QTc Interval Calculator (Bazett, Fridericia, Framingham, Hodges)

Overview

The QT interval measures how long the ventricles take to depolarize and repolarize. It shortens as heart rate rises and lengthens as it falls, so a raw QT cannot be compared across patients — you need QTc, the QT corrected to a rate of 60 bpm. This calculator takes the QT interval in milliseconds plus either heart rate (bpm) or the RR interval (ms) and returns all four standard corrections — Bazett, Fridericia, Framingham, and Hodges — alongside the AHA/ACCF/HRS sex-specific normal, borderline, and prolonged bands and the 500 ms Torsades de Pointes risk threshold. It also flags when the heart rate falls outside 60-100 bpm, where Bazett over- or under-corrects.

Formula

RR (s) = 60 ÷ heart rate (bpm) | Bazett: QTc = QT ÷ √RR | Fridericia: QTc = QT ÷ RR^(1/3) | Framingham: QTc = QT + 154 × (1 − RR) | Hodges: QTc = QT + 1.75 × (heart rate − 60) | e.g. QT 400 ms at 80 bpm → RR = 0.75 s, Bazett QTc = 400 ÷ √0.75 ≈ 461.9 ms

How to Use

1. 1Select sex — the normal QTc ceiling is 450 ms for men and 460 ms for women.
2. 2Enter the measured QT interval in milliseconds, usually the longest QT in lead II or V5.
3. 3Enter the heart rate in bpm, or switch the input mode and enter the RR interval in milliseconds.
4. 4Choose which correction drives the verdict (Bazett by default; Fridericia for drug trials).
5. 5Read all four QTc values, the verdict band, and the margin left to the normal ceiling.

Examples

Background

Bazett and Fridericia both published their power-law corrections in 1920. Bazett is simple enough that nearly every ECG machine prints it automatically, but its error grows quickly outside 60-100 bpm. The Framingham correction (Sagie et al., 1992) is a linear fit derived from 5,018 participants in the Framingham Heart Study, and the Hodges correction simply adds 1.75 ms for every beat above 60. The verdict bands follow the AHA/ACCF/HRS recommendations for ECG standardization: 450 ms in men and 460 ms in women mark borderline prolongation, 470 ms and 480 ms are the respective 99th percentiles and define prolongation, and 500 ms marks markedly increased Torsades de Pointes risk. At the short end, the European Society of Cardiology diagnoses short QT syndrome at a QTc of 340 ms or below. Regulatory QT studies for new drugs (ICH E14) use Fridericia rather than Bazett.

Tips

• ✔The QTc your ECG machine prints is almost always the Bazett value — cross-check with Fridericia whenever the rate is outside 60-100 bpm.
• ✔On a 25 mm/s tracing one small box is 40 ms, so ten boxes equal a 400 ms QT.
• ✔In atrial fibrillation the RR interval varies beat to beat; average several consecutive beats before entering QT and rate.
• ✔A wide QRS from left bundle branch block or ventricular pacing lengthens the measured QT and inflates QTc.
• ✔Hypokalemia, hypomagnesemia, hypocalcemia, antiarrhythmics, macrolide and quinolone antibiotics, and many antipsychotics are the usual culprits behind acquired QT prolongation.
• ✔An increase of 60 ms or more from a patient's own baseline after starting a drug warrants a conversation with the prescriber even if the absolute QTc stays under 500 ms.

FAQ