QTc Calculator

The QT interval on an electrocardiogram runs from the start of the QRS complex to the end of the T wave and reflects how long the ventricles take to recharge after each beat. A long QT matters because it can set the stage for torsade de pointes, a dangerous ventricular rhythm, and many medicines lengthen it. This page explains the four common correction formulas, the limits used to call a QTc normal or prolonged, and how to read results that disagree.

Two sister tools often come up alongside a QTc check. The eGFR Calculator estimates kidney function, which matters because some QT-prolonging drugs such as dofetilide and sotalol are cleared by the kidneys, and the CHA2DS2-VASc Calculator scores stroke risk for people with atrial fibrillation, who are among the most frequent users of rhythm drugs.

Why the QT interval needs correcting

The QT shortens as the heart speeds up and lengthens as it slows, so a raw QT of 400 ms means something different at 50 beats per minute than at 110. Correction formulas convert the measured QT to an estimate of what it would be at 60 beats per minute, where the RR interval, the time between two R waves, is exactly 1 second. That is why every formula returns the measured QT unchanged at 60 beats per minute.

The four QTc formulas

QT and QTc are in milliseconds and RR in seconds. Bazett and Fridericia published their formulas in 1920; the 2009 AHA/ACCF/HRS recommendations note that Bazett's was based on only 39 young subjects. The Framingham formula comes from the baseline ECGs of 5,018 Framingham Heart Study participants without coronary disease (Sagie and colleagues, 1992), and the Hodges formula is a linear correction based on heart rate. Framingham and Hodges are linear corrections, adding or subtracting a fixed amount per unit of RR interval and per beat per minute respectively, while Bazett and Fridericia divide by a power of RR.

Worked example: QT 400 ms at 75 beats per minute

A woman's ECG shows a QT of 400 ms and a heart rate of 75. The RR interval is 60 ÷ 75 = 0.8 s.

• Bazett: 400 ÷ √0.8 = 400 ÷ 0.8944 = 447 ms
• Fridericia: 400 ÷ ∛0.8 = 400 ÷ 0.9283 = 431 ms
• Framingham: 400 + 154 × 0.2 = 431 ms
• Hodges: 400 + 1.75 × 15 = 426.25, rounded to 426 ms

All four values sit below the 460 ms limit for women, so the QTc is within normal limits by every formula. The spread between them is 21 ms, typical for a rate this close to 60. This is the example the calculator loads by default.

When the formulas disagree: fast and slow heart rates

Now a man with a QT of 360 ms at 108 beats per minute (RR 0.556 s). Bazett gives 360 ÷ 0.7454 = 483 ms, above the 470 ms 99th percentile for men, while Fridericia gives 438 ms, Framingham 428 ms and Hodges 444 ms, all normal. One formula says prolonged and three say normal; the disagreement comes from Bazett overcorrecting at a fast rate.

At a slow rate the error flips. A man with a QT of 480 ms at 52 beats per minute (RR 1.154 s) gets 447 ms from Bazett, just under the 450 ms limit, but 458 ms from Fridericia, 456 ms from Framingham and 466 ms from Hodges, all in the borderline band. Here Bazett undercorrects and can hide a long QT. The Framingham authors described exactly this pattern: Bazett overcorrects at fast heart rates and undercorrects at slow ones.

QTc for a fixed QT of 400 ms at different heart rates

This table was computed with the calculator's formulas. Values are rounded to whole milliseconds.

The same 400 ms QT is normal by every formula at 60 and 70 beats per minute. At 80, Bazett already reaches 462 ms, borderline for both sexes, and at 100 it reports 516 ms, past the 500 ms line, while Framingham reports 462 ms. Below 60 the order reverses: Bazett and Framingham give the lowest values, and at 50 beats per minute every formula lands at 390 ms or less, the short-QT range.

Normal, borderline and prolonged QTc

Two AHA statements draw the line in slightly different places. The 2009 standardization recommendations call 450 ms in men and 460 ms in women prolonged; the 2010 statement on preventing torsade de pointes treats values above the 99th percentile, about 470 and 480 ms, as abnormally prolonged. The calculator labels the gap between them borderline instead of hiding the disagreement. Values in ms.

Each formula has its own upper limit of normal

Because the formulas treat heart rate differently, their normal ranges differ too. A 2016 study of ECGs from 6,609 hospital patients in Leuven, Belgium, derived the 99% upper limit of normal for each formula in its healthy subset:

Bazett's limits are the highest because it inflates QTc at the faster rates common in hospital patients. Fridericia and Framingham showed the best rate correction in that study and predicted 30-day and 1-year mortality better than Bazett, which overestimated how many patients had potentially dangerous prolongation.

Bazett or Fridericia: which formula to use

Bazett remains the default on many ECG machines and in older literature, so its value is often the one printed on a report. The 2009 AHA/ACCF/HRS recommendations advise using linear regression formulas rather than Bazett's and naming the correction method in every report. FDA guidance for drug trials asks for both Bazett and Fridericia and describes Fridericia as more accurate at high heart rates and below 60 beats per minute. In practice, use the same formula every time you compare two ECGs, and when Bazett and Fridericia disagree at a fast rate, give more weight to Fridericia or Framingham.

How to use this calculator

Enter the measured QT in milliseconds, then either the heart rate or the RR interval in milliseconds. Choose the sex so the right limits apply and pick the formula for the headline; all four values appear either way. If you have an earlier QTc from the same formula, enter it as a baseline to see the change. The table at the bottom repeats your QT at heart rates from 50 to 120 so you can see how much the correction itself contributes.

Who this calculator is for

It is built for nursing and medical students learning ECG interpretation, for pharmacists and clinicians cross-checking a machine-reported QTc during treatment with a QT-prolonging drug, and for patients who want to understand an ECG report. It is not a substitute for a clinician reading the full tracing.

Key terms

• QT interval: time from the start of the QRS complex to the end of the T wave.
• RR interval: time between two consecutive R waves; 60 divided by the heart rate, in seconds.
• QTc: the QT corrected to a heart rate of 60 beats per minute.
• Torsade de pointes: a polymorphic ventricular tachycardia linked to a long QT that can degenerate into ventricular fibrillation.
• Long QT syndrome: an inherited or drug-induced condition with a prolonged QT and a higher risk of torsade de pointes.

What changes the QTc

Beyond the formula and the heart rate, the 2010 AHA/ACCF statement lists risk factors for drug-induced torsade de pointes that often travel with a long QT: older age, female sex, heart disease, low potassium or magnesium, kidney or liver dysfunction, a slow heart rate or long pauses, more than one QT-prolonging drug at a time and genetic predisposition. The measurement matters as well: the 2009 recommendations warn against correcting the QT when the end of the T wave cannot be identified reliably.

Limitations

Every correction formula is an approximation fitted to a particular population, and none removes the effect of heart rate completely. Results depend on how the QT was measured, which lead was used and how clearly the end of the T wave can be seen. Correction is unreliable in atrial fibrillation and other irregular rhythms, the limits here are for adults, and a borderline value on its own does not diagnose long QT syndrome.

Related calculators

• Kidney function for drugs cleared by the kidneys: eGFR Calculator.
• Stroke risk in atrial fibrillation: CHA2DS2-VASc Calculator.
• Maximum heart rate and training zones: Target Heart Rate Calculator.

Sources

• Rautaharju PM, Surawicz B, Gettes LS, et al. (2009). AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part IV: The ST Segment, T and U Waves, and the QT Interval. Circulation 119:e241-e250. PubMed
• Drew BJ, Ackerman MJ, Funk M, et al. (2010). Prevention of Torsade de Pointes in Hospital Settings: a scientific statement from the AHA and ACCF. Circulation 121:1047-1060. Full text (PubMed Central)
• U.S. Food and Drug Administration (2005). E14 Clinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential for Non-Antiarrhythmic Drugs, guidance for industry. fda.gov
• Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D (1992). An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). American Journal of Cardiology 70:797-801. PubMed
• Vandenberk B, Vandael E, Robyns T, et al. (2016). Which QT Correction Formulae to Use for QT Monitoring? Journal of the American Heart Association 5:e003264. PubMed
• Bazett HC (1920). An analysis of the time-relations of electrocardiograms. Heart 7. Fridericia LS (1920). Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Medica Scandinavica 53. Hodges M, Salerno D, Erlien D (1983). Bazett's QT correction reviewed. Journal of the American College of Cardiology 1:694 (abstract).