Correct a measured QT interval for heart rate with four classic formulas side by side: Bazett, Fridericia, Framingham (Sagie) and Hodges — with sex-specific interpretation based on AHA/ACCF/HRS guidance.
Try an example:
RR interval: — · Heart rate: —
| Formula | QTc |
|---|---|
| Bazett — QT / √RR | — |
| Fridericia — QT / ∛RR | — |
| Framingham — QT + 154·(1 − RR) | — |
| Hodges — QT + 1.75·(HR − 60) | — |
Chart scale 300–600 ms. Each bar starts at 300 ms (x = 60) and ends at x = 60 + (QTc − 300) / 300 × 600.
—
—
Medical disclaimer: a single QTc number is never the whole story. QRS width, electrolytes, medications, and symptoms all matter — review every value with a clinician, and seek urgent care for concerning symptoms rather than relying on calculator output.
Most widely reported; overcorrects at fast heart rates, undercorrects at slow ones.
More stable at rate extremes; preferred in thorough QT drug studies.
With QT in milliseconds: QTc = QT + 154 × (1 − RR). Linear correction from 5,018 Framingham Heart Study subjects.
Linear in heart rate; a convenient bedside check. (Equivalent to QT + 0.00175 × (HR − 60) with QT in seconds.)
RR (seconds) = 60 / HR(bpm). At HR 60, RR = 1 s and every formula returns QTc = QT.
QT = 400 ms, heart rate = 120 bpm → RR = 60/120 = 0.5 s:
At 60 bpm (RR = 1 s) all four give exactly 400.0 ms — a good sanity check for any implementation.
Use Bazett when you need comparability with published cutoffs, drug labels, and automated ECG reports. Also check Fridericia whenever the heart rate is below 60 or above 100 bpm — when the two disagree substantially, Fridericia usually reflects reality better. Framingham is a solid independent cross-check, and Hodges works as a quick mental estimate.
The QTc is the QT interval corrected for heart rate. The raw QT interval on an ECG shortens when the heart beats faster and lengthens when it beats slower, so a correction formula normalizes every measurement to a reference rate of 60 beats per minute. That makes QT values comparable across different heart rates and lets clinicians judge whether repolarization is normal or prolonged.
Bazett (1920) is the most widely reported, but it overcorrects at fast heart rates and undercorrects at slow ones. Fridericia (1920) uses the cube root of RR and is more stable at rate extremes; regulators favor it in thorough QT drug studies. Framingham (Sagie et al., 1992) is a linear correction derived from 5,018 Framingham Heart Study subjects and behaves much like Fridericia. Hodges (1983) is also linear and handy as a quick mental check. A practical rule: use Bazett for comparability, and always glance at Fridericia when the rate is outside 60 to 100 bpm. Example: QT 400 ms at 120 bpm gives Bazett 565.7 ms, Fridericia 504.0 ms, Framingham 477.0 ms, Hodges 505.0 ms.
Per the AHA/ACCF/HRS 2009 recommendations: prolonged QT is longer than 450 ms in men and 460 ms or longer in women; a QTc of 390 ms or shorter is considered short in both sexes. A newer AHA/ACCF statement uses 99th percentile cutoffs above 470 ms (men) and above 480 ms (women), with values at or above 500 ms considered highly abnormal and carrying elevated torsades de pointes risk. This calculator labels 450 to 470 ms in men and 460 to 480 ms in women as borderline.
Bazett divides QT by the square root of the RR interval, a curve fitted to only 39 young subjects in 1920. At fast rates the square-root correction shrinks the denominator too aggressively, inflating QTc; at slow rates it does the opposite. The Framingham investigators showed this directly: Bazett overcorrects at fast heart rates and undercorrects at low heart rates, which is why linear or cube-root corrections are preferred when the rate is far from 60 bpm.
The QT interval runs from the start of the Q wave to the end of the T wave, usually measured in lead II or V5/V6, in milliseconds. The RR interval is the time between two consecutive R-wave peaks, also in milliseconds; heart rate equals 60,000 divided by the RR interval in ms. Enter either value: this calculator converts between heart rate and RR interval automatically (RR in seconds = 60 / heart rate).
A QTc of 500 ms or longer is considered markedly prolonged and is associated with a substantially elevated risk of torsades de pointes, a life-threatening arrhythmia. Values in this range, or a sudden increase of more than 60 ms from baseline, warrant urgent clinical review, especially when QT-prolonging drugs or electrolyte disturbances are involved. Do not delay care while rechecking numbers.
Yes. Switch the input mode to RR interval and enter the value in milliseconds; the calculator derives the heart rate as 60,000 divided by RR. Both modes feed the same four formulas. Example: QT 360 ms at 75 bpm (RR 800 ms) gives Bazett 402.5 ms, Fridericia 387.8 ms, Framingham 390.8 ms, and Hodges 386.2 ms.
No. It is an educational reference that performs arithmetic only. A QTc number never replaces a clinician reading the full ECG in context, including QRS width, electrolytes, medications, and symptoms. Always discuss results with a qualified healthcare professional, and seek urgent care for concerning symptoms rather than relying on any calculator output.
Free tool by NRTechStudio
Explore similar health tools and calculators. 100% free, accurate, and runs completely in your browser without sign-up.
2 taps, no typing needed — diagnostics are sent automatically.