QT / √RR
- Fridericia: QTc = QT / RR1/3
- Framingham: QTc = QT + 154 × (1 − RR)
- Hodges: QTc = QT + 1.75 × (HR − 60)
All outputs are presented in milliseconds (ms). In many practical settings, Fridericia or Framingham may be preferred at very high or very low heart rates because Bazett can overcorrect or undercorrect.
Quick Interpretation Guide
- Marked prolongation: QTc ≥ 500 ms (higher arrhythmic risk).
- Male reference: prolonged if > 450 ms.
- Female reference: prolonged if > 470 ms.
- Values in the border zones should be interpreted with clinical context and repeat ECG when needed.
Why QTc Matters in Practice
QT prolongation is often multifactorial. Common contributors include certain medications (for example, some antiarrhythmics, antipsychotics, macrolide antibiotics), electrolyte disturbances (low potassium, magnesium, or calcium), congenital channelopathies, and structural heart disease. A QTc estimate can support safer decisions such as medication adjustments and closer monitoring.
When to Be Extra Careful
- Starting a new QT-prolonging medication
- Combining multiple QT-prolonging drugs
- History of syncope, palpitations, or unexplained seizures
- Bradycardia, severe illness, or dehydration
- Known congenital long-QT syndrome
Step-by-Step Example
If QT is 420 ms and heart rate is 90 bpm:
- RR = 60 / 90 = 0.667 s
- Bazett QTc ≈ 514 ms (often high at faster rates)
- Fridericia QTc ≈ 481 ms
- Framingham QTc ≈ 471 ms
- Hodges QTc ≈ 473 ms
This is exactly why reporting multiple formulas can be useful: each method behaves differently across heart-rate ranges.
Common Errors in QT Calculation
1) Measuring the wrong QT endpoint
The end of the T wave can be subtle. Inconsistent lead choice or endpoint selection can create large differences in QTc.
2) Ignoring rhythm irregularity
In atrial fibrillation or ectopy, one beat can be misleading. Averaging beats can improve reliability.
3) Blindly trusting one formula
No formula is perfect in every scenario. Reviewing more than one estimate gives a more balanced interpretation.
4) Missing clinical context
QTc numbers are part of a bigger picture including symptoms, electrolytes, medications, and family history.
Final Thoughts
A practical calculo QT workflow is simple: measure accurately, calculate with at least one robust correction formula, and interpret in context. Use this page as a quick assistant for education and screening, then confirm findings with full clinical assessment when needed.