Bazett
— ms——
4 formulas · Formula review July 2026
QTc Calculator
Calculate the corrected QT interval from a measured QT and either heart rate or RR interval. The calculator shows Bazett, Fridericia, Framingham and Hodges results together, so you can see how formula choice changes the number.
Quick answer: QTc adjusts the measured QT interval for heart rate. Enter QT in milliseconds or ECG small boxes, then add heart rate or RR interval.
4
Formulas
QTc
Milliseconds
ECG
Box Mode
For adult education and calculation support. This tool does not diagnose long QT syndrome, assess medicine safety, or replace manual ECG review. Wide QRS, pacing, irregular rhythm and pediatric ECGs need a different approach.
Corrected QT interval
Calculated locallyEnter ECG Measurements
Corrected QT comparison
Four formulasQTc Results
Measured QT—
Heart rate—
RR interval—
Formula spread—
Fridericia
— ms——Framingham
— ms——Hodges
— ms——Do not treat one number as a diagnosis. Confirm the tracing, formula, rhythm and QRS width. Symptoms, medicines and electrolyte findings also matter.
What Does QTc Mean?
The QT interval runs from the start of ventricular depolarization to the end of ventricular repolarization on an ECG. It becomes shorter when the heart beats faster and longer when the heart beats slower. A QTc calculation applies a formula so the measured QT can be viewed in the context of heart rate.
QT is measured. QTc is calculated. An ECG machine may report both, but automated measurements should still be checked when the T-wave ending is unclear, the rhythm is irregular, or the QRS is wide.
How to Calculate QTc
First convert heart rate to an RR interval when RR is not already measured. Then apply a correction formula. QT is entered in milliseconds below, while RR is expressed in seconds.
RR (seconds) = 60 ÷ heart rate
| Formula | Equation | Input style |
|---|---|---|
| Bazett | QTcB = QT ÷ √RR | QT + RR |
| Fridericia | QTcF = QT ÷ ∛RR | QT + RR |
| Framingham | QTcFra = QT + 154 × (1 − RR) | QT in ms + RR in s |
| Hodges | QTcH = QT + 1.75 × (HR − 60) | QT in ms + heart rate |
For a measured QT of 400 ms and heart rate of 72 beats/min, RR is about 0.833 seconds. The estimated results are approximately 438 ms with Bazett, 425 ms with Fridericia, 426 ms with Framingham and 421 ms with Hodges. The difference is why a report should name the formula used.
Which QTc Formula Should You Use?
No single equation behaves the same at every heart rate and in every population. Bazett remains common, but studies have shown more rate dependence than several alternatives. Fridericia and Framingham performed well for rate correction in a large hospital ECG analysis. Formula choice still depends on the clinical setting and local protocol.
B
Bazett formula
Widely reported and easy to recognize. It may overcorrect at high heart rates and undercorrect at low rates.
F
Fridericia formula
Uses the cube root of RR. It often changes less than Bazett when heart rate moves away from 60 bpm.
Fr
Framingham formula
A linear RR correction. It is useful for comparison because it responds differently from power formulas.
H
Hodges formula
A linear heart-rate equation. It uses beats per minute directly instead of RR in the final equation.
How to Calculate QTc on an ECG
- 1
Choose a clear beat
Use a lead where the start of the QRS and the end of the T wave can be seen clearly. Do not guess through noise.
- 2
Measure QT
Measure from the first deflection of the QRS complex to the end of the T wave. A merged U wave can make the ending uncertain.
- 3
Check paper speed
At 25 mm/s, each small box is 40 ms. At 50 mm/s, each small box is 20 ms.
- 4
Measure rate or RR
Use the RR interval paired with the measured beat, or enter a reliable heart rate when the rhythm is regular.
- 5
Report the formula
State whether the result is QTcB, QTcF, QTcFra or QTcH. These values are not automatically interchangeable.
QTc Calculation Normal Range
A commonly cited adult context is 350–450 ms for men and 360–460 ms for women. These are not universal diagnostic cutoffs. Age, correction formula, ECG quality, heart rate, QRS width, medicines, electrolytes and the clinical question can change interpretation.
Below cited range<350–360 ms
Depends on the adult reference group and method.
Common adult context350–460 ms
The sex-specific upper ceiling is commonly cited as 450 or 460 ms.
Above cited ceiling>450–460 ms
Confirm measurement and interpret with the named formula.
Higher-risk threshold≥500 ms
Associated with increased ventricular arrhythmia risk and needs clinical review.
The calculator labels ranges only as context. It cannot determine whether a QTc result is caused by a medicine, electrolyte problem, congenital condition or measurement error.
QT Interval vs QTc Interval
Measured value
QT interval
Time from the start of the QRS to the end of the T wave on the ECG.
Reported in msCalculated value
QTc interval
The measured QT after a mathematical heart-rate correction has been applied.
Report the formulaQTc is not a direct physical measurement. A calculated QTc can change when the same QT and heart rate are entered into different formulas.
Cases That Need Extra Care
- Atrial fibrillation or irregular rhythm: a single RR interval may not represent the rhythm. Beat selection and averaging require a defined clinical method.
- Bundle branch block or paced rhythm: a wide QRS lengthens the QT because depolarization is prolonged. JT-based or wide-QRS methods may be needed.
- Pediatric and neonatal ECGs: adult reference labels do not apply. Age-specific assessment is required.
- Unclear T-wave ending: low amplitude, noise or a merged U wave can change the measured QT.
- Very fast or slow heart rate: formula disagreement often grows at rate extremes.
This standard calculator intentionally stops at standard QT correction. It does not pretend that ordinary QTc formulas solve AF, LBBB, RBBB, ventricular pacing or pediatric interpretation.
What Can Affect the QT Interval?
QT can be influenced by congenital long QT syndromes, medicines, heart rate, body temperature and electrolyte levels such as potassium, magnesium and calcium. An abnormal calcium result also needs correct interpretation because total calcium changes with albumin. Our corrected calcium calculator explains that estimate and its limits.
Do not stop or start a prescribed medicine from an online QTc result. A clinician may need to verify the ECG, medicine list, symptoms, kidney function and laboratory results.
Important Limitations
- Garbage in, garbage out: no formula can repair an incorrectly measured QT or RR interval.
- Formulas disagree: the same tracing can produce different corrected values.
- Reference ranges vary: a cutoff is not a diagnosis by itself.
- Automated ECG values need context: machine measurements may be wrong when wave endings are unclear.
- Symptoms matter: fainting, seizure-like episodes, palpitations or a family history of sudden death need professional assessment.
Seek urgent medical care for fainting, severe dizziness, chest pain, trouble breathing or a sustained rapid or irregular heartbeat. Do not wait for an online calculation.
QTc Calculator FAQs
How do you calculate QTc?
Measure the QT and RR intervals, then apply a correction equation. Bazett divides QT by the square root of RR. Fridericia uses the cube root. Framingham and Hodges use linear corrections.
What is a commonly cited adult QTc range?
A commonly cited context is 350–450 ms for men and 360–460 ms for women. Formula, age, rhythm, QRS width and clinical setting still matter.
Is Bazett or Fridericia better for QTc?
Bazett is widely used but can be more affected by fast or slow heart rates. Fridericia and Framingham often show less rate dependence. Report the formula rather than treating results as interchangeable.
How do you calculate QTc from ECG boxes?
At 25 mm/s, multiply the small-box count by 40 ms. At 50 mm/s, multiply by 20 ms. Then add heart rate or RR interval and apply a QT correction formula.
What does a QTc of 500 ms mean?
A QTc at or above 500 ms is associated with increased ventricular arrhythmia risk. It is not a diagnosis. The tracing, symptoms, medicines and electrolytes need prompt clinical review.
Evidence and Editorial Notes
- Rautaharju PM, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part IV. Circulation. 2009.
- Vandenberk B, et al. Which QT Correction Formulae to Use for QT Monitoring? Journal of the American Heart Association. 2016.
- Davies RA, et al. Canadian Cardiovascular Society Clinical Practice Update on Assessment and Management of the Patient With a Prolonged QT Interval. Canadian Journal of Cardiology. 2023.
- Merck Manual Professional: Electrocardiography and QT interval.
- Wang J, et al. Multidimensional Performance Evaluation of QTc Formulas. Journal of the American Heart Association. 2026.
External references are provided for transparency. A linked publication does not endorse this website. Formula and content review completed July 21, 2026.