qt interval calculator

What is the QT interval?

The QT interval is the portion of an electrocardiogram (ECG) that reflects the total time for ventricular depolarization and repolarization. In simple terms, it represents how long the lower chambers of the heart take to contract and then recover before the next beat.

Because QT length changes with heart rate, clinicians usually look at QTc (corrected QT), which adjusts the measured QT interval so values can be compared more consistently across different heart rates.

How this QT interval calculator works

This calculator takes your measured QT (in milliseconds) and heart rate (in beats per minute), then estimates QTc using one or more correction formulas. The formulas used are:

• Bazett: QTc = QT / √RR
• Fridericia: QTc = QT / RR^1/3
• Framingham: QTc = QT + 0.154 × (1 − RR)
• Hodges: QTc = QT + 1.75 × (HR − 60)

RR is the interval between beats in seconds (RR = 60 / HR). Different formulas can give slightly different answers, especially at very high or very low heart rates.

How to use the calculator

1. Enter the measured QT interval from the ECG (ms).
2. Enter the heart rate in bpm.
3. Choose sex if you want sex-specific interpretation ranges.
4. Select one formula or choose “All formulas”.
5. Click Calculate QTc to view results.

How to interpret results

QTc interpretation depends on context, measurement method, and patient factors. A common practical framework:

• Male: up to ~450 ms often considered normal, 451–470 ms borderline, >470 ms prolonged.
• Female: up to ~460 ms often considered normal, 461–480 ms borderline, >480 ms prolonged.
• Any sex: ≥500 ms is generally considered clearly high-risk and deserves urgent review.

Interpretation should always be linked to symptoms, medication exposure, electrolytes, congenital risk, and the quality of the ECG tracing.

Common causes of prolonged QTc

1) Medications

Many drugs can prolong QT, including some antiarrhythmics, certain antibiotics, antipsychotics, antidepressants, and antiemetics. Risk rises when multiple QT-prolonging drugs are combined.

2) Electrolyte disturbances

Low potassium, low magnesium, and low calcium can all increase QTc and arrhythmia risk.

3) Congenital long QT syndrome

Inherited ion channel disorders can produce persistently prolonged QTc and may require specialist management.

4) Bradycardia and structural illness

Slow heart rates, myocardial disease, endocrine conditions, and acute illness may influence QTc values.

Tips for better QT measurement

• Use a clean ECG lead where T-wave end is clear.
• Avoid including U waves as part of the T wave unless clinically indicated.
• Average multiple beats in irregular rhythms.
• Recheck QTc after correcting electrolytes or adjusting medications.

Important safety note

This tool is educational and not a diagnosis. If a person has syncope, palpitations, family history of sudden cardiac death, or a QTc near/above 500 ms, seek urgent professional medical evaluation.