QTc Interval Calculator: Bazett, Fridericia, Framingham & Hodges

Introduction to QTc interval correction across heart rates

The QT interval is the time from the beginning of ventricular depolarization to the completion of ventricular repolarization on an electrocardiogram. In routine ECG measurement, it usually runs from the earliest onset of the QRS complex to the point where the T wave returns to baseline. QT is recorded in milliseconds. A raw QT value cannot be compared fairly between recordings with different heart rates because QT normally shortens as rate rises and lengthens as rate slows.

QTc means corrected QT. It is an estimate intended to place QT measurements on a shared rate-related scale, conventionally the expected value at 60 beats per minute. It is useful for describing a measurement, but it is not a direct measurement and it is not a diagnosis. Each correction equation makes a different mathematical assumption about the relation between QT and cycle length. Their agreement near 60 bpm and disagreement farther from 60 bpm are clinically meaningful features of the estimate.

This QTc calculator displays Bazett, Fridericia, Framingham, and Hodges results together. Viewing all four makes formula sensitivity visible instead of presenting one number as unquestionably precise. The page keeps the units explicit: measured QT is milliseconds, RR is seconds, and heart rate is beats per minute.

Entering a QT interval and RR interval for QTc calculation

Enter the measured QT in milliseconds. On a standard tracing, choose a lead where the end of the T wave is visible and measure from QRS onset to T-wave end. A U wave, a low-amplitude T wave, baseline movement, or a biphasic T wave can make that endpoint uncertain. This calculator cannot determine the correct endpoint from an ECG image, so the measurement should be checked before the arithmetic is used for discussion.

For RR-based equations, enter the time between consecutive R peaks in seconds. For example, an interval of 800 ms must be entered as 0.8 seconds. If only a heart rate is known, leave RR blank and enter heart rate; the calculator converts it to cycle length using the relationship below.

Formula: RR = 60 / HR

Here RR is seconds and HR is beats per minute. When both RR and heart rate are supplied, the calculator uses RR for the equations and reports that both values were entered. At the common paper speed of 25 mm per second, one small 1 mm ECG square is 40 ms and one large 5 mm square is 200 ms. Two large squares plus one small square therefore represent 440 ms. A one-small-square placement error changes QT by 40 ms, which may be larger than the difference between correction methods near a normal rate.

QTc correction formulas and their units

All four displayed QTc equations preserve QT in milliseconds. Bazett, Fridericia, and Framingham use RR in seconds; Hodges uses heart rate directly. At 60 bpm, RR is one second. The rate conversion can also be written as follows.

Formula: HR = 60 / RR

Therefore every correction returns the measured QT unchanged when RR is one second and HR is 60 bpm. The equations separate as the rate moves away from that point.

Bazett QTc correction.

Bazett formula: QTc = QT / sqrt(RR). RR is in seconds.

Formula: QTc = QT / sqrt(RR)

Bazett divides QT by the square root of RR. It is widely reported by ECG machines, but it tends to over-correct at faster heart rates and under-correct at slower heart rates.

Fridericia QTc correction.

Fridericia formula: QTc = QT / RR^(1/3). RR is in seconds.

Formula: QTc = QT / (root(RR, 3))

Fridericia uses the cube root of RR. It often produces a less extreme estimate than Bazett when heart rate is elevated, although it remains an estimate rather than a universal correction.

Framingham QTc correction.

Formula: QTc = QT + 154 · (1 − RR)

The Framingham equation is linear in RR rather than based on a root. In this millisecond form, 154 is multiplied by one minus RR in seconds.

Hodges QTc correction.

Formula: QTc = QT + 1.75 · (HR − 60)

Hodges corrects against heart rate directly. The calculator derives HR from RR whenever RR is the supplied timing input. Formula differences are not a calculator error; they show that rate correction is model-dependent.

Comparing Bazett, Fridericia, Framingham, and Hodges estimates

Bazett is historically common and is often the automatically reported ECG value. At high heart rates its square-root correction can yield a noticeably larger QTc than the other methods. At slow rates it can yield a smaller value. Fridericia usually moderates that curvature, while Framingham and Hodges apply linear adjustments. No equation is perfect across every rhythm, patient population, or measurement circumstance.

For a QT of 420 ms and an RR of 0.8 seconds, the estimates are approximately 469.6 ms with Bazett, 452.4 ms with Fridericia, 450.8 ms with Framingham, and 446.3 ms with Hodges. The measured beat is identical in every calculation. The spread appears because the rate is 75 bpm rather than 60 bpm and the formulas describe the rate relationship differently.

Interpreting a QTc estimate in clinical context

QTc values need clinical context. Adult reference ranges vary by sex, age, guideline, correction method, ECG lead, and whether the QT was measured manually or automatically. Broad educational descriptions often identify values below about 450 ms in men and 460 ms in women as typical, while values around 470–480 ms or above may receive closer attention. A value of 500 ms or more is commonly treated as a higher-risk flag, particularly when other risk factors are present. These are not personal diagnostic thresholds.

A prolonged result can be associated with medicines, electrolyte disturbances, congenital long-QT conditions, myocardial disease, bradycardia, or measurement difficulty. A result near a threshold may move substantially when a different lead, beat, correction method, or preceding RR interval is selected. The result panel intentionally reports a range across four formulas to show that uncertainty instead of implying that one value settles the question.

Urgent medical assessment is appropriate for fainting, seizure-like episodes, sustained palpitations, or chest symptoms, particularly when an ECG has suggested a markedly prolonged QTc or when QT-prolonging medicines are involved. Do not use this page to begin, stop, or change medication.

Worked example: QT 420 ms with RR 0.8 seconds

Consider a regular ECG beat with QT = 420 ms and RR = 800 ms. First convert the cycle length to seconds: 800 ms is 0.8 s. The associated heart rate is 60 divided by 0.8, or 75 bpm. Bazett calculates 420 divided by the square root of 0.8, which is about 469.6 ms. Fridericia divides 420 by the cube root of 0.8, producing about 452.4 ms.

Framingham adds 154 × (1 − 0.8), or 30.8 ms, to the original QT for a result of 450.8 ms. Hodges adds 1.75 × (75 − 60), or 26.25 ms, to give about 446.3 ms. The highest and lowest values differ by more than 20 ms. This worked QTc example shows why a report should identify its correction formula and why a value near a clinical decision boundary deserves careful ECG review.

Measurement assumptions and QTc limitations

QTc arithmetic is only as sound as the measurement entered. Baseline artifact, biphasic or low-amplitude T waves, U waves, bundle branch block, pacing, and lead selection can all make the QT endpoint uncertain. A wide QRS can lengthen QT because of prolonged depolarization, a problem ordinary rate correction does not solve. Specialized approaches may be needed in that setting.

Irregular rhythms create another important limitation. In atrial fibrillation and frequent ectopy, the preceding RR interval changes from beat to beat. A single selected cycle may not represent the underlying rhythm. Clinicians may assess several beats or use protocol-specific approaches, but a simple single-beat calculator cannot resolve that complexity. Pediatric values, pregnancy, inherited arrhythmia syndromes, and drug-safety monitoring also need setting-specific interpretation.

The form applies guard rails: it rejects a cycle length implying a heart rate below 20 or above 250 bpm, which often means RR was entered in milliseconds instead of seconds, and it rejects a QT longer than the full RR cycle. Those checks catch obvious unit or placement problems; they do not establish that a remaining result is clinically normal.

Using this QTc interval calculator safely

Enter QT in milliseconds, then enter either RR in seconds or heart rate in bpm. Select Calculate QTc to see all four estimates, the derived heart rate, and the numerical spread. If both RR and heart rate were typed, read the message showing how they compare. Reset clears the form. Copy shareable link places the entered values in the page URL for convenient educational discussion.

Use the chart as a visual explanation rather than a prediction tool. It holds the entered QT constant while varying heart rate from 40 to 160 bpm, showing where the formulas converge and where they diverge. The vertical marker identifies the entered beat when its rate falls inside that chart range. Repeating a calculation with a corrected measurement is generally more useful than trying to infer clinical meaning from one decimal place.

QTc interval questions and source notes