QTc Calculator (Bazett + Fridericia + Framingham)
QTc calculator with Bazett, Fridericia and Framingham corrections. Enter QT and heart rate for the corrected QT and a sex-specific band from named AHA thresholds. Educational only.
QTc Calculator (Bazett, Fridericia, Framingham)
Please tick the acknowledgement above before calculating.
How to use the QTc calculator
Enter the QT and heart rate
Type the measured QT interval in milliseconds and the heart rate in beats per minute. The calculator converts heart rate to an RR interval (RR = 60 ÷ HR, in seconds) before correcting.
Choose a primary correction
All three corrections are shown, but the one you pick drives the headline and the band. Bazett is the one most often meant when a QTc is quoted without a formula; ICH E14 calls Fridericia more accurate when the heart rate is high or below 60 bpm. Whichever you pick, the page adds a note when the heart rate is below 60 or above 85 bpm, where the formulas part company.
Select sex
Thresholds for a prolonged QTc differ by sex, so choose male or female to get the correct band.
Acknowledge, then read the band
The result is graded short, not prolonged, prolonged, prolonged above the 99th percentile, or highly abnormal, and the band says whose threshold the value crossed. It is judged on the whole-millisecond figure shown, so the number and the band always agree. A long QTc raises the risk of a dangerous arrhythmia, but this page cannot see your ECG: interpretation and any action belong with a clinician reading the actual tracing.
QTc — correcting the QT interval for heart rate
Why the QT has to be corrected
The QT interval on an electrocardiogram measures the time the heart's ventricles take to depolarise and then recover — electrically, to fire and reset. It matters because an abnormally long QT can let the heart slip into torsades de pointes, a chaotic rhythm that can cause fainting or sudden cardiac death. The problem is that the raw QT naturally shortens as the heart speeds up and lengthens as it slows, so a single QT value means little without knowing the heart rate. The corrected QT, or QTc, normalises the interval to a standard rate of 60 beats per minute, making values comparable across different heart rates. The correction uses the RR interval (the time between beats, equal to 60 divided by the heart rate in seconds). This calculator uses three formulae: Bazett divides the QT by the square root of RR; Fridericia divides by the cube root of RR; and Framingham adds a linear term, 154 times (1 minus RR), to the QT.
Bazett's and Fridericia's formulae were both published in 1920; Framingham's came from a 1992 analysis of 5,018 ECGs from the Framingham Heart Study, at heart rates from 41 to 120 bpm. Bazett's is the one most often meant: the American Heart Association's 2010 statement on preventing torsade de pointes in hospital notes that, when not otherwise stated, QTc generally refers to the Bazett correction. Its weakness is well documented. ICH E14, the guideline drug regulators use for QT studies, says Bazett over-corrects at elevated heart rates and under-corrects below 60 bpm, and that Fridericia is more accurate in people with such rates; the AHA statement puts Bazett's over-long readings particularly above 85 bpm. In 2012 ICH stopped requiring Bazett-corrected data in every drug application, saying a Fridericia correction is likely to be appropriate in most situations. Showing all three side by side, as this tool does, makes the disagreement visible and helps avoid being misled by a single number.
"A QT means nothing without a heart rate. QTc normalises the interval to 60 beats per minute — and which formula you trust depends on how fast the heart was going."
What a prolonged QTc means
Where the line falls depends on who drew it and why. The AHA/ACCF/HRS 2009 recommendations on ECG interpretation consider a heart-rate-adjusted QT of 450 ms or longer in men and 460 ms or longer in women prolonged, and 390 ms or shorter a short QT. The AHA's 2010 torsade statement adds a second, higher line: approximate 99th-percentile values for otherwise healthy postpubertal individuals of 470 ms in men and 480 ms in women, above which a QTc should be considered abnormally prolonged, and it calls a QTc above 500 ms highly abnormal for both sexes. Drug trials use a third set: ICH E14 analyses a QTc above 450, 480 and 500 ms for everyone, not by sex, and says there is no consensus on the choice of upper limit. This calculator reports all of the AHA lines, so a value can be prolonged by the 2009 limit while still inside the 2010 99th percentile. It uses the same lines for all three formulae, because none of these sources gives formula-specific ones, though the 2009 recommendations prefer a linear-regression correction to Bazett's, and a 2016 Leuven study (Vandenberk et al.) found that Bazett's own healthy upper limit sat near 472 ms in men and 482 ms in women.
The AHA statement names the things that prolong the QT or raise the risk it carries: inherited long-QT syndromes, low potassium and low magnesium, slow heart rates, and drugs including antiarrhythmics, methadone and haloperidol. The number is only as good as the measurement. The same statement measures the QT from the start of the QRS complex to the end of the T wave, in the same lead each time; the 2009 recommendations use the lead showing the longest QT, usually V2 or V3. The 2010 statement also notes that ECG manufacturers use proprietary and often substantially different algorithms to measure it. Treat this calculator as an educational aid for understanding and converting between the corrections, not as a substitute for a clinician's interpretation of a real tracing.
Bazett and Fridericia both date from 1920, and agree only at 60 bpm
QTc corrects the QT interval for heart rate.
RR interval = 60 ÷ heart rate (in seconds).
Bazett (1920): QT ÷ √RR. An unqualified QTc generally means Bazett (AHA, 2010).
Fridericia (1920): QT ÷ ∛RR. More accurate than Bazett at altered heart rates (ICH E14).
Framingham (1992): QT + 154 × (1 − RR), fitted at 41–120 bpm.
At 60 bpm RR is 1 second, so all three return the raw QT.
Bazett over-corrects at high rates and under-corrects below 60 bpm (ICH E14).
Prolonged from 450 ms (men), 460 ms (women) (AHA/ACCF/HRS, 2009).
99th percentile ≈ 470 ms (men), 480 ms (women); above 500 ms is highly abnormal (AHA, 2010).
Since 2012 drug applications no longer need Bazett-corrected data by default (ICH E14 Q&A).
Frequently asked questions
QTc is the QT interval corrected for heart rate. The raw QT shortens as the heart speeds up and lengthens as it slows, so it can't be interpreted alone. The QTc normalises the interval to a heart rate of 60 beats per minute, allowing comparison across rates and assessment of whether the interval is dangerously long.
This page will not choose for you, and no single formula is correct for every ECG. Bazett is the one most often meant when a QTc is quoted without a formula (AHA, 2010). ICH E14 says it over-corrects at elevated heart rates and under-corrects below 60 bpm, and that Fridericia is more accurate in people with such rates; since 2012 ICH has said a Fridericia correction is likely to be appropriate in most drug-trial situations. The AHA/ACCF/HRS 2009 recommendations prefer a linear-regression correction, the family Framingham belongs to, over Bazett's. A 2016 Leuven cohort (Vandenberk et al.), which excluded heart rates above 90 bpm, found Fridericia and Framingham corrected best and Bazett worst. The calculator shows all three so you can compare; the one you pick in the dropdown drives the headline and the band.
It depends on whose line you use. The AHA/ACCF/HRS 2009 recommendations call a QTc of 450 ms or longer in men and 460 ms or longer in women prolonged, and 390 ms or shorter short. The AHA's 2010 torsade statement puts the 99th percentile for healthy postpubertal adults at about 470 ms in men and 480 ms in women, and calls a QTc above 500 ms highly abnormal for both sexes. Drug trials under ICH E14 look at 450, 480 and 500 ms without separating the sexes. This calculator reports each of those AHA lines and names the one the value crossed. The bands are a guide to reading the number, not a diagnosis.
A prolonged QT means the ventricles take longer to reset electrically, creating a window in which an early beat can trigger torsades de pointes, a fast, chaotic ventricular rhythm. Torsades can cause fainting and, occasionally, degenerate into ventricular fibrillation and sudden cardiac death. The AHA's 2010 statement cites data from congenital long-QT syndrome in which a QTc above 500 ms was associated with a 2- to 3-fold higher risk of torsade.
The AHA's 2010 statement names inherited long-QT syndromes; low potassium and low magnesium; low calcium, which also prolongs the QT but has been linked to torsade only in rare cases; slow heart rates; and drugs, including antiarrhythmics, some antibiotics such as erythromycin and clarithromycin, some antipsychotics such as haloperidol, and methadone. Drugs differ widely in risk: the same statement calls torsade with moxifloxacin very rare. This list is for understanding, not a reason to start, stop or change any medicine.
The AHA's 2010 statement measures the QT from the start of the QRS complex to the end of the T wave, in a lead whose T wave is at least 2 mm tall with a well-defined end, and asks for the same lead in the same patient over time, with the lead recorded. The AHA/ACCF/HRS 2009 recommendations use the lead showing the longest QT, usually V2 or V3. Where the end of the T wave is hard to see, for example when a U wave runs into it, the 2010 statement draws a line from the T-wave peak along its steepest downslope. Irregular rhythms are where the sources part: the 2009 recommendations say rate correction should not be attempted when RR-interval variability is large, as often in atrial fibrillation, while the 2010 statement offers averaging the QTc at the shortest and longest RR intervals. Feed a wrong QT in and you get a wrong QTc out.
At exactly 60 bpm, where RR is 1 second, all three formulae return the raw QT unchanged. The differences grow as the heart rate moves away from 60: at fast rates Bazett reads higher than Fridericia or Framingham, and at slow rates Bazett reads lower than Fridericia, while Framingham can land on either side of Bazett. That is why the result adds a note below 60 bpm and above 85 bpm, whichever formula you picked.
Treat it as a starting point. The AHA's 2010 statement notes that manufacturers of electrocardiographs use proprietary and often substantially different algorithms to measure the QT, so two machines can print different values for the same tracing, and that some 12-lead algorithms label a QTc above 440 ms as borderline even though roughly 10% to 20% of people exceed it. When the QTc matters clinically, a clinician checks the measurement on the tracing rather than trusting the printout alone.
No. This tool does arithmetic on numbers you type. It cannot see an ECG, diagnose long-QT syndrome or judge whether a medicine is safe for you. Interpreting a QTc needs the actual tracing, an accurate measurement and clinical context, and any change to medication is a decision for a doctor. If you have been told you have a long QT, or you take a drug that can prolong it, follow your clinician's advice rather than a calculator.
No. The QT, heart rate, and sex you enter are processed in your browser only. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs entirely on your device.
Method & sources
How it computes
RR = 60/HR (s). Bazett QT/√RR; Fridericia QT/∛RR; Framingham QT + 154×(1 − RR), fitted at 41–120 bpm and flagged outside. Bands on the whole-ms value shown: short ≤390 ms; prolonged ≥450 ms (men) or ≥460 ms (women) (AHA/ACCF/HRS 2009); above the 99th percentile >470 / >480 ms and highly abnormal >500 ms (Drew 2010, AHA). Notes below 60 bpm (ICH E14) and above 85 bpm (Drew 2010), whatever formula is chosen.
What this tool implements
- Bazett QT/√RR and Fridericia QT/∛RR (both 1920); Framingham QT + 154 × (1 − RR) (Sagie 1992), fitted to heart rates of 41–120 bpm and flagged outside that range
- AHA/ACCF/HRS 2009 lines: a QT of 390 ms or less is short; 450 ms or longer in men and 460 ms or longer in women is prolonged
- Drew et al. 2010 (AHA/ACCF) lines: above the approximate 99th percentile, 470 ms (men) and 480 ms (women), is abnormally prolonged; above 500 ms is highly abnormal, either sex
- One set of lines for all three formulae, and no borderline tier, because no source used gives either; the band is judged on the whole-millisecond value the page prints
- Heart-rate notes whatever formula is selected: below 60 bpm (ICH E14: Bazett under-corrects) and above 85 bpm (Drew 2010: Bazett reads overlong)
Sources
- Bazett HC. Heart 1920;7:353-370
- Fridericia LS. Acta Med Scand 1920;53:469-486
- Sagie A et al. Am J Cardiol 1992;70(7):797-801. PMID 1519533
- Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, part IV: the ST segment, T and U waves, and the QT interval. J Am Coll Cardiol 2009;53(11):982-991; also Circulation 2009;119:e241-e250. https://www.jacc.org/doi/10.1016/j.jacc.2008.12.014
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation 2010;121(8):1047-1060. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3056123/
- ICH E14. The Clinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential for Non-Antiarrhythmic Drugs, Step 4, 12 May 2005, sections 3.1.1 and 3.2.2. https://database.ich.org/sites/default/files/E14_Guideline.pdf
- ICH E14 Implementation Working Group. E14 Questions & Answers (R3), question 1.5 (April 2012), dated 10 December 2015. https://database.ich.org/sites/default/files/E14_Q%26As_R3_Q%26As…
- Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? J Am Heart Assoc 2016;5(6):e003264. https://doi.org/10.1161/JAHA.116.003264
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