Corrected QT Interval Calculator: How to Use It

A corrected QT interval calculator adjusts the raw QT interval from an ECG for heart rate, giving a rate-independent value (QTc) that helps assess arrhythmia risk. You enter the measured QT interval in milliseconds and the heart rate in beats per minute (or the preceding RR interval), and the calculator returns the corrected QT in milliseconds using a standard formula like Bazett’s, Fridericia’s, or Framingham’s.

Formula

The QT interval on an ECG represents the time from the start of ventricular depolarization (Q wave) to the end of repolarization (T wave). Because the QT interval naturally shortens as heart rate increases and lengthens as it decreases, a raw measurement is hard to interpret without correction. The most common correction formulas are:

• Bazett’s formula: QTc = QT / √(RR), where RR is the interval between two consecutive R waves in seconds. In terms of heart rate (HR in bpm), RR = 60 / HR, so QTc = QT / √(60 / HR). This is the most widely used in clinical practice, though it tends to overcorrect at high heart rates and undercorrect at low rates.
• Fridericia’s formula: QTc = QT / ∛(RR) = QT / ∛(60 / HR). It uses the cube root, which is more accurate at higher heart rates.
• Framingham formula: QTc = QT + 0.154 × (1 − RR) × 1000 (with RR in seconds). This linear regression formula is derived from a large population and is often preferred for rates outside the normal range.

All formulas take the QT interval in milliseconds and the heart rate (or RR interval) and output a corrected QT in milliseconds. The calculator above shows all three results so you can compare them.

Worked Example

Using the default values from the calculator: QT = 400 ms and heart rate = 70 bpm.

1. Bazett: RR = 60 / 70 = 0.857 s. QTc = 400 / √(0.857) = 400 / 0.926 = 432 ms.
2. Fridericia: QTc = 400 / ∛(0.857) = 400 / 0.950 = 421 ms.
3. Framingham: QTc = 400 + 0.154 × (1 − 0.857) × 1000 = 400 + 0.154 × 0.143 × 1000 = 400 + 22 = 422 ms.

The calculator rounds to the nearest millisecond, giving 432, 421, and 422 ms respectively. Note that the three formulas agree closely at a normal heart rate of 70 bpm, but they can diverge by 20–30 ms at extreme heart rates.

When to Use a Corrected QT Calculator

Clinicians use QTc to screen for long QT syndrome, a condition that predisposes to torsades de pointes and sudden cardiac death. You would use this calculator when you have a 12-lead ECG printout and need to interpret the QT interval in the context of the patient’s heart rate. It’s also relevant when monitoring patients on medications known to prolong the QT interval, such as certain antiarrhythmics, antipsychotics, and antibiotics. However, the calculator is a mathematical tool only—it does not diagnose or interpret the ECG tracing. A physician must verify that the QT interval is measured correctly (e.g., using lead II or V5, with a clear T wave end) and that the heart rate is stable.

How to Read the Result

The QTc value is compared to established thresholds. Generally, a QTc above 450 ms in men and 460 ms in women is considered prolonged, though some guidelines use 440 ms and 460 ms respectively. Values above 500 ms are associated with a significantly higher risk of torsades de pointes. But these are population averages; a single prolonged QTc does not confirm a diagnosis. The result should be interpreted alongside the patient’s symptoms, family history, electrolyte levels (potassium, calcium, magnesium), and any medications. Also note that the correction formula itself can introduce error—Bazett’s formula, for example, overcorrects at heart rates above 100 bpm, so a high heart rate might falsely elevate QTc.

Common Mistakes and Limitations

• Using the wrong unit: The QT interval must be in milliseconds (ms), not seconds. If you enter 0.4 instead of 400, the result will be off by a factor of 1000.
• Measuring QT incorrectly: The QT interval should be measured from the onset of the QRS complex to the end of the T wave, where it returns to the isoelectric baseline. If the T wave is not distinct (e.g., in bundle branch block or with U waves), the measurement is unreliable.
• Ignoring heart rate variability: The correction formulas assume a stable heart rate. If the patient has atrial fibrillation or frequent ectopy, the QT interval varies beat-to-beat, and a single measurement may be misleading.
• Choosing the wrong formula: Bazett is standard, but at heart rates above 100 bpm or below 60 bpm, Fridericia or Framingham is more accurate. Many modern ECG machines report both.
• Rounding too early: Do not round the RR interval before the calculation; keep full precision until the final step.

Alternatives and Related Calculators

If you need to calculate the QT interval from an ECG tracing, you might also want a QT dispersion calculator, which measures the difference between the longest and shortest QT intervals across leads. For pediatric patients, the QTc for children uses the same formulas but with different normal ranges. If you have the RR interval directly instead of heart rate, many calculators accept that input; the formulas above can be rearranged to use RR in seconds directly. Some clinical guidelines recommend using a nomogram (a graphical chart) instead of a formula, but the calculator here gives the same numeric results.

Accuracy and Safety Notes

This calculator is for educational and reference purposes only. It is not a medical device and should never replace professional ECG interpretation. The QTc value is only one piece of the arrhythmia risk assessment. A normal QTc does not guarantee safety, and a prolonged QTc does not always lead to arrhythmia. Always consult a qualified healthcare provider for any clinical decision. If you are measuring the QT interval yourself, ensure the ECG paper speed is 25 mm/s and the calibration is correct (10 mm/mV).

Frequently Asked Questions

What is the normal range for QTc?

For adults, a QTc below 450 ms in men and below 460 ms in women is generally considered normal. Some sources use 440 ms as the upper limit for both. Values above these thresholds are considered prolonged, and values above 500 ms are associated with a high risk of torsades de pointes. However, these are population-based cutoffs; individual variation exists.

Why do different formulas give different QTc values?

Each formula was derived from different populations and uses a different mathematical adjustment for heart rate. Bazett uses a square root, Fridericia a cube root, and Framingham a linear correction. At normal heart rates (60–80 bpm) they agree closely, but at extremes they can differ by 20–30 ms. Clinicians often report the formula used and may choose one based on the patient’s heart rate.

Can I use this calculator if I have the RR interval instead of heart rate?

Yes. If you have the RR interval in seconds, you can convert it to heart rate by HR = 60 / RR. For example, an RR of 0.8 s corresponds to 75 bpm. Then enter that heart rate into the calculator. Alternatively, you can substitute RR directly into the formulas: Bazett QTc = QT / √(RR), Fridericia QTc = QT / ∛(RR), and Framingham QTc = QT + 0.154 × (1 − RR) × 1000.

What is the difference between QT and QTc?

QT is the raw interval measured from the ECG, which varies with heart rate. QTc is the corrected value, adjusted to what the QT would be at a heart rate of 60 bpm. This allows comparison across different heart rates and helps identify true abnormalities. A raw QT of 400 ms at 60 bpm is normal, but the same raw QT at 100 bpm is abnormally long.

Does this calculator work for children?

The formulas are the same, but the normal ranges differ. In children, the QTc tends to be slightly longer than in adults, and the upper limit of normal is often considered 460 ms. The same formulas (Bazett, Fridericia, Framingham) are used, but interpretation should be based on age-specific reference values.

Why is Bazett’s formula so commonly used if it’s less accurate at high heart rates?

Bazett’s formula was published in 1920 and is simple to compute manually, which made it the default for decades. It is still used because most clinical guidelines and drug-safety studies reference it. However, many modern ECG machines and textbooks recommend using Fridericia’s formula when the heart rate is above 100 bpm or below 60 bpm, as it is more accurate in those ranges.

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in qt | QT interval | ms | 400 | 0
in hr | Heart rate | bpm | 70 | 0
out qtc_b | QTc (Bazett) | ms | qt / sqrt(60 / hr) | 0
out qtc_f | QTc (Fridericia) | ms | qt / cbrt(60 / hr) | 0
out qtc_fram | QTc (Framingham) | ms | qt + 0.154 * (1 - 60 / hr) * 1000 | 0