How many small squares on an electrocardiogram (ECG) does a normal QT (QT interval) interval typically span?

QT Interval Measurement on ECG

Direct Answer to Box Counting

A normal QT interval typically spans approximately 8-10 small squares (0.04 seconds each) on a standard ECG at normal heart rates, translating to roughly 320-400 milliseconds. However, this is highly dependent on heart rate, and the QT interval must be corrected (QTc) for accurate clinical interpretation 1, 2.

Understanding ECG Paper Calibration

• Standard ECG paper runs at 25 mm/second, where each small square (1 mm) represents 0.04 seconds (40 milliseconds) 3
• Each large square (5 mm) represents 0.20 seconds (200 milliseconds) 3
• At a heart rate of 60 beats per minute with an RR interval of 1.0 second, a normal uncorrected QT interval of 400 ms would span exactly 10 small squares 3

Rate-Dependent Variation in Box Count

The number of boxes the QT interval occupies varies significantly with heart rate, making raw box counting unreliable without rate correction:

• At faster heart rates (>90 bpm), the QT interval physiologically shortens, spanning fewer boxes 4, 5
• At slower heart rates (<60 bpm), the QT interval lengthens, spanning more boxes 4, 5
• This is why Bazett's formula (QTc = QT/√RR) or other correction formulas are essential for clinical interpretation 3, 1

Practical Measurement Approach

The American Heart Association and American College of Cardiology recommend measuring QT interval from the earliest onset of the QRS complex to the latest offset of the T wave, ideally using global measurements across multiple leads 3:

• Measure in leads II, V5, or V6, using the longest value obtained 3
• Use the tangent method: draw a line tangent to the steepest downslope of the T wave to identify its end point where it intersects the baseline 1, 6
• Avoid measuring in leads with prominent U waves; if unavoidable, use leads aVR or aVL where U waves are typically absent 3

Normal Values After Rate Correction

After correcting for heart rate, normal QTc values are:

• Men: ≤450 ms (approximately 11.25 small squares at 60 bpm) 3, 1, 2
• Women: ≤460 ms (approximately 11.5 small squares at 60 bpm) 3, 1, 2
• Children <12 years: approximately 450 ms regardless of gender 3, 2

Common Pitfalls to Avoid

Critical errors in QT measurement include:

• Counting boxes without correcting for heart rate, which renders the measurement clinically meaningless 1, 2
• Using Bazett's formula at heart rates <50 bpm or >90 bpm, where it significantly over- or under-corrects 4, 5
• Relying solely on automated computer measurements without visual validation, as algorithms vary and may be inaccurate 3, 1
• Measuring in different leads for serial comparisons in the same patient, which introduces variability 1, 2
• Failing to account for QRS prolongation in bundle branch blocks, which artificially lengthens the QT interval 3

Clinical Context

The raw box count is only a starting point; clinical interpretation requires:

• Rate correction using appropriate formulas (Fridericia is more accurate than Bazett at non-normal heart rates) 2, 5
• Gender-specific thresholds after puberty (8-10 ms difference emerges at ages 12-16) 3, 2
• Recognition that athletes may have longer uncorrected QT intervals due to bradycardia, but normal QTc values 3, 2