How to Calculate QTc (Bazett, Fridericia, Framingham, Hodges)
QTc is the QT interval corrected for heart rate, calculated as QTc = QT ÷ √RR (Bazett), where RR is the interval between beats in seconds (RR = 60 ÷ heart rate). Three other formulas, Fridericia, Framingham, and Hodges, correct the same QT differently and can disagree with Bazett by tens of milliseconds at fast or slow heart rates.
This guide walks through all four formulas, shows a worked comparison table at a fixed QT across a range of heart rates, and covers the mistakes that lead to a wrong QTc reading in practice.
Why QT needs correcting at all
The QT interval on an ECG measures the time from the start of the Q wave to the end of the T wave, the total duration of ventricular depolarization and repolarization. It is not a fixed number: QT shortens as the heart rate increases and lengthens as it slows down. A QT of 400 ms might be entirely normal at a heart rate of 60 and clearly abnormal at a heart rate of 100.
That rate dependence makes raw QT useless for comparison. You cannot judge a single reading against a fixed threshold, or compare two ECGs taken at different heart rates, without first correcting for rate. QTc solves that by asking: what would this QT be if the heart rate were 60 bpm? The four formulas below all answer that question, just with different assumptions about how QT and heart rate relate.
- Bazett: QTc = QT ÷ √RR
- Fridericia: QTc = QT ÷ ∛RR
- Framingham: QTc = QT + 154 × (1 − RR)
- Hodges: QTc = QT + 1.75 × (HR − 60)
Bazett and Fridericia are power-law corrections built around the RR interval in seconds. Framingham and Hodges are additive corrections built directly around how far the rate sits from 60 bpm. That difference in approach is exactly why they diverge at extreme heart rates, and it’s the point the table below makes concrete.
Worked comparison: QT fixed at 400 ms
Hold QT constant at 400 ms and vary only the heart rate. Every formula agrees at 60 bpm, because RR equals 1 second there and every correction term reduces to zero (or, for Bazett and Fridericia, to dividing by 1). Move away from 60 bpm and the formulas pull apart.
| HR (bpm) | RR (s) | Bazett | Fridericia | Framingham | Hodges |
|---|---|---|---|---|---|
| 40 | 1.500 | 327 | 349 | 323 | 365 |
| 60 | 1.000 | 400 | 400 | 400 | 400 |
| 75 | 0.800 | 447 | 431 | 431 | 426 |
| 100 | 0.600 | 516 | 474 | 462 | 470 |
| 130 | 0.462 | 589 | 518 | 483 | 523 |
Two things stand out. First, at 60 bpm all four values are identical: 400 ms, matching the input QT exactly, because RR = 1 makes every correction term vanish. That row is a useful sanity check for any QTc calculator, including this one: it should reproduce the raw QT unchanged at exactly 60 bpm.
Second, Bazett diverges the most as the heart rate moves in either direction, and it diverges in opposite ways depending on which direction. At the fast end, HR 130, Bazett gives 589 ms, a markedly prolonged reading, while Fridericia gives only 518 ms, borderline at most. Bazett overcorrects at high heart rates, pushing QTc up further than the other three formulas agree it should go. At the slow end, HR 40, Bazett gives 327 ms, well below Fridericia’s 349, Framingham’s 323, and Hodges’s 365. There Bazett undercorrects, which means it can understate a QT that is actually prolonged in a bradycardic patient.
Calculate with your own values
Reference thresholds
These bands apply to the Bazett QTc, the value most commonly reported on ECG machines and cited in guidelines:
| Normal | Borderline | Prolonged | |
|---|---|---|---|
| Male | ≤ 430 ms | 431 to 450 ms | > 450 ms |
| Female | ≤ 450 ms | 451 to 470 ms | > 470 ms |
A QTc of 500 ms or more is markedly prolonged in either sex and warrants particular caution, since it carries a real risk of torsades de pointes, a dangerous polymorphic ventricular arrhythmia.
Common mistakes
Trusting the machine’s default Bazett value at heart-rate extremes. Bazett is the oldest correction (1920) and still the one printed on most ECG machines and cited in most guidelines, which is precisely why its bias matters. As the table above shows, a tachycardic patient’s automated QTc can read as prolonged when Fridericia, Framingham, and Hodges all say borderline or normal. A bradycardic patient’s automated QTc can read as normal when the other three formulas say otherwise, false reassurance in exactly the setting where you cannot afford it. When the heart rate sits well outside 60 to 100 bpm, check a second formula rather than reading the default number in isolation. Fridericia in particular is favored in drug-safety and QT studies for tracking the other formulas more closely away from 60 bpm.
Applying the wrong sex-specific threshold. The normal and prolonged cutoffs differ between men and women, and mixing them up, or not adjusting expectations at all, changes whether a given QTc gets flagged. Confirm which threshold applies before calling a value normal or abnormal.
Feeding the formula a poorly measured QT. No correction formula can fix a bad measurement. Common sources of error include measuring from the wrong lead, including a prominent U wave as if it were part of the T wave, and poor determination of exactly where the T wave ends against the baseline. The QTc is only as reliable as the QT interval that goes into it.
Reacting to one isolated reading. A single prolonged QTc should prompt a check for reversible causes before anything else: QT-prolonging drugs (amiodarone, sotalol, haloperidol, macrolide or fluoroquinolone antibiotics, methadone, among others), electrolyte disturbances (low potassium, magnesium, or calcium), and, if those are ruled out, a possible congenital long QT syndrome. It is also worth repeating the measurement or averaging a few beats rather than acting on one strip alone.
Frequently asked questions
Which QTc formula should I use?
Bazett is the most widely used and the default on most ECG machines, so it is the number you will see most often. Fridericia is generally more accurate away from 60 bpm and is preferred in many drug-safety studies. Framingham and Hodges, both additive corrections, avoid Bazett’s tendency to blow up at extreme heart rates. When the heart rate is close to 60 to 100 bpm the formulas agree closely enough that the choice rarely matters; outside that range, checking more than one is worthwhile.
Why do all four formulas give the same answer at 60 bpm?
At a heart rate of 60 bpm, RR equals exactly 1 second. Dividing by the square root or cube root of 1 leaves QT unchanged, and Framingham’s and Hodges’s correction terms both depend on RR or heart rate being different from that reference point, so they reduce to zero. QTc simply equals QT at 60 bpm under every formula.
Why does Bazett read differently from the other formulas at fast or slow heart rates?
Bazett corrects using the square root of RR, a stronger correction than Fridericia’s cube root and a fundamentally different shape than the additive corrections used by Framingham and Hodges. That stronger, non-linear correction overcorrects at fast heart rates, producing an inflated QTc, and undercorrects at slow heart rates, producing a QTc that reads lower than it probably should. The worked table above shows the size of that gap at HR 130 and HR 40.
What causes a genuinely prolonged QTc?
The most common causes are QT-prolonging medications (certain antiarrhythmics, antipsychotics, and antibiotics, plus methadone), electrolyte disturbances such as low potassium, magnesium, or calcium, and congenital long QT syndrome. A prolonged QTc from any of these causes raises the risk of torsades de pointes, so identifying and addressing a reversible cause is the first step after confirming the reading.