QTc calculator
Enter QT and heart rate, get the corrected QT.
Why correct the QT interval?
The QT interval gets shorter as the heart beats faster, so it is adjusted to what it would be at 60 beats a minute (RR = 1 second) before it is compared with normal limits.
Which formulas does the calculator use?
Bazett QT ÷ √RR, Fridericia QT ÷ ∛RR, Framingham QT + 154 × (1 − RR) and Hodges QT + 1.75 × (heart rate − 60), with QT in milliseconds and RR = 60 ÷ heart rate in seconds.
Example
QT 400 ms at 75 beats a minute (RR 0.8 s): Bazett 447 ms, Fridericia 431 ms, Framingham 431 ms, Hodges 426 ms. Bazett gives higher values at fast heart rates.
What QTc is prolonged?
The 2009 AHA/ACCF/HRS recommendations consider a QTc of 450 ms or more in men and 460 ms or more in women prolonged. A 2010 AHA/ACCF statement calls a QTc above 500 ms highly abnormal in both sexes.
Can I use this to read my own ECG?
No — measuring QT takes care, and results depend on the method. Discuss your ECG with your doctor.
Why do the formulas give different answers?
Each one makes a different assumption about how QT shortens as the heart rate rises. At 60 beats a minute they all agree (QT 400 ms gives 400 ms in every formula). The further the rate is from 60, the further apart they drift: for QT 340 ms at 110 beats a minute the answers range from 410 to 460 ms.
How each correction works
RR is the time between two heartbeats in seconds, 60 divided by the heart rate. The Bazett and Fridericia formulas divide QT by a root of RR (the square root and the cube root). The Framingham and Hodges formulas are linear: they add a fixed amount for each unit by which the heart rate or RR differs from 60 beats a minute (RR of 1 second). All four return the QT you would expect at 60 beats a minute, in milliseconds, so every formula leaves a QT measured at exactly 60 beats a minute unchanged.
Worked examples at slow and fast rates
A QT of 440 ms at 50 beats a minute (RR 1.2 s) corrects to 402 ms by Bazett, 414 ms by Fridericia, 409 ms by Framingham and 423 ms by Hodges. A QT of 340 ms at 110 beats a minute (RR 0.55 s) corrects to 460, 416, 410 and 428 ms. Bazett gives the highest value at the fast rate, and published studies have found that it overestimates QT at heart rates of about 100 and above; no single replacement is agreed on.
Common mistakes
Entering QT in seconds instead of milliseconds is the first: 0.4 s is 400 ms. Using the heart rate of a different moment from the QT measurement is the second; both come from the same ECG complex or average. Comparing a result with a threshold from a different formula is the third, since the 450 and 460 ms limits quoted in the FAQ are for corrected values and different formulas can put the same ECG on either side of a limit. Finally, a calculator cannot replace a trained reading: QT measurement depends on where the end of the T wave is judged to be.
Sources: Rautaharju PM et al., AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part IV, Circulation 2009 (QTc limits and criticism of power-function corrections); the original formulas of Bazett (1920), Fridericia (1920), Sagie et al. (Framingham, 1992) and Hodges (1983). Related: mean arterial pressure calculator, target heart rate calculator, heart rate reserve calculator and anion gap calculator.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.