QTc Calculator (Bazett, Fridericia, Framingham, Hodges)

Understanding the QTc calculator — concept and principle

The QT interval on an electrocardiogram (ECG) runs from the start of the QRS complex to the end of the T wave and reflects how long the heart's ventricles take to depolarise and recover. Because QT naturally shortens as heart rate rises, it is "corrected" to a standard heart rate of 60 beats per minute, giving the QTc. Clinicians check it when starting QT-prolonging medicines or after electrolyte disturbances. This calculator applies four widely used correction formulas side by side.

At 75 bpm the four formulas stretch QT 400 ms to 426–447 ms; an ECG must still be read by a professional.

How it is calculated

First the RR interval (time between beats) is found in seconds: RR = 60 ÷ HR.

Bazett (1920): QTc = QT ÷ √RR
Fridericia (1920): QTc = QT ÷ ∛RR
Framingham (Sagie, 1992): QTc = QT + 154 × (1 − RR), with QT in ms
Hodges (1983): QTc = QT + 1.75 × (HR − 60)

QT is entered in milliseconds and heart rate in beats per minute. The flag uses the Fridericia value with the limit for the sex you choose: 450 ms for men, 460 ms for women, and above 500 ms high risk.

The principle

At 60 bpm the RR interval is exactly 1 second, so every formula returns the measured QT unchanged. At faster rates RR is below 1 and the formulas lengthen QT; at slower rates they shorten it. Bazett's square-root formula over-corrects at fast heart rates and under-corrects at slow ones. Fridericia's cube root and the linear Framingham and Hodges formulas behave more evenly across rates, which is why many drug-safety studies prefer Fridericia.

Worked example

Inputs: QT 400 ms, heart rate 75 bpm.
RR = 60 ÷ 75 = 0.8 s.
Bazett: 400 ÷ √0.8 = 400 ÷ 0.894 = 447 ms.
Fridericia: 400 ÷ ∛0.8 = 400 ÷ 0.928 = 431 ms.
Framingham: 400 + 154 × 0.2 = 431 ms.
Hodges: 400 + 1.75 × 15 = 426.25, shown as 426 ms.
With sex set to male, Fridericia 431 ms is below 450 ms, so the flag reads not prolonged.

Each point is the calculator re-run with only “QT interval (ms)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

QT interval (ms)	Bazett
100	112
200	224
300	335
400	447
500	559
600	671
800	894

How to read the result

Where does your result fall?

Commonly used upper limits are about 450 ms for men and 460 ms for women; values above 500 ms are associated with a markedly higher risk of the dangerous arrhythmia torsades de pointes. A rise of 60 ms or more from a person's own baseline is also considered significant.

Common mistakes

Measuring QT in a lead where the end of the T wave is unclear, or including a U wave.
Taking QT from a single beat in atrial fibrillation instead of averaging several beats.
Relying on Bazett alone when the heart rate is fast.

Keep in mind: This is not a diagnosis. QT measurement needs a good-quality ECG read by a trained professional, and decisions about medicines must be made by a doctor or pharmacist.

Frequently asked questions

Which formula should I trust most?

Fridericia or Framingham is usually more reliable than Bazett away from 60 bpm; the key is to use the same formula when comparing ECGs over time.

Why does a wide QRS matter?

Bundle branch block lengthens QT without the same risk, so clinicians use adjusted methods in that situation.

Sources: Rautaharju PM et al., AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part IV, Circulation 2009 · Sagie A et al., An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study), American Journal of Cardiology 1992

Checked 8 Nov 2026. Published formulas cited on the page. For information only — not medical advice.

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