Fridericia Formula


A formula for calculating the QT interval on an electrocardiogram (ECG) according to heart rate. Using this formula, Fridericia postulated that QT values >0.43 seconds were considered prolonged

History of the Fridericia Formula

1891Augustus Desiré Waller provided a series of values for the duration of mechanical systole with different heart rates. He demonstrated that the period of mechanical contraction (systole) was shortened at faster heart rates

1920 – The Danish physician Louis Sigurd Fridericia (1881 – 1947) hypothesised a relationship between the duration of the ventricular electrocardiogram (s) and the duration of the pulse period (p). He studied 50 healthy individuals (age 3-81 years; 28 males and 32 females) at rest to determine the accuracy and the error rate in the measurement of both the pulse period (p) and the duration of the ventricular electrocardiogram (s).

He found that QT could be predicted accurately from the following formula: s = K (p0.3558) which he then simplified (p0.3558) to ³√p without any significant error; then calculated k (=8.22) for normal healthy individuals.

Fridericia 1

He concluded that in a normal, resting subject, the duration of electrical systole on an ECG is proportional to the cube root of the duration of the pulse period.


By applying this equation to his data set, Fridericia defined an average (Gauss) error of 0.015 seconds for a single duration. Variations of the observed duration of electrical systole >3 times the average error were considered to be pathological. Therefore the upper value for a ‘normal QT interval’ at a heart rate of 60 bpm was defined as 0.427 seconds (0.382 sec+0.045 s), and so QT values>0.43 seconds were deemed prolonged.

Associated Persons


Charbit B et al in a study of 108 patients found that automatic QT correction using Bazett formula had a sensitivity for detection of QT prolongation of 54% while automatic QT correction using Fridericia formula had 100% sensitivity


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Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM with a passion for rugby; medical history; medical education; and informatics. Asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | vocortex |

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