QTc Calculator & Normal QTc Range

What is a normal QTc?

The QT interval on an electrocardiogram (ECG) measures the time from the start of the heart's electrical activation (the beginning of the QRS complex) to the end of its recovery (the end of the T wave). It represents how long the ventricles take to "recharge" after each beat. Because the QT interval naturally shortens as the heart beats faster and lengthens as it slows, it is corrected for heart rate to give the QTc, so that values can be compared across different heart rates.

A normal QTc for adults, according to the 2009 AHA/ACCF/HRS recommendations on ECG interpretation:

A QTc of 500 ms or more is associated with a substantially higher risk of a dangerous ventricular arrhythmia called torsades de pointes. A short QT is uncommon and is clinically important mainly in the rare inherited short-QT syndrome.

The calculator above takes a QT interval (in ms or seconds) and a heart rate, and calculates QTc with the Bazett, Fridericia, Framingham and Hodges formulas. It interprets the result with the formula you choose (Fridericia by default, because it performs better across a wider range of heart rates) using sex-specific thresholds.

QTc formulas compared

In all equations, RR is the interval between heartbeats in seconds (RR = 60 ÷ heart rate), QT is in milliseconds, and HR is beats per minute. At a heart rate of exactly 60 bpm, RR is 1 second and all four formulas give QTc = QT.

How much the formulas differ

The table below shows the QTc for a measured QT of 400 ms at different heart rates:

At 100 bpm, Bazett labels this QT as markedly prolonged (516 ms), while Fridericia gives 474 ms and Framingham 462 ms. That difference is exactly why many clinicians and regulatory guidance documents prefer Fridericia or another linear formula when heart rates are fast, and why the AHA/ACCF/HRS statement recommends that a linear regression correction be used rather than Bazett. At slow heart rates the reverse happens: Bazett makes a long QT look shorter.

Worked examples

• QT 400 ms, heart rate 75 bpm (man): RR = 0.8 s. Bazett = 400 ÷ √0.8 = 447 ms; Fridericia = 400 ÷ ∛0.8 = 431 ms. Normal by Fridericia; Bazett is borderline.
• QT 440 ms, heart rate 90 bpm (woman): RR = 0.667 s. Bazett = 539 ms; Fridericia = 504 ms. Markedly prolonged by both – needs review.
• QT 480 ms, heart rate 60 bpm (man): RR = 1.0 s. All formulas = 480 ms. Prolonged.
• QT 0.36 s, heart rate 50 bpm: enter 0.36 with the "s" unit. RR = 1.2 s; Fridericia = 360 ÷ ∛1.2 = 339 ms. Short by the 390 ms threshold – usually insignificant at a slow rate, but interpret with the whole ECG.

How to measure the QT interval

QT is measured from the beginning of the QRS complex to the end of the T wave, ideally in the lead with the longest QT, often lead II or V5. Practical points:

• Find the end of the T wave using the "tangent method": draw a line along the steepest part of the T wave's downslope and see where it meets the baseline.
• Exclude U waves – small waves after the T wave – unless they merge with the T wave.
• Average several beats, especially if the rhythm is irregular.
• Use the RR interval preceding the measured QT, or an average in atrial fibrillation.
• On standard ECG paper at 25 mm/s, one small square = 40 ms and one large square = 200 ms.

Automated ECG machines report QT and QTc, usually with Bazett. Their measurements are generally reliable in sinus rhythm but can be wrong with flat T waves, U waves, bundle branch block or noise, so the automated value should be checked by hand when it matters.

Everyday factors that change the QT interval

QT is not fixed. Even in healthy people it varies by a few tens of milliseconds over a day:

• Sleep: QT tends to be longer at night, when the heart rate is slower and vagal tone higher.
• Meals: QTc can shorten slightly after meals.
• Hormones: women have longer QTc than men on average after puberty, partly due to the effects of sex hormones on the heart's ion channels.
• Age: QTc lengthens slightly with age.
• Body position and activity: changing from lying to standing alters heart rate and autonomic tone, which affects QT before the heart rate settles.
• Measurement differences: different leads, observers and machines can produce differences of 10–20 ms.

Because of this natural variation, a single borderline value is usually confirmed with a repeat ECG, ideally recorded at rest after several minutes lying quietly.

Special situations

Bundle branch block and wide QRS

When the QRS is wide (for example in bundle branch block or with ventricular pacing), part of the QT is prolonged by slow ventricular activation rather than slow recovery. Alternatives include measuring the JT interval (QT minus QRS duration) or using specific correction formulas. Interpretation is best done by a clinician experienced in ECGs.

Atrial fibrillation

With an irregular rhythm, QT varies beat to beat. Clinicians often average QT and RR over several beats, or measure QT after the longest and shortest RR intervals and average the corrected values.

Very slow or fast heart rates

At heart rates under about 50 or over about 100 bpm, the choice of formula matters more. Fridericia and the linear formulas are less distorted than Bazett at these extremes.

Children

Children have faster heart rates, and Bazett is still commonly used in paediatric practice; paediatric reference values and expertise apply.

Change from baseline matters too

An absolute QTc value is only part of the picture. The 2010 American Heart Association scientific statement on preventing torsade de pointes in hospital settings highlights two warning signs: a QTc over 500 ms, and an increase of more than 60 ms from the person's own baseline. A rise from 410 to 480 ms after a new medicine, for example, is more concerning than a stable 470 ms that has been present for years. This is why clinicians often compare a new ECG with an old one, and why it helps to measure QT the same way, in the same lead and with the same correction formula each time.

Reading QTc on your ECG report

An automated ECG report typically lists several intervals:

If the report shows QT and heart rate, you can enter them in the calculator above to see how the result changes with Fridericia or Framingham. If your heart rate was fast when the ECG was taken – for example from anxiety, fever or caffeine – a "borderline long" Bazett QTc may look normal with Fridericia.

A little history

Both Bazett and Fridericia published their corrections in 1920, using small numbers of healthy people and hand-measured ECGs. Bazett's square-root formula became the default because it was simple to calculate before computers. Fridericia's cube-root correction was largely overlooked in English-language practice until an English translation of his paper was published in 2003. The Framingham formula, published by Sagie and colleagues in 1992, was derived from over 5,000 participants in the Framingham Heart Study and showed that a linear correction fitted the data better than Bazett. Today, software can apply any of these instantly, which is why calculators like this one show all four side by side.

What prolongs the QT interval?

Combinations are especially important: a QT-prolonging medicine plus low potassium and a slow heart rate is a classic setting for torsades de pointes. Our BMP blood test page explains potassium, and the magnesium converter covers magnesium units. This page does not give medication advice – if you take a medicine known to affect the QT interval, your prescriber or pharmacist can tell you whether ECG monitoring is needed.

What shortens the QT interval?

Short QT is less common. Causes include high calcium, high potassium, digoxin effect, fast heart rates (before correction), and the rare inherited short-QT syndrome. Isolated values just under 390 ms in healthy people with normal ECGs are usually not significant.

Symptoms that matter

A long QT itself does not cause symptoms. The concern is the arrhythmia it can trigger. Seek urgent medical care for:

• Fainting or near-fainting, particularly during exercise, emotion, being startled (such as an alarm clock) or during sleep.
• Palpitations with dizziness.
• Seizure-like episodes without a known cause.
• A family history of unexplained sudden death in young relatives, or of drowning in a strong swimmer.

QTc in drug safety and research

Regulatory guidance for new medicines requires a "thorough QT" assessment to detect even small increases in QTc. These studies commonly report QTcF (Fridericia) or study-specific corrections, because Bazett can produce artificial changes simply from changes in heart rate. If you see "QTcF" in a report, it means Fridericia was used; "QTcB" means Bazett.