ECG · Heart-rate correction
QTc calculator: Bazett and Fridericia.
Enter the QT interval in milliseconds and the heart rate to see the corrected QT by four formulas at once, so you can see how much the choice of formula moves the answer.
A QT of 400 ms at 75 bpm corrects to 447.2 ms by Bazett, 430.9 ms by Fridericia, 430.8 ms by Framingham and 426.3 ms by Hodges. Same ECG, four answers, which is why the formula matters.
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How the four QTc formulas work
All four correct the measured QT for heart rate, because QT shortens as the heart speeds up. With QT and RR in seconds, as set out in the 2016 Vandenberk comparison under Sources: Bazett is QT / RR1/2, Fridericia is QT / RR1/3, Framingham is QT + 0.154 × (1 - RR), and Hodges is QT + 0.00175 × (60/RR - 60). In this calculator QT is in milliseconds, so the Framingham term is 154 × (1 - RR) ms and the Hodges term is 1.75 × (heart rate - 60) ms. RR is 60 divided by the heart rate, so 75 bpm is an RR of 0.8 s.
At a heart rate of exactly 60 bpm the RR interval is 1 second and all four formulas return the measured QT unchanged. Away from 60 they diverge. The Vandenberk paper describes Bazett as having a known overcorrection at high heart rates and undercorrection at lower ones, and found Fridericia and Framingham gave the best rate correction. That is why this page puts four results next to each other instead of picking one for you.
Worked example
QT 400 ms, heart rate 75 bpm, so RR = 60 / 75 = 0.8 s. Bazett: 400 / √0.8 = 400 / 0.8944 = 447.2 ms. Fridericia: 400 / 0.81/3 = 400 / 0.9283 = 430.9 ms. Framingham: 400 + 154 × 0.2 = 430.8 ms. Hodges: 400 + 1.75 × 15 = 426.3 ms. The spread between the highest and lowest is about 21 ms, which is large enough to move a result across a cut-off.
The cut-off line in the result uses 450 ms for men and 470 ms for women, the cutoffs that the Vandenberk study states it used. Those are study cut-offs, not a clinical rule from this page, and the same paper reports that the 99% upper limits in its healthy subset were higher with Bazett (472 ms for men, 482 ms for women). Atrial fibrillation, wide QRS and measuring across several beats raise further questions this tool does not address: it takes one QT and one rate and does the arithmetic.
Common mix-ups
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Often assumedBazett is the standard formula, so its QTc is accurate at any heart rate.
What the source saysThe authors describe a known overcorrection at high heart rates and undercorrection at lower rates, with a QTc/RR slope of -0.071. Fridericia and Framingham showed the best rate correction. [1]
Checked against these sources on .
Frequently asked questions
What are the Bazett and Fridericia formulas?
With QT and RR in seconds, Bazett is QT divided by the square root of RR and Fridericia is QT divided by the cube root of RR. RR is 60 divided by the heart rate in beats per minute.
Why do the formulas give different answers?
Each assumes a different relationship between QT and heart rate. They agree at 60 bpm, where RR is 1 second, and spread out as the rate moves away from 60.
Which QTc formula is best?
A 2016 comparison in the Journal of the American Heart Association found Fridericia and Framingham gave the best rate correction, and described Bazett as over-correcting at high heart rates and under-correcting at lower ones. Which to use in practice is a clinical and protocol decision.
What QTc is considered prolonged?
The 2016 study used cut-offs of 450 ms for men and 470 ms for women. Other guidelines and drug labels use their own thresholds, so use the one your clinician or protocol specifies.
How do I get the heart rate from the RR interval?
Heart rate in beats per minute is 60 divided by the RR interval in seconds. An RR of 0.8 s is 75 bpm.
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Not medical advice
This calculator does arithmetic on the numbers you type and does not diagnose or treat anything. Results depend on the quality of the inputs and the equation used. Lab values, drug doses and ECG readings must be interpreted by a clinician who knows the patient; contact your care team or emergency services for anything urgent.