Corrected QT Interval CalculatorFree online fitness & health calculator — instant results, no sign-up
Calculate Bazett and Fridericia corrected QT intervals from QT and heart rate.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Corrected QT Interval Calculator
The Corrected QT Interval Calculator estimates Bazett and Fridericia corrected QT intervals from a measured QT interval in milliseconds and a heart rate in beats per minute. It displays both QTc estimates so you can compare how the two formulas adjust QT for heart rate. This tool provides educational estimates for adults and does not replace ECG review.
- Measured QT interval (ms)
- The interval from the start of the QRS complex to the end of the T wave on an ECG, entered in milliseconds (ms), with a default of 400.
- Heart rate (bpm)
- The heart rate associated with the measured QT interval, entered in beats per minute (bpm), with a default of 70.
Formula and method
RR = 60 / HR; QTc_Bazett = QT / sqrt(RR); QTc_Fridericia = QT / RR^(1/3). Use RR numerically in seconds and QT in milliseconds to obtain QTc in milliseconds. With QT = 400 ms and HR = 70 bpm: QTc_Bazett = 400 / sqrt(60/70) = 432.05 ms; QTc_Fridericia = 400 / (60/70)^(1/3) = 421.09 ms.
- QTMeasured, uncorrected QT interval in milliseconds.
- HRHeart rate in beats per minute.
- RREstimated time between successive R waves in seconds, calculated as 60 divided by heart rate.
- QTc_BazettHeart-rate-corrected QT interval in milliseconds using Bazett's square-root correction.
- QTc_FridericiaHeart-rate-corrected QT interval in milliseconds using Fridericia's cube-root correction.
The calculator estimates the RR interval from the entered heart rate, then divides the measured QT by the square root of RR for Bazett and by the cube root of RR for Fridericia. Calculations retain precision until the final QTc results are rounded to two decimal places. Both inputs must be positive.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Measured QT interval400 ms
- Heart rate70 bpm
Results
- QTc Bazett432.05 ms
- QTc Fridericia421.09 ms
- NoteCorrection formulas differ, especially at extreme heart rates; ECG review is required.
- NoteEducational estimate for adults; not a diagnosis or treatment recommendation.
How to use the Corrected QT Interval Calculator
- 1Obtain the measured QT interval and corresponding heart rate from the same ECG recording.
- 2Enter the measured QT interval in milliseconds; the default is 400 ms.
- 3Enter the heart rate in beats per minute; the default is 70 bpm.
- 4Calculate and compare the two results: the default inputs give QTc Bazett = 432.05 ms and QTc Fridericia = 421.09 ms.
- 5Read both notes and have the ECG reviewed by a qualified clinician before drawing clinical conclusions.
Reading your result
The QTc Bazett row shows the square-root-corrected QT interval: 432.05 ms for the default inputs. The QTc Fridericia row shows the cube-root-corrected QT interval: 421.09 ms for the same inputs. These are alternative estimates, not separate ECG measurements or values to average. The first Note row states: "Correction formulas differ, especially at extreme heart rates; ECG review is required." The second Note row states: "Educational estimate for adults; not a diagnosis or treatment recommendation." Neither QTc value alone establishes whether an ECG is normal or whether treatment is needed.
Limitations
The calculation assumes that the entered heart rate represents the rhythm associated with the QT measurement and uses RR = 60/HR rather than a directly measured beat-specific RR interval. Irregular rhythms, rapidly changing heart rates, and measurement errors can reduce reliability. Bazett commonly overcorrects at high heart rates and undercorrects at low heart rates; Fridericia also has limitations. Wide QRS complexes and paced rhythms require additional clinical assessment. Interpretation depends on factors such as age, sex, symptoms, medications, electrolytes, and ECG quality. Two-decimal output reflects numerical rounding, not ECG measurement accuracy. This adult educational tool should not be used to diagnose long QT syndrome or to start, stop, or adjust medication.
Common questions
Why do the default inputs produce two different QTc values?+
With QT = 400 ms and heart rate = 70 bpm, Bazett divides QT by the square root of 60/70, while Fridericia divides it by the cube root. The different correction factors produce 432.05 ms and 421.09 ms, respectively.
Should I enter the QT or QTc printed on my ECG?+
Enter the measured, uncorrected QT interval in milliseconds. Entering an already corrected QTc would apply a second correction and produce a misleading result.
Which correction should I use at very high or very low heart rates?+
Do not automatically select one result as definitive. Bazett is especially sensitive to extreme heart rates, and Fridericia is often less rate-dependent, but neither is universally reliable; the appropriate interpretation requires ECG review and clinical context.
Can I use an average heart rate when my rhythm is irregular?+
An average heart rate can yield a numerical estimate, but it may not represent the RR interval associated with the measured QT. Irregular rhythms require clinician-guided beat selection and assessment rather than reliance on this two-input estimate.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.