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Clinical reference calculator · GeraClinic

QTc Calculator (Bazett & Fridericia)

The corrected QT interval (QTc) adjusts the measured QT for heart rate. This tool reports both the Bazett (QT ÷ √RR) and Fridericia (QT ÷ ∛RR) corrections, with sex-specific prolongation thresholds.

Quick answer

The corrected QT interval (QTc) adjusts the measured QT for heart rate. Bazett’s formula is QT ÷ √RR and Fridericia’s is QT ÷ ∛RR, with RR in seconds (RR = 60 ÷ heart rate). Normal is under 450 ms in men and under 460 ms in women; a QTc over 500 ms markedly raises the risk of torsades de pointes.

QTc inputs

Validated formula

Bazett: QTc = QT (ms) / √(RR in seconds). Fridericia: QTc = QT (ms) / (RR in seconds)^(1/3). RR (s) = 60 / heart rate (bpm). Bazett over-corrects at heart rates >85/min; Fridericia is preferred at high rates.

Interpretation

  • Normal— QTc <450 ms (men) / <460 ms (women) — AHA/ACCF/HRS 2009.
  • Prolonged— QTc ≥450 ms (men) / ≥460 ms (women).
  • Markedly prolonged (99th percentile)— QTc >470 ms (men) / >480 ms (women).
  • High torsades risk— QTc >500 ms — substantially increased risk of torsades de pointes.

How to use the QTc (Bazett & Fridericia) calculator

  1. 1Enter the QT interval. Enter the measured QT interval in milliseconds.
  2. 2Enter the RR interval. Enter the RR interval in seconds, or derive it from heart rate (RR = 60 ÷ heart rate).
  3. 3Read both corrections. The tool shows the Bazett and Fridericia QTc with sex-specific thresholds. Fridericia is preferred at higher heart rates, where Bazett over-corrects.

Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.

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Frequently asked questions

What is a normal QTc?⌄

Using the AHA/ACCF/HRS 2009 thresholds, a QTc under 450 ms in men and under 460 ms in women is normal. A QTc of 470 ms (men) or 480 ms (women) and above is markedly prolonged, and over 500 ms carries a high torsades risk.

Should I use Bazett or Fridericia?⌄

Bazett is the classic formula but over-corrects at fast heart rates and under-corrects at slow ones. Fridericia is more accurate at higher rates. This tool shows both so they can be compared.

How do I get the RR interval?⌄

RR in seconds equals 60 divided by the heart rate in beats per minute. For example, a heart rate of 75 gives an RR of 0.8 seconds.

Sources & validation

This calculator reproduces the published QTc (Bazett & Fridericia) score, validated for rate-corrected QT interval for prolongation / torsades risk.

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