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Corrected QT interval (QTc — Bazett & Fridericia)

QT measured from QRS onset to end of the T wave (lead II or V5), U wave excluded. Accepts ms or s; QTc is returned in the same unit (ms).

Enter the heart rate in beats/min, or the R–R interval directly (unit ms or s). Internally converted to RR in seconds (RR = 60 / HR).

Independent clinical validation: pending

Two independent clinical validators will be named here once review is complete.

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How it is calculated

Convert the heart rate to the R–R interval in seconds: RR = 60 ÷ HR (if you enter the R–R interval directly it is used as given). Bazett QTc = QT ÷ √RR. Fridericia QTc = QT ÷ RR^(1/3) (the cube root of RR). Both use RR in seconds and return QTc in the same unit as the QT you enter (milliseconds). At a heart rate of 60 (RR = 1 s) both corrections equal the raw QT.

Limitations and notes

Not a clinical device: a prolonged QTc is a screening flag, not a diagnosis of long QT syndrome (established with full criteria — Schwartz score, symptoms, family history, genetics). Formula choice changes the answer: Bazett over-corrects at fast rates and under-corrects at slow rates, so it systematically over-calls prolongation in children (who run high heart rates); when HR is outside ~60–100 bpm, Fridericia is the more defensible correction. Interpretation bands are applied ONLY to the Bazett QTc, because the pediatric 440/460/480 ms thresholds (Phan 2015; Andršová 2020) were derived with Bazett and the research states they are not equivalent for a Fridericia QTc; the Fridericia value is reported without bands. Adult sex-specific cutoffs (Goldenberg 2006) must not be applied to children. QTc is only as good as the QT measurement (lead, tangent method, U-wave exclusion, machine vs manual). [NEEDS SOURCE]: an authoritative pediatric QT/HR reference-interval table — the QT (~200–700 ms) and HR (~30–250 bpm) input bounds here are engineering sanity guards, not values from a fetched normative table. [NEEDS SOURCE]: the exact Goldenberg 2006 age/sex-binned millisecond boundaries could not be retrieved from an open full-text source.

Accepted input ranges

  • Measured QT interval — 200–700 ms
  • Heart rate (or R–R interval) — 30–250 bpm

References

  1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370. Reprinted Ann Noninvasive Electrocardiol. 1997;2(2):177–194.DOI 10.1111/j.1542-474X.1997.tb00325.x
  2. Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469–486. English translation Ann Noninvasive Electrocardiol. 2003;8(4):343–351.DOI 10.1111/j.0954-6820.1920.tb18266.x
  3. Phan DQ, Silka MJ, Lan Y-T, Chang R-KR. Comparison of formulas for calculation of the corrected QT interval in infants and young children. J Pediatr. 2015;166(4):960–964.PMID 25648293DOI 10.1016/j.jpeds.2014.12.037
  4. Andršová I, Hnatkova K, Helánová K, et al. Problems with Bazett QTc correction in paediatric screening of prolonged QTc interval. BMC Pediatr. 2020;20:558.PMID 33317470DOI 10.1186/s12887-020-02460-8
  5. Goldenberg I, Moss AJ, Zareba W. QT interval: how to measure it and what is "normal." J Cardiovasc Electrophysiol. 2006;17(3):333–336.PMID 16643414DOI 10.1111/j.1540-8167.2006.00408.x

Version and changelog

v1.0.0 · General

  • 2026-07-25 v1.0.0 — Initial release: QTc by Bazett and Fridericia with pediatric Bazett-based prolongation bands.

Important

For use by qualified health professionals as an informational and educational aid. It supports clinical judgment and does not replace it. Verify every result independently before making a clinical decision. This is not a medical device.