About the QTc Calculator
The QT interval on an ECG — from the start of the QRS complex to the end of the T wave — shortens as heart rate rises, so a raw QT cannot be compared between readings taken at different rates. The corrected QT (QTc) adjusts the measurement to what it would be at 60 beats per minute, making it possible to judge whether repolarisation is prolonged.
This calculator is for clinicians, nursing and medical students, and ECG learners. Enter the QT in milliseconds and either the heart rate or the RR interval; you get the QTc from all four widely used correction formulas side by side, plus an interpretation using common sex-specific cut-offs (above 450 ms in men and 460 ms in women is usually called prolonged, and above 500 ms markedly prolonged).
Bazett is the most familiar formula but over-corrects at fast heart rates and under-corrects at slow ones; Fridericia and Framingham are often preferred when the heart rate is above about 90 bpm. Always measure QT carefully (ideally in lead II or V5) and interpret it in clinical context.
With the default inputs, the qtc — bazett is 447 ms. Change any value above to recalculate instantly.
How to use the qtc calculator
- 1Measure the QT interval on the ECG in milliseconds.
- 2Choose whether you will enter heart rate or the RR interval.
- 3Enter the heart rate (bpm) or RR interval (ms).
- 4Select sex for the prolonged-QT threshold.
- 5Compare the QTc from each formula and read the interpretation.
Formula and method
All four formulas estimate what the QT interval would be at a heart rate of 60 bpm, where the RR interval is exactly 1 second. RR is in seconds (60 ÷ heart rate) and QT in milliseconds. Bazett divides by the square root of RR, Fridericia by the cube root, while Framingham and Hodges use linear corrections based on RR and heart rate respectively.
At 60 bpm all four give the same answer. As the rate moves away from 60 they diverge: Bazett is most sensitive to heart rate and tends to overestimate QTc in tachycardia. Interpretation uses common thresholds — prolonged above 450 ms (men) or 460 ms (women), markedly prolonged above 500 ms, and short below 350 ms.
- QT
- Measured QT interval (ms)
- RR
- Interval between R waves, in seconds (60 ÷ HR)
- HR
- Heart rate (beats per minute)
Worked examples
QT 400 ms at 75 bpm
RR = 60 ÷ 75 = 0.8 s. Bazett gives 400 ÷ √0.8 = 447 ms, Fridericia 400 ÷ ∛0.8 = 431 ms, Framingham 400 + 154 × 0.2 = 431 ms, and Hodges 400 + 1.75 × 15 = 426 ms — all normal for a man.
Tachycardia: QT 360 ms at 100 bpm (female)
RR = 0.6 s. Bazett inflates the QTc to 465 ms (prolonged for a woman), while Fridericia gives 427 ms and Framingham 422 ms — a classic case where Bazett over-corrects at a fast rate.
Entering an RR interval of 1000 ms
An RR interval of 1 second equals 60 bpm, where no correction is needed, so every formula returns the measured 440 ms.
Frequently asked questions
What is a normal QTc?+
A QTc up to about 450 ms in men and 460 ms in women is generally considered normal. Values above 500 ms are associated with a substantially higher risk of dangerous arrhythmias such as torsades de pointes.
Which QTc formula is best?+
Bazett is the most widely printed on ECG machines, but it over-corrects at fast heart rates. Many cardiologists and drug-safety studies prefer Fridericia or Framingham, especially when the heart rate is above about 90 bpm.
How do I convert heart rate to RR interval?+
RR in seconds = 60 ÷ heart rate. For example, 75 bpm gives an RR of 0.8 s (800 ms). Enter either value in this calculator and it converts the other for you.
What can prolong the QT interval?+
Common causes include certain medications (some antiarrhythmics, antibiotics, antipsychotics and antiemetics), low potassium, magnesium or calcium, congenital long QT syndrome, and some heart conditions. A clinician should review any prolonged QTc.
Where should the QT be measured?+
QT is usually measured in lead II or V5, from the start of the QRS complex to the end of the T wave, ideally averaged over several beats. The tangent method helps define the end of the T wave.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.