QTc Bazett Formula Calculator

Bazett’s QTc is the correction every ECG cart prints and the one that behaves worst. Enter the QT with either the heart rate or the RR interval, and see how far it sits from Fridericia on the same trace.

Last updated 8 Oct 2026

Bazett QTc

QT / √RR

Measured QT interval (ms)Earliest QRS onset to latest T-wave offset, in the lead where the QT is longest (usually II, V5 or V6), by the tangent method. Do not include a U wave; where T and U merge the measurement is unreliable and so is everything below it.

Heart rate or RR intervalFrom the beat the QT was measured on. In sinus rhythm use the RR interval immediately preceding that beat.

Rate unitSet the unit to match what you are holding. Every rate correction here was published with RR in SECONDS, so a heart rate typed into an RR field is the commonest way these formulas are got wrong: at 100 bpm it divides the QT by 10 instead of by 0.775. Both quantities are printed back under the answer.

SexOnly the reported upper limit changes with sex, not the arithmetic. The male QTc shortens at puberty and the female QTc does not, so adult women sit about 10 ms higher.

516ms (Bazett)Example

QT 400 ms, heart rate 100 bpm, male

Formula

QTc_Bazett = QT / √RR (RR in seconds)
RR (s) = 60 / heart rate (bpm)

RR in seconds: the commonest way this formula is implemented wrongly. At 100 bpm the divisor is √0.6 = 0.775; a heart rate typed into the RR field divides by √100 = 10 and returns 40 ms
over- and under-correction: ICH E14’s own words: Bazett “overcorrects at elevated heart rates and under corrects at heart rates below 60 bpm”. The error runs in the alarming direction when the patient is tachycardic, which is when a QTc tends to be checked
450 and 460 ms: the upper limits the AHA/ACCF/HRS 2009 statement reports for men and women; 500 ms is ICH E14’s threshold of particular concern, with a rise over 60 ms from baseline as the companion rule. Thresholds for a prolonged QTc are method-dependent — a cart’s own algorithm and a manual measurement disagree — and a local convention takes precedence.
why it survives: every historical dataset, drug label and published threshold is in Bazett, so abandoning it breaks comparability with the literature. ICH E14 requires both Bazett and Fridericia in a submission for that reason

Worked example

QT 400 ms, heart rate 100 bpm, male
RR = 60 / 100 = 0.600 s, and √0.600 = 0.7746
QTc = 400 / 0.7746 = 516 ms
Fridericia on the same trace: 400 / 0.600^1/3 = 474 ms
The two differ by 42 ms on one unchanged measurement. 516 ms is above ICH E14's 500 ms threshold of particular concern; 474 ms is not
Framingham gives 462 ms and Hodges 470 ms, so Bazett is the outlier of the four, and the outlier in the alarming direction
Change the rate to 60 bpm: RR = 1.000 s and all four return 400 ms

The same 400 ms QT, by Bazett and by Fridericia

This calculator’s own arithmetic on one unchanged 400 ms measurement. The two agree only at 60 bpm, where RR is 1 second and both reduce to the measured QT. By 120 bpm they are 62 ms apart; at 40 bpm Bazett is 23 ms the lower. The gap is the arithmetic, not the patient.
Heart rate (bpm)	RR (s)	Bazett (ms)	Fridericia (ms)	Difference (ms)
40	1.50	327	349	-23
50	1.20	365	376	-11
60	1.00	400	400	+0
80	0.75	462	440	+22
100	0.60	516	474	+42
120	0.50	566	504	+62
150	0.40	632	543	+90

Reported upper limits for the QTc, and who publishes them

Four numbers for one measurement, because they answer different questions — a population limit, an observed arrhythmia range, a regulatory reporting category, a diagnostic criterion. None is a treatment threshold. The 1920 original was not reachable from here; the formula was taken from the ICH E14 text, which quotes it.
Source	Men	Women	What the figure is for
AHA/ACCF/HRS 2009, part IV	450 ms	460 ms	Upper limit of normal on a resting 12-lead ECG
AHA/ACCF/HRS 2009, part IV	500 ms	500 ms	Markedly prolonged — the range in which torsades is reported
ICH E14	450 / 480 / 500 ms	450 / 480 / 500 ms	Categorical analysis thresholds in a drug study. 500 ms is a “threshold of particular concern” and the usual discontinuation criterion, with a rise over 60 ms from baseline
2022 ESC ventricular arrhythmia guideline	480 ms	480 ms	QTc of 480 ms or more on repeated ECGs, as one route to a diagnosis of long QT syndrome

The correction everyone uses and nobody defends

Bazett published his correction in 1920 from 39 young subjects, dividing the QT interval by the square root of the RR interval in seconds. It is still what almost every ECG cart prints, what almost every drug label quotes and what almost every published threshold was derived against — and it is the worst-behaved correction in common use. ICH E14, which governs how drug-induced QT prolongation is assessed, puts it without hedging: Bazett’s correction “overcorrects at elevated heart rates and under corrects at heart rates below 60 bpm and hence is not an ideal correction”.

The size of that error decides cases. Take one unchanged 400 ms QT. At 60 bpm every correction returns 400 ms, because RR is exactly one second and each formula reduces to the measured value. At 100 bpm Bazett gives 516 ms and Fridericia 474. At 120 bpm it is 566 against 504. The 500 ms figure ICH E14 calls a threshold of particular concern falls between those pairs, so the choice of formula — not the patient — decides which side of it the number lands on. Below 60 bpm the error runs the other way and is quietly reassuring: at 40 bpm Bazett gives 327 ms where Fridericia gives 349.

The second trap is arithmetic. Bazett takes RR in seconds. A reader holding a heart rate of 100 and typing it into an RR field divides by 10 rather than 0.775 and gets 40 ms, which is obviously wrong; the same slip with RR in milliseconds gives 16 ms, and the reverse slip gives a number large enough to look like a crisis. This page takes whichever quantity you hold and prints both back.

So why keep it? Comparability. Every threshold in the literature, every label warning and every historical dataset is in Bazett, and a reader checking a patient against a published figure needs the correction that figure was derived with — which is why ICH E14 asks for both. The practical position is to compute both, read Bazett against the published thresholds, and treat a large gap between the two as a warning that the heart rate is doing the work. The Fridericia, Framingham and Hodges calculator computes all three alternatives on one trace. Neither number is a decision: QTc moves by tens of milliseconds through the day, automated and manual measurements disagree, and a merged T and U wave makes the whole measurement unreliable. This supports a clinician’s judgement rather than replacing it. An ECG interval is read alongside the patient, the drug chart and the electrolytes, and never instead of them.

Frequently asked questions

What is Bazett’s formula for QTc?

QTc = QT divided by the square root of the RR interval, with RR in seconds. At 100 bpm the RR interval is 0.6 s, so a measured QT of 400 ms corrects to 400 / 0.7746 = 516 ms. Entering the heart rate instead of the RR interval is the commonest implementation error.

Why is Bazett’s formula considered inaccurate?

It was fitted to 39 young subjects in 1920 and the square-root relationship does not hold across the population. ICH E14 states that it “overcorrects at elevated heart rates and under corrects at heart rates below 60 bpm”. On one 400 ms QT, Bazett and Fridericia differ by 42 ms at 100 bpm and 62 ms at 120 bpm.

Which QT correction should I use instead?

ICH E14 describes Fridericia’s cube-root correction as “more accurate than Bazett’s correction in subjects with such altered heart rates” and requires both in a submission. In practice compute both and read published thresholds against the correction they were derived with.

What QTc counts as prolonged?

The AHA/ACCF/HRS 2009 statement reports 450 ms in men and 460 ms in women as upper limits, with 500 ms as markedly prolonged. ICH E14 uses 450, 480 and 500 ms as categorical thresholds; the 2022 ESC guideline uses 480 ms or more on repeated ECGs for long QT syndrome. Thresholds for a prolonged QTc are method-dependent — a cart’s own algorithm and a manual measurement disagree — and a local convention takes precedence.

Why do men and women have different QTc limits?

The QTc of boys shortens at puberty and the female QTc does not, so adult women sit roughly 10 ms higher. That is why the reported upper limit is 460 ms in women against 450 in men, and part of why women are over-represented in drug-induced torsades de pointes.

Related calculators

References

Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
International Council for Harmonisation. E14 Clinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential for Non-Antiarrhythmic Drugs. Guidance for industry, US Food and Drug Administration, October 2005.
Rautaharju PM, Surawicz B, Gettes LS, et al. 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.
Zeppenfeld K, Tfelt-Hansen J, de Riva M, et al. 2022 ESC guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Eur Heart J. 2022;43(40):3997–4126.

Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/