# Corrected QT Interval Calculator

> Calculate corrected QT interval using Bazett, Fridericia, Framingham, and Hodges formulas from QT and heart rate

**Category:** Health
**Keywords:** QT interval, QTc, Bazett, Fridericia, Framingham, Hodges, cardiology, ECG, EKG, heart rate, arrhythmia, long QT
**URL:** https://complete.tools/corrected-qt-interval-calculator

## The four correction formulas

**Bazett (1920) — most widely used:**
```
QTcB = QT / √(RR)
```
Simple and familiar, but overcorrects at high heart rates and undercorrects at low rates. Remains the clinical standard despite its limitations.

**Fridericia (1920) — preferred at extreme rates:**
```
QTcFri = QT / ∛(RR)
```
Uses the cube root rather than square root. Better performance at heart rates below 50 or above 90 bpm. Preferred in many research settings.

**Framingham (1992):**
```
QTcFra = QT + 0.154 × (1 − RR)
```
Linear correction derived from the Framingham Heart Study. Less sensitive to extreme rates than Bazett.

**Hodges (1983):**
```
QTcH = QT + 1.75 × (HR − 60)
```
Linear formula using heart rate directly. Simple to calculate mentally. Performs similarly to Fridericia in most ranges.

RR interval is calculated as 60 ÷ heart rate (seconds).

## Normal QTc reference ranges

Reference ranges differ slightly by sex due to hormonal effects on cardiac repolarisation:

**Males:**
- Normal: ≤440 ms
- Borderline: 441–460 ms
- Prolonged: >460 ms

**Females:**
- Normal: ≤450 ms
- Borderline: 451–470 ms
- Prolonged: >470 ms

Values above 500 ms carry substantially elevated arrhythmia risk and usually prompt immediate clinical review.

## Clinical uses

QTc calculation is important in several clinical situations:

- **Drug safety monitoring:** Many medications (antiarrhythmics, antipsychotics, antibiotics, antiemetics) prolong the QT interval. Baseline and serial QTc checks guide prescribing decisions.
- **Electrolyte disorders:** Hypokalaemia, hypomagnesaemia, and hypocalcaemia extend the QT interval and increase arrhythmia risk.
- **Congenital long QT syndrome (LQTS):** Genetic channelopathy diagnosed partly by persistently prolonged QTc on resting ECG.
- **Pre-operative assessment:** Some anaesthetic agents affect QTc; baseline measurement helps risk-stratify patients.

## How to use

1. Select the patient's sex (affects normal range interpretation)
2. Enter the measured QT interval in milliseconds from the ECG
3. Enter the patient's heart rate in beats per minute
4. Click "Calculate QTc" to see all four corrected values
5. Each result is labelled Normal, Borderline, or Prolonged based on sex-specific thresholds
6. Click "Start Over" to enter new values

## FAQs

**Q:** Which formula should I use?


**A:** Bazett is the most widely used in clinical practice and familiar to most clinicians. Fridericia is preferred when heart rate is below 50 or above 90 bpm, as Bazett overcorrects at extremes. When in doubt, look at all four values — if they all agree, you can be confident in the result.


**Q:** How do I measure the QT interval from an ECG?


**A:** Measure from the start of the Q wave (or R wave if no Q wave is visible) to the end of the T wave, at the point where the T wave returns to the isoelectric baseline. Use the lead with the longest visible QT interval, commonly lead II or V5. Avoid including U waves.


**Q:** What is the RR interval?


**A:** The RR interval is the time between two consecutive R wave peaks on the ECG, equal to 60 divided by the heart rate in seconds. At 60 bpm, RR = 1.0 second. At 75 bpm, RR ≈ 0.8 seconds.


**Q:** Can a short QTc be dangerous too?


**A:** Yes. Short QT syndrome (QTc below approximately 340 ms) is a rare but serious condition associated with atrial fibrillation and sudden death. This calculator flags prolonged values; for clinical evaluation of short QTc, consult a cardiologist.


**Q:** Why do the four formulas give different results?


**A:** Each formula uses a different mathematical relationship to model how the QT interval should vary with heart rate. They agree closely at normal heart rates (60–80 bpm) but diverge at rates outside that range. The variation highlights the imprecision of all correction methods.

---

*Generated from [complete.tools/corrected-qt-interval-calculator](https://complete.tools/corrected-qt-interval-calculator)*