QTc Calculator

Why the QT interval gets corrected

The QT interval on an ECG is the time the ventricles take to depolarise and then recover, and it has one inconvenient property: it changes with heart rate. The faster the heart beats, the shorter the QT, and the slower it beats, the longer the QT. So a raw QT of 400 ms means something very different at 50 beats per minute than it does at 110.

That is the problem the corrected QT, the QTc, exists to solve. It mathematically adjusts the measured QT to what it would be at a standard heart rate of 60 beats per minute, so that two readings taken at different rates can actually be compared. This calculator takes your QT and heart rate, works out the RR interval between beats, and then reports the QTc four different ways. Why four? That turns out to be the whole story.

Four formulas, four answers

There is no single agreed way to correct the QT, because no formula is perfect across every heart rate. Over a century, several have been proposed, and this tool runs the four best known. First it finds the RR interval, the gap between beats, from your heart rate (RR in milliseconds = 60000 divided by heart rate). Then:

• Bazett divides QT by the square root of RR (in seconds). Published in 1920, it is by far the most common, and it is almost certainly the one your ECG machine prints. Its weakness is well documented: it overcorrects at fast heart rates and undercorrects at slow ones, so it drifts exactly where accuracy matters most.
• Fridericia divides QT by the cube root of RR. Also from 1920, it holds up much better at heart rate extremes, which is why regulators favour it for drug-safety studies.
• Framingham uses a straight-line correction derived from the Framingham Heart Study, and performs similarly well to Fridericia across a range of rates.
• Hodges uses a different linear correction based directly on heart rate, and in large comparisons has fit the data well overall.

The reason this matters is easiest to see in numbers. Take a QT of 360 ms at a heart rate of 100, which gives an RR of 600 ms. The four corrections come out as:

That is a spread of over 40 ms from the same beat. Bazett alone reads near the borderline-prolonged zone, while the other three sit comfortably normal. This is why you compare formulas before calling a QTc prolonged, especially away from 60 beats per minute: at fast rates Fridericia or Framingham are the safer read, and reacting to Bazett alone can mean stopping a needed medication over a number the correction inflated.

When a QTc is long enough to worry about

The thresholds vary a little by source, but as a common guide, a QTc above roughly 450 ms in men or 470 ms in women is considered prolonged, with the range just below that counted as borderline. A QTc of 500 ms and above is the figure that draws real concern, as it carries a substantially higher risk of torsades de pointes, a dangerous ventricular arrhythmia.

That arrhythmia risk is the reason anyone cares about this number. A long QTc can be inherited (the congenital long QT syndromes), but far more often it is acquired, driven by medications or by electrolytes. A great many common drugs prolong the QT, including certain antiarrhythmics, antipsychotics and antibiotics, and their effects add up when combined. Low potassium, magnesium or calcium all lengthen it too. So a prolonged QTc is usually a prompt to review the medication list and check electrolytes, not a diagnosis in itself.

Getting the measurement right

The correction is only as good as the QT you feed it, and measuring QT is genuinely easy to get wrong. A few points that matter:

• Measure the QT across several beats and take a representative value, using a lead with a clear end to the T wave, often lead II or V5 to V6.
• Do not include a U wave in the measurement, which is a common way to overestimate QT.
• A wide QRS complex, as in bundle branch block, lengthens the QT for reasons unrelated to repolarisation, so the plain QTc can mislead and other approaches are needed.

And the honest framing: this tool does arithmetic on numbers you provide. It does not read an ECG, it cannot judge the clinical situation, and it is not a diagnosis. It is a reference and teaching aid, and the interpretation, and any decision that follows, belongs to a clinician looking at the whole picture.

Questions people ask

Which QTc value should I actually use?

If the heart rate is close to 60, the formulas mostly agree and it barely matters. Away from 60, particularly at fast rates, Fridericia or Framingham are generally the more reliable read, while Bazett tends to overstate the QTc. A sensible habit is to look at more than one: if they agree, you can trust the result, and if they diverge, that divergence is itself the warning.

Why is this different from the number on the ECG printout?

Most ECG machines report the Bazett QTc by default. If your printout disagrees with, say, the Fridericia value here, that is expected rather than an error, and it is exactly the reason this tool shows several formulas side by side.

What is a normal QTc?

Broadly, under about 450 ms in men and under about 470 ms in women, though the exact cutoffs differ between sources and settings. What matters as much as the single number is the trend and the context, especially any QT-prolonging drugs or electrolyte problems.

References

Where the figures come from. The comparison of the four correction formulas, and the finding that Bazett performs worst at heart rate extremes while Fridericia and Framingham correct better, draws on the work of Vandenberk and colleagues and the earlier comparison by Luo and colleagues.

1. Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? Journal of the American Heart Association. 2016;5(6):e003264. https://doi.org/10.1161/JAHA.116.003264
2. Luo S, Michler K, Johnston P, Macfarlane PW. A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs. Journal of Electrocardiology. 2004;37(Suppl):81-90.