EKG Calculator

The QT interval physiologically lengthens at slow heart rates and shortens at fast ones. Without correction, comparing QT values across different heart rates would be meaningless. Bazett's 1920 formula is the most commonly cited but tends to overcorrect at high HR. Modern guidelines often prefer Fridericia or Framingham corrections for more reliable results.

Each EKG interval times a specific electrical event in the heart, measured horizontally across the tracing in milliseconds. The PR interval captures the time from the start of atrial depolarization to the start of ventricular depolarization — essentially how long the signal takes to cross the AV node. The QRS duration measures ventricular depolarization, and the QT interval spans the full cycle of ventricular depolarization plus repolarization.

Each interval maps to a labeled segment of the waveform:

• PR interval: from the beginning of the P wave to the beginning of the QRS complex
• QRS duration: from the start of the Q wave to the end of the S wave
• QT interval: from the start of the Q wave to the end of the T wave

Standardized definitions for these measurements come from the American Heart Association (AHA), the American College of Cardiology (ACC), and the Heart Rhythm Society (HRS), whose joint recommendations govern how ECG intervals are recorded and reported.

To calculate QTc, measure the QT interval and the RR interval in seconds, then apply a correction formula. The RR interval is the distance between two consecutive R waves; QTc adjusts the raw QT so it can be compared regardless of heart rate.

Work through it in order:

• Measure QT from the start of the Q wave to the end of the T wave (e.g., 400 ms = 0.40 s)
• Measure RR between two R waves, or derive it as 60,000 divided by heart rate (at 75 bpm, RR = 800 ms = 0.80 s)
• Apply Bazett: QTc = QT / √RR = 0.40 / √0.80 = 0.447 s = 447 ms
• Apply Fridericia: QTc = QT / ∛RR = 0.40 / ∛0.80 = 431 ms

A QTc near 447 ms falls in the borderline range. This calculator runs both formulas automatically, but understanding the arithmetic helps you sanity-check any tool. The American Heart Association provides the reference standards behind these calculations.

Fridericia is generally more accurate than Bazett at very fast or very slow heart rates, which is why many modern protocols favor it. Bazett divides QT by the square root of RR, while Fridericia divides by the cube root, producing a flatter, more stable correction curve.

The practical differences matter most at the extremes:

• At high heart rates, Bazett tends to overcorrect, reporting a longer QTc than reality and potentially triggering false alarms
• At low heart rates, Bazett tends to undercorrect, understating true prolongation
• Fridericia (and the Framingham linear correction) reduces this rate dependence

Despite its limitations, Bazett remains the historical clinical standard cited in most references and drug labeling. Guidance from the American College of Cardiology and the Heart Rhythm Society recognizes both; when heart rate is far from 60 bpm, cross-check Bazett against Fridericia rather than relying on a single value.

A normal QTc is generally under 440 ms in men and under 460 ms in women, with women having a physiologically longer baseline. Values between these cutoffs and roughly 460–480 ms are considered borderline, and higher values are prolonged.

The widely used thresholds break down as:

• Normal: below 440 ms (men) / below 460 ms (women)
• Borderline: approximately 440–460 ms
• Prolonged: above 460 ms (men) / above 480 ms (women)
• Severely prolonged: above 500 ms, regardless of sex

A QTc above 500 ms carries the highest risk for torsades de pointes, a dangerous ventricular arrhythmia. These sex-specific cutoffs are drawn from consensus documents by the American Heart Association, American College of Cardiology, and Heart Rhythm Society. Always interpret a single number alongside the patient's symptoms, medications, and prior tracings rather than in isolation.

A normal PR interval runs from 120 to 200 milliseconds; anything longer than 200 ms indicates first-degree AV block. The PR interval reflects how long the electrical impulse takes to travel from the atria through the AV node to the ventricles, so a prolonged value signals delayed conduction.

Read the PR interval against these landmarks:

• 120–200 ms: normal AV conduction
• Greater than 200 ms: first-degree AV block (delayed but every impulse conducts)
• Progressively lengthening or dropped beats: higher-degree (second- or third-degree) block
• Shorter than 120 ms: possible pre-excitation, as seen in Wolff-Parkinson-White

First-degree block is often benign, but higher-degree blocks can cause symptomatic bradycardia. The conduction-system framework and these cutoffs follow American Heart Association and Heart Rhythm Society standards. This calculator classifies your PR value automatically, but a full rhythm strip is needed to distinguish first-degree block from more serious types.

A QRS duration of 120 milliseconds or more indicates a bundle branch block or another intraventricular conduction delay. The QRS represents how quickly electrical activation spreads through the ventricles; a widened complex means the signal is taking an abnormal, slower path.

Interpret QRS width in tiers:

• Under 120 ms: normal ventricular conduction
• 120 ms or greater: bundle branch block (right or left) or nonspecific delay
• Markedly wide (often above 150 ms): frequently associated with left bundle branch block

Distinguishing left from right bundle branch block requires examining QRS morphology in specific leads, not just the width. A newly wide QRS with chest symptoms can also signal acute ischemia and warrants urgent evaluation. These duration thresholds come from American Heart Association and American College of Cardiology ECG standardization recommendations. The calculator flags a wide QRS, but the underlying cause must be confirmed on the full 12-lead tracing.

Dozens of common medications and several electrolyte disturbances can prolong the QT interval, raising the risk of dangerous arrhythmia. Because effects are additive, patients on multiple QT-prolonging drugs or with abnormal electrolytes face the highest risk.

Frequently implicated contributors include:

• Certain antibiotics (macrolides such as azithromycin, and fluoroquinolones)
• Some antipsychotics and antiemetics
• Antiarrhythmics such as sotalol and amiodarone
• Low potassium, magnesium, or calcium (hypokalemia, hypomagnesemia, hypocalcemia)
• Congenital long QT syndrome and hypothermia

The U.S. Food and Drug Administration (FDA) requires QT-liability testing for new drugs, and the National Institutes of Health (NIH) maintains resources on inherited long QT syndromes. Before prescribing a QT-prolonging medication to a patient with a borderline QTc, review the full medication list and correct electrolytes. This tool measures QTc but cannot assess drug interactions — always consult current clinical references.

Heart rate equals 60,000 divided by the RR interval in milliseconds, because there are 60,000 milliseconds in one minute. This lets you convert a measured beat-to-beat distance into beats per minute, or work backward from a known heart rate to the RR interval.

The relationship works both directions:

• Heart rate (bpm) = 60,000 / RR (ms) — an 800 ms RR gives 75 bpm
• RR (ms) = 60,000 / heart rate — a 60 bpm rate gives a 1,000 ms RR

A quick bedside shortcut on standard paper is to divide 300 by the number of large boxes between R waves. Normal resting adult heart rate is roughly 60 to 100 bpm per the American Heart Association; below 60 is bradycardia and above 100 is tachycardia. Accurate RR measurement matters because QTc correction depends directly on it — an error in RR propagates straight into the QTc result.

The most common QTc errors come from mismeasuring the QT interval or applying the wrong units in the formula. Small measurement slips are amplified by the correction math, so precision at the source is essential.

Watch for these frequent pitfalls:

• Mixing milliseconds and seconds — Bazett and Fridericia expect RR in seconds; feeding raw milliseconds gives nonsense results
• Ending the QT at the wrong point — the interval ends where the T wave returns to baseline, not at its peak; a prominent U wave is easily mistaken for the T-wave end
• Using Bazett at extreme heart rates without cross-checking Fridericia
• Measuring in a single lead — the longest QT across several leads is more reliable
• Ignoring sex-specific cutoffs, applying the male threshold to a female patient or vice versa

Guidance from the American Heart Association and Heart Rhythm Society stresses consistent lead selection and clear T-wave endpoints. When a QTc looks surprisingly high or low, re-measure before acting on it, and treat the tool's output as a check on your own reading, not a replacement for it.

A QTc above 500 milliseconds, or an increase of more than about 60 ms from a patient's baseline, is a red flag that warrants prompt clinical attention. At this level the risk of torsades de pointes rises sharply, and immediate steps may be needed to lower the danger.

High-risk situations to escalate include:

• QTc consistently above 500 ms on repeat tracings
• A large jump from a prior baseline QTc
• Prolonged QTc plus symptoms such as syncope, palpitations, or seizures
• Prolongation combined with low potassium or magnesium

Typical urgent measures — stopping offending drugs and correcting electrolytes — should be directed by a qualified clinician, not by a calculator. The American Heart Association, American College of Cardiology, and Heart Rhythm Society publish the consensus behind these risk thresholds. This EKG calculator is an educational and workflow aid for trained users; it does not diagnose, and any concerning result must be confirmed and managed by a licensed healthcare professional.