QTc Calculator
Corrected QT interval by Bazett, Fridericia, Framingham and Hodges at once, with the reason Bazett over-corrects at fast heart rates and what to use instead.
QTc Calculator: with the default inputs, corrected qt is 400.
As reported on the ECG, in milliseconds.
The RR interval is derived from this: RR in seconds = 60 ÷ heart rate.
Only used to pick the published upper limit of normal, which differs between men and women.
All four are computed; this one becomes the headline number.
milliseconds, by the formula you chose.
- Bazett
- 400
- Fridericia
- 400
- Framingham
- 400
- Hodges
- 400
- Spread across the four formulas
- 0Highest minus lowest, in ms. Small near 60 bpm, large at the extremes.
- RR interval
- 1seconds.
- Published upper limit of normal for this formula and sex
- 47299% upper limit from a healthy cohort (JAHA, 2016).
- Against that upper limit
- At or below the 472 ms upper limit published for Bazett in men
Assumptions
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement.
- The QT interval is entered in milliseconds and the RR interval is derived as 60 ÷ heart rate, so a regular rhythm is assumed.
- Formulas are as published for QT and RR in seconds and restated here in milliseconds: Bazett QT/√RR, Fridericia QT/∛RR, Framingham QT + 154(1 − RR), Hodges QT + 1.75(HR − 60).
- Upper limits of normal are the 99% values from a healthy age- and sex-matched subset of 212 adults in Vandenberk et al., JAHA 2016, and are specific to each formula.
- No correction accounts for a wide QRS, ventricular pacing, or beat-to-beat variation in atrial fibrillation.
| Formula | Calculation | QTc (ms) | Upper limit of normal (ms) |
|---|---|---|---|
| Bazett (1920) | 400 ÷ √1 | 400 | 472 men, 482 women |
| Fridericia (1920) | 400 ÷ ∛1 | 400 | 448 men, 468 women |
| Framingham (Sagie et al., 1992) | 400 + 154 × (1 − 1) | 400 | 449 men, 467 women |
| Hodges (1983) | 400 + 1.75 × (60 − 60) | 400 | 446 men, 465 women |
Upper limits are the 99% values from a healthy age- and sex-matched subset in Vandenberk et al., JAHA 2016. Note how much higher Bazett's own upper limit is than the others — 472 ms in men against 446-449 — which is why applying a single 450 ms cut-off to a Bazett QTc flags people who are not prolonged.
How this is worked out
The formula
RR interval (seconds) = 60 ÷ heart rate (bpm) With QT and QTc in milliseconds and RR in seconds: Bazett QTc = QT ÷ √RR Fridericia QTc = QT ÷ ∛RR Framingham QTc = QT + 154 × (1 − RR) Hodges QTc = QT + 1.75 × (heart rate − 60) At 60 bpm, RR = 1 s and all four return the uncorrected QT.
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Measured QT interval
- As reported on the ECG, in milliseconds.from 100 to 800 · defaults to 400
- Heart rate
- The RR interval is derived from this: RR in seconds = 60 ÷ heart rate.from 0 to 300 · defaults to 60
- Sex
- Only used to pick the published upper limit of normal, which differs between men and women.Male · Female
- Headline formula
- All four are computed; this one becomes the headline number.Bazett — the clinical default, and the least accurate · Fridericia — best rate correction · Framingham — best rate correction · Hodges
What you get back
- Corrected QTmain answer
- milliseconds, by the formula you chose.
- Bazett
- Fridericia
- Framingham
- Hodges
- Spread across the four formulas
- Highest minus lowest, in ms. Small near 60 bpm, large at the extremes.
- RR interval
- seconds.
- Published upper limit of normal for this formula and sex
- 99% upper limit from a healthy cohort (JAHA, 2016).
- Against that upper limit
What this assumes
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement.
- The QT interval is entered in milliseconds and the RR interval is derived as 60 ÷ heart rate, so a regular rhythm is assumed.
- Formulas are as published for QT and RR in seconds and restated here in milliseconds: Bazett QT/√RR, Fridericia QT/∛RR, Framingham QT + 154(1 − RR), Hodges QT + 1.75(HR − 60).
- Upper limits of normal are the 99% values from a healthy age- and sex-matched subset of 212 adults in Vandenberk et al., JAHA 2016, and are specific to each formula.
- No correction accounts for a wide QRS, ventricular pacing, or beat-to-beat variation in atrial fibrillation.
About this calculator
This is an estimate for education. It does not diagnose anything and does not replace clinical judgement.
Why the QT needs correcting at all
The QT interval is the time from the start of ventricular depolarisation to the end of repolarisation. It shortens as the heart speeds up, so a raw QT is uninterpretable without knowing the rate it was measured at. Correction rescales it to what it would have been at 60 beats per minute, so that measurements at different times and different rates can be compared.
Four corrections are in common use, and this page computes all of them.
The Bazett problem
Bazett is the default in nearly every hospital, and it is the least accurate of the four. This is the single most common error in QTc interpretation, so it is worth being precise about.
Bazett divides the QT by the square root of the RR interval. ICH E14 — the international regulatory guidance on assessing QT prolongation, adopted by the FDA — states it directly: "Bazett's correction overcorrects at elevated heart rates and under corrects at heart rates below 60 beats per minute and hence is not an ideal correction. Fridericia's correction is more accurate than Bazett's correction in subjects with such altered heart rates."
The clinical consequence is asymmetric and it runs in the dangerous direction both ways. In a tachycardic patient Bazett inflates the QTc and manufactures prolongation that is not there, which leads to withholding a first-choice drug for no reason. In a bradycardic patient it deflates the QTc and can hide real prolongation.
The size of the effect is not subtle. In 6,609 patients at a university hospital, Bazett differed from Fridericia by a mean of 8.8 ms and from Hodges by 9.6 ms, and its residual dependence on rate was an order of magnitude worse than either. Fridericia and Framingham showed the best rate correction and were significantly better predictors of 30-day mortality. In a point-prevalence study of patients on haloperidol, switching from Bazett to an optimal correction halved the number flagged as at risk of harmful prolongation.
Thresholds, and why the formula has to travel with the number
There is a widely quoted rule that a QTc above 450 ms in men or 460 in women is prolonged, and that above 500 is dangerous. ICH E14 uses 450, 480 and 500 ms as categorical analysis thresholds and singles out 500 ms during therapy as a threshold of particular concern.
But a threshold only means something alongside a correction formula. In the healthy subset of that 6,609-patient cohort, the 99% upper limit of normal for a Bazett QTc was 472 ms in men and 482 in women — well above the conventional 450/460 lines — while for Fridericia, Framingham and Hodges it sat between 446 and 468. Applying a 450 ms cut-off to a Bazett-corrected value therefore flags a substantial number of entirely normal people. A QTc reported without saying which formula produced it is not a usable number.
What correction cannot fix
Where the QRS is wide — bundle branch block, ventricular pacing — the QT contains prolonged depolarisation as well as repolarisation, and no rate correction addresses that. In atrial fibrillation the RR interval varies beat to beat, so any single correction is an average over a moving target. And automated QT measurements from ECG machines are frequently wrong at the point where they matter most, which is why a QTc that changes a decision gets measured by hand.
Frequently asked questions
▸What is the Bazett formula for QTc?
QTc = QT divided by the square root of the RR interval in seconds. With RR = 60/heart rate, a QT of 400 ms at 100 bpm gives 400 ÷ √0.6 = 516 ms.
▸Why does Bazett over-correct at high heart rates?
The square-root relationship is too steep for fast rates. ICH E14 states that Bazett over-corrects above 60 bpm and under-corrects below it, so at tachycardia it reports prolongation that is not present. Fridericia and Framingham correct better.
▸What QTc counts as prolonged?
It depends which formula produced it. ICH E14 uses 450, 480 and 500 ms as analysis thresholds and treats above 500 ms as of particular concern, but published 99% upper limits of normal are 472 ms for Bazett in men against 446-449 for the other three formulas.
▸Which QT correction should I use?
For rate correction, Fridericia or Framingham. Bazett remains the clinical default largely by inertia; where a decision hangs on the number, use one of the others and say which you used.
▸Does QTc mean anything in atrial fibrillation or bundle branch block?
Less than you would like. In atrial fibrillation the RR interval varies beat to beat so a single correction is an average, and a wide QRS adds depolarisation time to the QT that no rate correction removes.
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