AMT · ECG calculation support
QTc calculator
Enter QT and R–R as small squares on the ECG. Check the paper speed first. All four corrections update together as you type.
Your measurements
From QRS onset to the end of the T wave.
The preceding R–R interval for the measured beat.
1 small square (1 mm) = 40 ms.
Four commonly used corrections are shown together. No single formula is the definitive result for every patient.
Fractional squares are accepted. No patient identifiers are needed; entered values are not saved or transmitted.
Enter both intervals to calculate QTc.
QTc results
All QTc results are in ms. In the formulas: QT = ms, RR = seconds, HR = bpm.
Bazett
ms
QTcB = QT / √RR
Fridericia
ms
QTcF = QT / ∛RR
Framingham
ms
QTcFra = QT + 154 × (1 − RR)
Nonpositive mathematical result
Hodges
ms
QTcH = QT + 1.75 × (HR − 60)
Nonpositive mathematical result
- Measured QT
- ms
- Measured R–R
- ms
- Heart rate from R–R
- bpm
- R–R in the formulas
- s
Positive QTc estimates rounded to the nearest millisecond. These are separate estimates, not a score to average. No normal/abnormal classification is applied.
One or more linear corrections gave a nonpositive mathematical result. This is not a physiologically interpretable QTc. Check measurements and paper speed; the correction is being extrapolated beyond a meaningful range.
Check QT and R–R measurements: QT is at least as long as R–R. These values require clinical review.
Before interpreting the number
For clinician calculation support only. Verify paper speed and manual QT/R–R measurements. Results do not diagnose or exclude long QT syndrome.
- Correction formulas have limitations. Bazett can overestimate QTc at faster heart rates and underestimate it at slower rates. Formula performance depends on heart rate, age and clinical setting. Pediatric evidence is not uniform across ages; adult-derived Framingham and Hodges should not be assumed validated for every infant or child. Do not choose a formula simply because it gives the lowest number.
- Irregular rhythms, ectopic beats, rapidly changing heart rate, wide QRS complexes and ventricular pacing require clinician-specific assessment beyond this calculator.
- Confirm the end of the T wave and do not include a separate U wave. A single entered beat does not replace appropriate ECG review or averaging where indicated.
How it is calculated
One small square is 1 mm. Duration per square = 1 ÷ paper speed in seconds. QT = QT squares × 1000 ÷ speed (ms). RR = R–R squares ÷ speed (s). Heart rate = 60 ÷ RR (bpm).
Bazett: QTc (ms) = QT (ms) ÷ √RR (s). Fridericia: QTc (ms) = QT (ms) ÷ ∛RR (s). Framingham: QTc (ms) = QT (ms) + 154 × [1 − RR (s)]. Hodges: QTc (ms) = QT (ms) + 1.75 × [HR (bpm) − 60]. Calculations retain full precision before display rounding. Formula-specific reference values are not interchangeable; this tool does not apply cutoffs.
Sources and limitations
- AHA/ACCF/HRS recommendations on ECG interpretation, Part IV (2009): QT measurement and rate-correction limitations. Listing Bazett first here is not a claim that it is the statement’s preferred correction.
- Vandenberk et al. (2016): Which QT Correction Formulae to Use for QT Monitoring?, with the published Hodges correction (2018).
- Sagie et al. (1992): Framingham Heart Study.
- Phan et al. (2015): comparison in infants and young children.
- Andršová et al. (2020): Problems with Bazett QTc correction in paediatric screening.