QTc corrects the QT interval for heart rate. Bazett: QT ÷ √RR. Fridericia: QT ÷ ∛RR. Framingham: QT + 154 × (1 − RR). Hodges: QT + 1.75 × (HR − 60), with QT in ms and RR in seconds. A QTc of 450 ms or more in men or 460 ms or more in women is prolonged, and over 500 ms is highly abnormal.
This QTc calculator runs all four formulas side by side, so you can see how much they disagree at your patient's heart rate. Fridericia is shown as the headline result because it corrects rate better than Bazett in comparative studies, but every value is labelled against the AHA/ACCF/HRS limits.
How to use this calculator
- Measure the QT interval by hand, from the start of the QRS complex to the end of the T wave, in the lead with the longest well-defined QT (often lead II or V5).
- Enter the heart rate, or switch to RR interval and enter the time between two R waves in milliseconds.
- Choose sex, which sets the prolonged limit (450 ms for men, 460 ms for women).
- Compare the four QTc values. If they disagree across a threshold, the heart rate is probably far from 60 bpm.
The four QTc formulas
| Formula | Equation (QT in ms, RR in s) | Original source |
|---|---|---|
| Bazett | QT ÷ √RR | Bazett, Heart 1920 |
| Fridericia | QT ÷ ∛RR | Fridericia, Acta Med Scand 1920 |
| Framingham | QT + 154 × (1 − RR) | Sagie et al., Am J Cardiol 1992 |
| Hodges | QT + 1.75 × (HR − 60) | Hodges et al., J Am Coll Cardiol 1983 |
The equations are written as in Vandenberk and colleagues' 2016 comparison in the Journal of the American Heart Association, converted to milliseconds. Hodges was originally written with heart rate; the other three use the RR interval. RR in seconds is 60 divided by heart rate.
Bazett and Fridericia divide by a power of RR, so they stretch QT a lot when the heart is fast. Framingham and Hodges add or subtract a straight-line amount, which behaves more evenly across rates.
Normal and prolonged QTc limits
| QTc | Men | Women | Source |
|---|---|---|---|
| Short | ≤ 390 ms | ≤ 390 ms | AHA/ACCF/HRS 2009 |
| Within normal limits | 391–449 ms | 391–459 ms | AHA/ACCF/HRS 2009 |
| Prolonged | ≥ 450 ms | ≥ 460 ms | AHA/ACCF/HRS 2009 |
| Approximate 99th percentile, healthy post-pubertal | 470 ms | 480 ms | AHA/ACCF 2010 |
| Highly abnormal | > 500 ms | > 500 ms | AHA/ACCF 2010 |
The 2010 AHA/ACCF statement on torsade de pointes in hospital settings says a QTc above the 99th percentile should be considered abnormally prolonged, puts that percentile at about 470 ms for males and 480 ms for females, and adds that a QTc over 500 ms is associated with a 2- to 3-fold higher risk of torsade de pointes in congenital long-QT data. It also treats a rise of 60 ms or more from a pre-drug baseline as a warning sign.
Worked example
A QT of 360 ms at a heart rate of 100 bpm. The RR interval is 60 ÷ 100 = 0.6 s.
- Bazett: 360 ÷ √0.6 = 465 ms — prolonged for a man.
- Fridericia: 360 ÷ ∛0.6 = 427 ms — within normal limits.
- Framingham: 360 + 154 × (1 − 0.6) = 422 ms.
- Hodges: 360 + 1.75 × (100 − 60) = 430 ms.
Bazett labels this tracing prolonged, while the other three put it in the normal range. At fast heart rates that disagreement is common, and it is the main reason many cardiologists prefer Fridericia or a linear formula when the heart rate is above 100 bpm.
How the formulas compare across heart rates
Each row is a QT that is fairly typical for that heart rate, corrected by each formula (ms).
| QT (ms) | HR (bpm) | Bazett | Fridericia | Framingham | Hodges |
|---|---|---|---|---|---|
| 440 | 50 | 402 | 414 | 409 | 423 |
| 400 | 60 | 400 | 400 | 400 | 400 |
| 380 | 75 | 425 | 409 | 411 | 406 |
| 360 | 90 | 441 | 412 | 411 | 413 |
| 340 | 110 | 460 | 416 | 410 | 428 |
| 320 | 130 | 471 | 414 | 403 | 443 |
At 60 bpm every formula gives the measured QT. Below 60, Bazett gives the lowest value; above 60 it gives the highest, and the gap widens as the rate climbs.
Measuring QT accurately
- Pick the right lead. Use the lead with the longest clear QT; lead II and V5 are common choices.
- Find the end of the T wave. The tangent method draws a line down the steepest slope of the T wave's downstroke to the baseline.
- Leave out U waves. A separate U wave after the T wave should not be counted.
- Average several beats. Especially with sinus arrhythmia or atrial fibrillation.
- Check the machine. Automated QT and QTc are a starting point, not a final answer.
Common mistakes
- Mixing units. The formulas need RR in seconds. Entering 800 instead of 0.8 gives nonsense.
- Trusting Bazett at high heart rates. It can label a normal QT as prolonged above about 100 bpm.
- Using the same limit for everyone. Women's prolonged limit is 10 ms higher.
- Ignoring a wide QRS. Bundle branch block lengthens QT; consider the JT interval.
- Comparing QTc values from different formulas. Use the same formula for serial ECGs on the same patient.
More clinical tools: ABPI calculator, anion gap, corrected sodium and Centor score.
For healthcare professionals and students only. A QTc value supports ECG interpretation; it is not a diagnosis, and this tool gives no advice on medicines. Fainting, palpitations or a seizure with a long QT needs urgent assessment. Patients should see their own clinician.
Frequently asked questions
What is the QTc formula?
QTc is the QT interval corrected for heart rate. Bazett: QTc = QT ÷ √RR. Fridericia: QTc = QT ÷ ∛RR. Framingham (Sagie 1992): QTc = QT + 154 × (1 − RR). Hodges: QTc = QT + 1.75 × (heart rate − 60). QT and QTc are in milliseconds and RR is in seconds (RR = 60 ÷ heart rate).
What is a normal QTc?
The 2009 AHA/ACCF/HRS ECG standardization statement treats a QTc of 450 ms or more in men and 460 ms or more in women as prolonged, and 390 ms or less as short. Values between those limits are within normal limits.
What QTc is dangerous?
The 2010 AHA/ACCF statement on preventing torsade de pointes says a QTc over 500 ms is considered highly abnormal in both sexes, and that data from congenital long-QT studies associate it with a 2- to 3-fold higher risk of torsade de pointes. The same statement flags an increase of 60 ms or more from the pre-drug baseline.
Which QTc formula is best?
Bazett is the most widely printed on ECG machines but over-corrects at fast heart rates and under-corrects at slow ones. In Vandenberk and colleagues' 2016 study, Fridericia and Framingham gave the best rate correction and Bazett performed worst; the authors suggested Fridericia could replace Bazett for hospital QT monitoring. The 2009 AHA/ACCF/HRS statement also recommends a linear regression correction rather than Bazett.
Why does my ECG machine show a different QTc?
Most machines use Bazett and their own automated QT measurement. Automated QT can differ from a careful manual reading, especially with flat T waves or U waves. Measure QT yourself in the lead with the longest clear QT and recalculate.
How do I convert heart rate to RR interval?
RR (seconds) = 60 ÷ heart rate in beats per minute. At 60 bpm, RR is 1.0 s and every formula returns the measured QT unchanged. At 100 bpm, RR is 0.6 s.
Can I calculate QTc in atrial fibrillation?
With caution. In AF the RR interval changes from beat to beat. A Polish commentary on the 2009 recommendations notes the experts advise against rate correction when QT varies widely, as in atrial fibrillation. A common approach is to average QT and RR over several beats, but the result is less reliable.
Does a wide QRS change the QTc?
Yes. A wide QRS (for example a bundle branch block) lengthens QT without a repolarisation problem. Many clinicians look at the JT interval (QT minus QRS) instead. This calculator does not adjust for QRS width.
What heart rate range are the formulas valid for?
All four formulas work best near 60 bpm, where corrections are small. They diverge most below 50 and above 100 bpm, and the calculator warns you when the rate is outside 50–100 bpm.
Is QTc used to dose medicines?
QTc is used to monitor the safety of QT-prolonging medicines, but this calculator gives no dosing advice. Decisions about starting, stopping or adjusting any medicine belong to the treating clinician and pharmacist.
Sources & method
- Rautaharju PM, Surawicz B, Gettes LS (2009), Circulation 119:e241–e250 — AHA/ACCF/HRS recommendations for the standardization and interpretation of the ECG, Part IV: ST segment, T and U waves, and the QT interval
- Drew BJ, Ackerman MJ, Funk M et al. (2010), Circulation — Prevention of torsade de pointes in hospital settings: an AHA/ACCF scientific statement (QTc > 500 ms, 99th percentiles, +60 ms)
- Vandenberk B, Vandael E, Robyns T et al. (2016), Journal of the American Heart Association — Which QT correction formulae to use for QT monitoring? (formula definitions and comparison)
- Sagie A, Larson MG, Goldberg RJ et al. (1992), American Journal of Cardiology 70:797–801 — An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)
- Stanford Rogers Lab EP Tools — QTc calculator (formula set and Rautaharju 2009 reference ranges)
- Kardiologia Polska commentary on the AHA/ACCF/HRS 2009 Part IV recommendations (linear correction, ≥450/≥460 ms)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.