QTc calculator with four correction formulas
For healthcare professionals and students. Check results against your local protocols and clinical judgement; this tool does not give medical advice.
This QTc calculator corrects a measured QT interval for heart rate using the four formulas people most often ask about — Bazett, Fridericia, Framingham and Hodges — and shows them side by side. Enter the QT in milliseconds or seconds and either the heart rate or the RR interval, choose which formula to headline, and the tool gives each corrected value, the working with your numbers, and how the result compares with the sex-specific cut-offs in the AHA/ACCF/HRS recommendations.
Seeing all four at once matters because they diverge as the heart rate moves away from 60 bpm. A small chart places each value on a 300–600 ms scale with the prolonged and 500 ms reference lines.
How to use the QTc Calculator
- Enter the QT interval and choose ms or s.
- Choose whether you have the heart rate or the RR interval (ms or s), and enter it.
- Pick the sex so the right prolonged-QTc cut-off is used.
- Choose the headline formula your department uses; the other three stay visible below.
- Read the QTc, the chart and any warnings, and copy the summary if you need it in the notes.
QTc Calculator features
Four formulas together
Bazett, Fridericia, Framingham and Hodges from the same input, with the headline one highlighted on the chart.
Heart rate or RR
Use bpm, RR in milliseconds or RR in seconds; QT in ms or s.
Sex-specific cut-offs
Prolonged at 450 ms or more in men and 460 ms or more in women, short at 390 ms or less, as recommended in 2009.
Full working
Every formula is written out with your QT and RR, so you can check a value against a hand calculation.
Rate warnings
Flags fast and slow rates where Bazett is least reliable, implausible values, unit mix-ups and a QTc over 500 ms.
QTc calculation formulas
RR is the interval between beats in seconds, RR = 60 ÷ heart rate. QT and QTc are in milliseconds. At 60 bpm, RR = 1 s and all four formulas return the measured QT unchanged.
| Formula | Equation | Source |
|---|---|---|
| Bazett | QTc = QT ÷ √RR | Bazett HC, Heart 1920;7:353–370 (reprinted Ann Noninvasive Electrocardiol 1997) |
| Fridericia | QTc = QT ÷ ∛RR | Fridericia LS, Acta Med Scand 1920;53:469–486 |
| Framingham | QTc = QT + 154 × (1 − RR) | Sagie A et al., Am J Cardiol 1992;70:797–801 (0.154 s × (1 − RR) in seconds) |
| Hodges | QTc = QT + 1.75 × (HR − 60) | Hodges M et al., J Am Coll Cardiol 1983;1:694 (abstract) |
Worked example and why the formulas disagree
A QT of 400 ms at 75 bpm (RR 0.8 s) gives Bazett 400 ÷ √0.8 = 447 ms, Fridericia 400 ÷ 0.928 = 431 ms, Framingham 400 + 154 × 0.2 = 431 ms and Hodges 400 + 1.75 × 15 = 426 ms. At 100 bpm the same QT gives 516, 474, 462 and 470 ms — a 54 ms spread that can move a patient across a threshold.
Rautaharju PM, Surawicz B, Gettes LS, et al. Circulation 2009;119:e241–e250 note that Bazett over-corrects at high heart rates and under-corrects at low ones, and recommend linear regression functions rather than Bazett for rate correction. Many departments document Fridericia, particularly at fast rates. Whichever you use, use the same formula when comparing serial ECGs.
Interpreting QTc: cut-offs used here
The AHA/ACCF/HRS 2009 recommendations propose that a QTc of 450 ms or longer in men and 460 ms or longer in women be considered prolonged, and a QT of 390 ms or less be considered short. Drew BJ et al., Circulation 2010;121:1047–1060 describe a QTc over 500 ms as associated with a higher risk of torsade de pointes. These are reference points, not a diagnosis: drug effects, electrolytes, bundle branch block (a wide QRS lengthens QT) and atrial fibrillation (variable RR) all change how a number should be read.
QTc Calculator FAQ
How do you calculate QTc with Bazett’s formula?
Divide the QT by the square root of the RR interval in seconds. For QT 400 ms at 75 bpm, RR is 0.8 s and QTc = 400 ÷ 0.894 = 447 ms. The QTc calculator above does it for any rate and shows the other three formulas too.
Which QTc formula is best?
There is no single answer. Bazett is the most widely used and printed on many ECG machines, but it over-corrects at fast rates; Fridericia and Framingham behave better away from 60 bpm. Follow your local protocol and keep to one formula for serial comparison.
What is a normal QTc?
The AHA/ACCF/HRS 2009 recommendations treat 450 ms or more in men and 460 ms or more in women as prolonged, and 390 ms or less as short. Values above 500 ms carry a higher risk of torsade de pointes.
Can I enter the RR interval instead of the heart rate?
Yes. Choose RR interval in milliseconds or seconds; the tool converts it to heart rate for Hodges and shows both in the working.
How do I handle atrial fibrillation?
With an irregular rhythm a single QT and RR pair is unreliable. A common approach is to average several beats, but follow your local guidance; the tool corrects whatever pair you enter.
Does a wide QRS affect QTc?
Yes. Bundle branch block lengthens the QT without a matching repolarisation change, so a QTc can look prolonged. Interpret it with the full ECG; for other bedside numbers see the corrected calcium calculator, as calcium also changes the QT.