Scores & indices
QTc calculator
The QT means nothing until it is corrected for rate — Bazett and Fridericia side by side, with the sex-specific limits and the 500 ms line that matters for torsades.
How to read the result
QTc (Bazett) (ms) falls into these published bands.
An ECG after starting a QT-prolonging antiemetic: measured QT 360 ms at a heart rate of 100/min.
QTc 465 ms (Bazett) — borderline; 427 ms by Fridericia
A 38 ms divergence from one measurement: Bazett overcorrects in tachycardia and can manufacture apparent prolongation, so Fridericia is the number to trust at this rate — while drugs and electrolytes still get reviewed on their own merits.
Who this is for
For clinicians reading ECGs around QT-prolonging drugs — antiemetics, antipsychotics, macrolides, methadone — and working up syncope, electrolyte derangement, or suspected long QT. Enter the measured QT and heart rate; both Bazett and Fridericia are shown, because outside 60–100 bpm the choice of formula changes the answer.
How it works
What it calculates
The rate-corrected QT interval by Bazett (the everyday standard) and Fridericia (more accurate outside 60–100 bpm), from the measured QT and heart rate.
Interpreting the result
Usual upper limits are about 450 ms in men and 470 ms in women; above 500 ms the risk of torsades de pointes rises steeply. Review QT-prolonging drugs and correct potassium, magnesium, and calcium.
Measurement matters
Measure the QT manually in a lead with a clear T-wave end (II or V5) — automated readings mis-measure at extremes. Bazett overcorrects in tachycardia; trust Fridericia there.
Built on published evidence
- Bazett 1920; Fridericia 1920
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Bazett or Fridericia — which QTc should I use?
Bazett (QT/√RR) is the everyday standard within 60–100 bpm, but it overcorrects in tachycardia and undercorrects in bradycardia — prefer Fridericia (QT/∛RR) outside that range. The tool computes both so the divergence is visible rather than hidden.
What counts as a prolonged QTc?
Usual upper limits are about 450 ms in men and 470 ms in women. Above 500 ms the risk of torsades de pointes rises steeply — that is the threshold at which causes need acting on now, not at the next review.
How should the QT be measured?
Manually, in a lead with a clearly defined T-wave end — typically II or V5 — because automated readings mis-measure at extremes of rate and morphology. The correction is only as good as the interval you feed it.
What do I actually do about a long QTc?
Review and rationalise QT-prolonging drugs, and correct potassium, magnesium, and calcium — the reversible causes come first, and at marked prolongation (over 500 ms) they come urgently.
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