QTc Interval Calculator
Calculate the corrected QT interval (QTc) from the QT interval and heart rate using Bazett's formula, QTc = QT / √RR.
How to use this tool
- Enter the measured QT interval in milliseconds.
- Enter the heart rate in beats per minute.
- Select sex to apply the matching prolonged-QTc threshold.
- Read the corrected QT (QTc) and its interpretation.
The QTc interval calculator corrects a measured QT interval for heart rate using Bazett's formula, giving the rate-independent QTc that is screened for long-QT.
Formula
QTc = QT ÷ √RR, with RR = 60 ÷ heart rate (seconds)
This is Bazett's correction. QT and QTc are in milliseconds and RR is in seconds, so at a heart rate of 60 bpm RR = 1 s and QTc equals the raw QT.
How it works
The QT interval on an ECG shortens as heart rate rises, so a raw QT cannot be compared between people at different rates. Bazett's formula corrects QT to the value it would have at 60 bpm by dividing by the square root of the RR interval (the time between beats, in seconds). The result, QTc, is what clinicians screen for QT prolongation.
Bazett's correction is the most widely used but it over-corrects at fast heart rates and under-corrects at slow ones; alternatives such as Fridericia are preferred at extreme rates. Commonly cited upper limits are about 450 ms for men and 470 ms for women, with values above 500 ms carrying higher arrhythmia risk. Thresholds and measurement technique vary.
This calculator is provided for general information and education only and is not medical advice. Clinical formulas are screening and estimation tools, not diagnoses, and they assume valid, correctly-measured inputs. Always consult a qualified clinician before making any decision about your health.
Worked example
QT 400 ms at a heart rate of 60 bpm
- RR interval = 60 ÷ 60 = 1.0 second
- √RR = √1.0 = 1.0
- QTc = QT ÷ √RR = 400 ÷ 1.0 = 400 ms
- Round to two decimals = 400.00 ms
Corrected QT (QTc) = 400.00 ms (Within typical range)
QTc (Bazett) for a fixed QT of 400 ms across heart rates
| Heart rate (bpm) | RR (s) | QTc (ms) |
|---|---|---|
| 50 | 1.200 | 365.15 |
| 60 | 1.000 | 400.00 |
| 75 | 0.800 | 447.21 |
| 100 | 0.600 | 516.40 |
Key terms
- QT interval
- The time on the ECG from the start of the QRS complex to the end of the T wave, representing ventricular depolarisation and repolarisation.
- QTc
- The QT interval corrected for heart rate, allowing comparison across different rates.
- RR interval
- The time between two consecutive R waves (heartbeats), equal to 60 divided by the heart rate in beats per minute.
- Bazett's formula
- The heart-rate correction QTc = QT / √RR, the most commonly used QTc method.
Frequently asked questions
- What QTc is considered prolonged?
- Commonly cited upper limits are about 450 ms for men and 470 ms for women; a QTc above 500 ms is generally regarded as markedly prolonged and higher risk. Thresholds vary by source.
- Why correct the QT for heart rate at all?
- The raw QT naturally shortens as heart rate increases, so without correction you cannot tell whether a short QT reflects a fast heart rate or a genuinely short interval.
- Is Bazett's formula the best correction?
- It is the most common but not always the most accurate. At very fast or very slow heart rates, formulas such as Fridericia or Framingham track the true QTc more closely.