QTc Calculator (ECG)
Calculate corrected QT (QTc) from a measured QT interval and either heart rate or RR interval.
Educational tool only. ECG interpretation should always be confirmed by a qualified clinician.
What does “calculating QT” usually mean?
In clinical ECG work, “calculating QT” typically refers to estimating the corrected QT interval (QTc) rather than using the raw QT value alone. The QT interval changes with heart rate, so a QT of 420 ms may be normal at one rate and concerning at another. QT correction formulas adjust for this and help with screening for repolarization abnormalities, medication effects, and arrhythmia risk.
Why QT correction matters
A prolonged QTc can increase the risk of dangerous ventricular rhythms such as torsades de pointes. A shortened QTc can also be clinically meaningful in specific settings. Because of this, QT assessment is common when:
- Reviewing ECGs before or after starting QT-prolonging medications
- Monitoring electrolyte disturbances (low potassium, magnesium, or calcium)
- Evaluating syncope, palpitations, or family history of sudden cardiac death
- Following patients with congenital long QT syndromes
Common QTc formulas
Formulas used by the calculator
- Bazett: QTc = QT / √RR
- Fridericia: QTc = QT / RR^(1/3)
- Framingham: QTc = QT + 154 × (1 − RR)
- Hodges: QTc = QT + 1.75 × (HR − 60)
Units: QT in milliseconds, RR in seconds, HR in beats per minute.
No single formula is perfect for every rhythm or heart rate. Bazett is widely used but can over-correct at high rates and under-correct at low rates. Seeing multiple formulas side-by-side often gives a better quick picture.
How to use this calculator
Step 1: Enter measured QT
Type the QT interval from the ECG in milliseconds.
Step 2: Enter HR or RR
Enter either heart rate (bpm) or RR interval (ms). If both are entered, RR is used directly because it is closer to the actual beat-to-beat measurement.
Step 3: Review QTc and interpretation
The calculator returns four QTc estimates and a quick interpretation band (normal, borderline, prolonged, or short) based on your selected sex category.
Practical interpretation ranges (quick guide)
- Male: often considered prolonged if QTc > 450 ms
- Female: often considered prolonged if QTc > 470 ms
- Unspecified mixed threshold: many quick screens use > 460 ms
These are practical screening cutoffs, not absolute diagnostic boundaries. Clinical context, ECG quality, and rhythm type matter.
Common pitfalls when calculating QT
- Using a poor-quality tracing with uncertain T-wave end
- Not accounting for irregular rhythm (e.g., atrial fibrillation)
- Relying on one formula without considering rate extremes
- Ignoring medication interactions or electrolyte abnormalities
- Assuming calculator output replaces physician interpretation
Bottom line
Calculating QT is really about estimating QTc accurately and interpreting it carefully. Use a consistent method, verify measurements, and always pair the number with clinical judgment. A calculator is useful for speed and consistency, but final decisions should be made by trained professionals using the full clinical picture.