What is the normal QT (QT interval) interval in milliseconds and number of small boxes on an electrocardiogram (ECG) for males and females?

Normal QT Interval in Males and Females

The normal QT interval is ≤450 ms for men and ≤460 ms for women, with values ≤390 ms considered abnormally short for both genders. 1

Normal QT Interval Values

In Adults:

• Males: ≤450 ms (normal), 451-480 ms (borderline prolonged), >480 ms (prolonged)
• Females: ≤460 ms (normal), 461-480 ms (borderline prolonged), >480 ms (prolonged)
• Both genders: ≤390 ms is considered abnormally short 1
• High-risk prolongation: >500 ms for both genders 1

In Children:

• Under 12 years: Approximately 450 ms 1
• 12-16 years: Gender differences begin to emerge with an 8 ms difference between males and females 2

QT Interval in Small Boxes on ECG

On a standard ECG (paper speed 25 mm/s, 1 mm = 0.04 seconds):

• Normal QT interval for men (≤450 ms) equals ≤11.25 small boxes
• Normal QT interval for women (≤460 ms) equals ≤11.5 small boxes

Age and Gender Considerations

• QT interval shortens in males during adolescence due to testosterone effects 2, 1
• The gender difference is approximately 6-10 ms in older age groups and 12-15 ms in younger adults 2
• Gender differences become small after 40 years of age and practically disappear in older men and women 2, 1

Important Clinical Considerations

QT Correction Formulas

• Linear regression formulas are recommended over Bazett's formula for QT correction 2, 1
• Bazett's formula can produce artificially prolonged QTc values, especially at heart rates >85 bpm 1
• Fridericia's formula (QTcF = QT/∛RR) performs better than Bazett's at heart rates outside the normal range 1

Measurement Technique

• Measure QT interval in leads with T-wave amplitude of at least 2 mm (typically leads II, V3, or V5) 1
• Measure from beginning of QRS complex to end of T wave 1
• Avoid leads with U waves for accurate assessment 1

Pitfalls to Avoid

1. Inaccurate heart rate correction: Do not attempt rate correction when RR interval variability is large (e.g., atrial fibrillation) 2, 1
2. Using Bazett's formula at extreme heart rates: Can produce false QT prolongations at heart rates >100 bpm or <60 bpm 1
3. First-time ECGs after syncope: Must be interpreted with caution, particularly with borderline QT intervals 3
4. Ventricular conduction defects: QT interval naturally prolongs and requires adjustment 2, 1

Remember that approximately one-third of pediatric patients in emergency department settings may exhibit QTc values ≥440 ms, with significant normalization on follow-up evaluations 3. Therefore, a single borderline measurement should not be overinterpreted without follow-up.