This QTc calculator corrects the ECG’s QT interval for heart rate — because QT naturally shortens as the heart speeds up — reporting all four standard corrections (Bazett, Fridericia, Framingham, Hodges) so their disagreements are visible, not hidden.
Reference bands (adults): normal QTc ≲ 450 ms (men) / 470 ms (women); borderline 450–470 / 470–480; prolonged above those, with >500 ms marking high torsades risk. For education only — QT measurement technique (lead choice, U-wave handling, averaging) moves values by tens of ms; clinical decisions belong to clinicians reading the actual ECG.
Four Corrections, One Honest Spread
Bazett: QT ÷ √RR (1920, still the default on ECG printouts — and known to overcorrect at high rates, undercorrect at low). Fridericia: QT ÷ ∛RR — better behaved at extremes, preferred in drug trials. Framingham: QT + 154(1 − RR). Hodges: QT + 1.75(HR − 60). Showing all four replaces false precision with the true state of the art. Rate context itself is on our Heart Rate Calculator. Part of our Health Calculators collection.
Worked Examples (Verified)
- QT 400 ms at HR 60 (RR = 1.0 s): every formula returns 400 ms — corrections vanish at 60 bpm by construction; the anchor point of the whole system
- QT 400 ms at HR 75 (RR = 0.8 s): Bazett 447 ms · Fridericia 431 ms · Framingham 431 ms · Hodges 426 ms — the widget’s defaults: notice Bazett already running hottest at a modest rate
- At HR 100 the Bazett–Fridericia gap widens past 25 ms on the same tracing — formula choice is a clinical decision at rate extremes, which is why trials specify theirs in advance
Why Anyone Corrects the QT
- A prolonged QTc signals delayed ventricular repolarization — the substrate for torsades de pointes; it’s monitored for congenital long-QT, electrolyte disturbances (low K⁺/Mg²⁺/Ca²⁺) and a long list of QT-prolonging drugs.
- Measurement is the fragile step: the QT is read in the lead with the longest interval (often II or V5), U waves excluded, averaged over 3–5 beats — a sloppy tangent line costs more accuracy than formula choice ever will.
- Wide QRS (bundle branch block, pacing) inflates QT mechanically — JT-interval corrections exist for exactly that case.
- Sex and age matter to the reference bands; pediatric ranges differ substantially from adult ones.
Related Tools
- Heart Rate Calculator — rates, max and zones
- Blood Pressure Calculator — the other vital-sign number
- Health Calculators — the full collection
Frequently Asked Questions
How do you calculate QTc?
Correct the measured QT for the RR interval (60 ÷ heart rate, in seconds). Bazett: QT ÷ √RR. Fridericia: QT ÷ ∛RR. For QT 400 ms at 75 bpm (RR 0.8 s): Bazett 447 ms, Fridericia 431 ms.
What is a normal QTc?
Roughly ≤450 ms in men and ≤470 ms in women, with 450–480 the borderline zone and >500 ms considered high risk for torsades. Boundaries vary slightly between guidelines — and always presume a well-measured QT.
Which QTc formula is best?
Bazett is the entrenched default but distorts at rate extremes (overcorrecting when fast, undercorrecting when slow). Fridericia is generally preferred in research and at high/low rates; Framingham and Hodges perform well in validation cohorts. This calculator shows all four precisely because “best” is context-dependent.
Why does QT need correcting at all?
Because repolarization time physiologically tracks cycle length: a heart beating at 120 gives a shorter raw QT than the same heart at 50, with no change in underlying risk. Correction re-expresses every measurement at the 60 bpm reference so intervals can be compared to norms.
Can this calculator diagnose long QT syndrome?
No. Diagnosis integrates repeated ECGs, the Schwartz score, history (syncope, family sudden death), electrolytes, medications and often genetics. A computed QTc here is arithmetic on numbers you supply — interpretation belongs to a clinician with the tracing.
Is this QTc calculator free?
Yes — free, instant, no sign-up.