How do I manually calculate the QT interval and corrected QT (QTc) on a 12‑lead ECG?

How to Manually Calculate the QT Interval and Corrected QT (QTc)

Measure the QT interval from the beginning of the QRS complex to the end of the T wave in a lead with a T-wave amplitude ≥2 mm and a well-defined T-wave end, then apply Fridericia's formula (QTc = QT/∛RR) rather than Bazett's formula to correct for heart rate. 1

Step 1: Select the Appropriate Lead

• Choose a lead from the 12-lead ECG where the T-wave amplitude is at least 2 mm and the T-wave end is clearly visible 1
• Lead II is conventionally preferred because U waves are less prominent and the T-wave end corresponds to the end of significant repolarization 2
• Alternatively, leads V3 or V5 may be used if they show clearer T-wave morphology 3
• Document which lead you use and measure the same lead consistently for all serial measurements in that patient 1

Step 2: Identify the QT Interval Boundaries

Beginning of QT (QRS onset)

• Mark the earliest deflection from the isoelectric baseline that represents the start of ventricular depolarization 1
• This point is usually readily apparent on the ECG 1

End of QT (T-wave offset)

• Identify where the T wave returns to the isoelectric baseline 1
• When the T-wave end is difficult to determine (biphasic, notched, or low-amplitude T waves), draw a tangent line from the peak of the T wave following the steepest downslope; the intersection of this line with the isoelectric baseline marks the T-wave end 1
• Exclude discrete U waves that occur after the T wave returns to baseline 4
• When T waves have superimposed U waves, use the tangent method described above 1

Step 3: Measure the QT Interval

• Measure from the beginning of the QRS complex to the end of the T wave 1
• Express the measurement in milliseconds (ms) or seconds (s) 1
• Enhance visualization by increasing waveform size (standardization from 1 to 2,3, or 4) and recording speed (from 25 to 50 mm/s) when using electronic calipers 1

Step 4: Measure the Preceding RR Interval

• Measure the RR interval immediately preceding the QT interval you just measured 1
• Express in seconds for use in correction formulas 5

Step 5: Calculate the Corrected QT (QTc)

Recommended Formula: Fridericia's Cube-Root Formula

• QTc = QT / ∛RR (where RR is in seconds) 3, 5
• Fridericia's formula is preferred over Bazett's, especially at heart rates >80 bpm, because it provides more accurate rate correction 3, 6

Alternative Quick Approximation

• For each 10 bpm increase in heart rate above 60 bpm, add 5% to the uncorrected QT 3
• For each 10 bpm decrease in heart rate below 60 bpm, subtract 5% from the uncorrected QT 3

Why Not Bazett's Formula?

• Bazett's formula (QTc = QT/√RR) systematically overcorrects at heart rates >85 bpm and undercorrects at low heart rates 3, 5, 6, 7
• This can lead to falsely elevated QTc values and unnecessary interventions 3

Special Situations

Bundle Branch Block or Wide QRS (≥120 ms)

• The prolonged QRS artificially increases the total QT interval 1
• Method 1: Subtract the difference in QRS width before and after the block from the measured QT 1
• Method 2: Measure the JT interval (from the end of QRS to the end of T wave) instead, which eliminates the QRS contribution entirely 1
• Apply whichever method you choose consistently for serial measurements 1

Atrial Fibrillation or Irregular Rhythms

• QT varies beat-to-beat with varying RR intervals, making single-beat measurement unreliable 1
• Method 1: Identify the shortest and longest RR intervals, calculate QTc for each, then average the two values 1
• Method 2: Print a long rhythm strip and determine whether the interval from R wave to T-wave peak (or nadir) exceeds 50% of the average RR interval; if so, QTc likely exceeds 500 ms 1

Critical Pitfalls to Avoid

• Do not rely on automated ECG measurements without manual verification, as computer algorithms vary substantially between manufacturers and may mislabel QTc >440 ms as "borderline" when this value is exceeded by 10-20% of the population 1, 3
• Do not switch between different ECG machines for serial measurements, as proprietary algorithms produce different values even from identical cardiac activity 1, 8
• Do not measure QT in the presence of new bundle branch block without adjusting for QRS duration 1, 3
• Do not use Bazett's formula at heart rates >85 bpm, as it produces falsely elevated QTc values 3, 6

Normal Values and Thresholds

• Normal upper limits: <450 ms for males, <460 ms for females 1, 3
• 99th percentile thresholds: 470 ms for males, 480 ms for females 1, 3
• High-risk threshold: QTc >500 ms or an increase >60 ms from baseline significantly increases torsades de pointes risk 1, 3