The QT interval is measured from the very beginning of the QRS complex to the end of the T wave on an ECG tracing. It represents the total time your heart’s ventricles take to electrically activate and then reset, and getting an accurate measurement matters because a prolonged QT interval raises the risk of dangerous heart rhythms. The process involves picking the right ECG lead, identifying two landmarks, reading the distance on the paper, and then correcting for heart rate.
The Two Landmarks You Need to Find
Every QT measurement comes down to two points on the ECG strip. The starting point is the onset of the QRS complex, the first deflection away from the flat baseline that signals the ventricles are firing. This is usually easy to spot because the QRS complex is the tallest, sharpest waveform on the tracing.
The ending point is where things get tricky. You need to identify where the T wave finishes and the baseline resumes. Two established methods exist for pinpointing this:
- Tangent method: Draw an imaginary line along the steepest part of the T wave’s downslope. Where that line crosses the baseline is your endpoint. This is the most widely recommended technique because it’s reproducible and less affected by low-amplitude T wave tails that drag out the measurement.
- Threshold method: Simply follow the T wave until it physically returns to the baseline. This is more intuitive but can overestimate the interval when the T wave has a long, gradual tail.
In both methods, the “baseline” is defined as the voltage level right at the onset of the QRS complex, not some other flat segment of the tracing.
Which ECG Lead to Use
Not every lead on a 12-lead ECG shows the T wave equally well. Lead II and leads V5 or V6 typically produce the tallest, most clearly defined T waves, making the endpoint easier to identify. If you’re doing a quick manual check, start with lead II. If the T wave is flat or hard to read there, switch to V5 or V6. Whichever lead gives you the most distinct T wave is the one to measure from.
Some clinicians measure the QT in multiple leads and use the longest value, since a short or absent T wave in one lead can cause you to underestimate the true interval.
Reading the ECG Paper
Standard ECG paper runs at 25 millimeters per second. Each small square on the grid is 1 mm wide, which equals 0.04 seconds (40 milliseconds). Each large square contains five small squares and equals 0.20 seconds (200 milliseconds). To get your QT interval, count the number of small squares between your two landmarks and multiply by 40 milliseconds. For example, 10 small squares equals a QT of 400 ms.
If you prefer to measure in millimeters with a ruler, simply divide the distance by 25 to convert to seconds, then multiply by 1,000 for milliseconds.
Dealing With U Waves
U waves are small, low-amplitude bumps that sometimes appear after the T wave. They can make it hard to tell where the T wave truly ends. The international guideline from ICH E14, used in drug safety evaluations worldwide, is clear: discrete U waves should be excluded from the QT measurement. If a U wave interrupts the T wave before it returns to baseline, measure to the lowest point (the nadir) between the T wave and the U wave, not to the end of the U wave. Including U waves inflates the measurement and can lead to a false diagnosis of QT prolongation.
Correcting for Heart Rate
The raw QT interval naturally shortens when the heart beats faster and lengthens when it beats slower. To compare measurements across different heart rates, you correct the QT to produce a value called the QTc. This requires one more measurement from the ECG: the RR interval, which is the distance between two consecutive R waves (the tall peaks of the QRS complex). Measure it the same way you measured the QT, in seconds.
Bazett’s Formula
The most commonly used correction is Bazett’s formula: QTc equals the QT interval divided by the square root of the RR interval (both in seconds). It’s the default on most automated ECG machines, but it has a well-documented flaw. Bazett overcorrects at fast heart rates and undercorrects at slow ones. In one study, it had only 54% sensitivity for detecting true QT prolongation, compared to 100% for the Fridericia formula. It also overestimates the number of patients with dangerous prolongation, which can lead to unnecessary medication changes.
Fridericia’s Formula
Fridericia’s formula divides the QT interval by the cube root of the RR interval instead of the square root. This single change makes it significantly more accurate across a wider range of heart rates. Research in the Journal of the American Heart Association found that Fridericia was a better predictor of 30-day mortality than Bazett, with a hazard ratio of 5.95 compared to 4.49. Many guidelines now recommend Fridericia over Bazett, especially when heart rate is above 100 or below 60 beats per minute.
Framingham Formula
The Framingham formula takes a different mathematical approach: QTc equals QT plus 0.154 multiplied by (1 minus the RR interval). It performed as well as or slightly better than Fridericia in predicting mortality (hazard ratio of 7.31) and is considered one of the most reliable options. It’s less commonly built into ECG machines but worth knowing about if you’re doing calculations yourself.
A Practical Step-by-Step Walkthrough
Here’s the full process from start to finish:
- Select your lead. Use lead II first. If the T wave is flat or ambiguous, try V5 or V6.
- Mark the QRS onset. Find the very first deflection of the QRS complex away from the baseline.
- Mark the T wave end. Use the tangent method: visually trace a line along the steepest part of the T wave’s downslope and note where it hits the baseline. Exclude any U wave.
- Count the small squares between those two marks. Multiply by 40 to get the QT in milliseconds.
- Measure the RR interval. Count the small squares between two consecutive R wave peaks. Multiply by 40 for milliseconds, then divide by 1,000 to convert to seconds.
- Apply a correction formula. Divide the QT (in seconds) by the cube root of the RR interval for Fridericia, or by the square root for Bazett.
A normal QTc is generally considered to be under 450 ms for men and under 460 ms for women. Values above 500 ms are associated with a meaningfully higher risk of a dangerous arrhythmia called torsades de pointes.
Why Automated Readings Can Be Wrong
ECG machines print a computer-generated QTc on every tracing, but these automated readings are frequently inaccurate. The algorithm can misidentify the end of the T wave, especially when T waves are flat, notched, or followed by U waves. Most machines also default to Bazett’s formula, which compounds the problem at extreme heart rates. Studies consistently show Bazett to be unreliable compared to Fridericia and Framingham in these situations. If the clinical stakes are high, such as when a patient is on a medication known to prolong the QT, a manual measurement using the tangent method and a more accurate correction formula is worth the extra minute.