QTc Calculator
Correct a measured QT interval for heart rate with Bazett, Fridericia, Framingham and Hodges side by side, using editable prolongation thresholds.
Measurements from the ECG
Thresholds (editable — societies and protocols differ)
Bazett (1920) QTc
462 ms
All four corrections
| Formula | Working | QTc | Where it behaves |
|---|---|---|---|
| Bazett (1920) | 400 ÷ √0.750 | 462 ms | The original and still the most quoted. It over-corrects above about 90 bpm and under-corrects below about 60, which is why a tachycardic patient so often gets an alarming QTc. |
| Fridericia (1920) | 400 ÷ 0.750^(1/3) | 440 ms | A cube-root correction that stays much flatter across heart rate. Regulatory thorough-QT studies and most modern drug-safety work prefer it. |
| Framingham (Sagie 1992) | 400 + 154 × (1 − 0.750) | 439 ms | A linear correction fitted on 5,018 Framingham Heart Study subjects. Performs well in the 50–90 bpm range it was derived in. |
| Hodges (1983) | 400 + 1.75 × (80 − 60) | 435 ms | Corrects directly off heart rate rather than RR. Simple to do in your head and reasonably stable at the extremes. |
Worked check: a QT of 400 ms at 80 bpm gives an RR of 0.75 s, so Bazett returns 462 ms, Fridericia 440 ms, Framingham 439 ms and Hodges 435 ms — the same four values MDCalc's QTc calculator publishes for those inputs. At 80 bpm, Bazett here sits 22 ms above Fridericia.
Estimates only, not medical advice, and no substitute for a cardiologist reading the actual tracing. The correction is only as good as the QT you measured: automated ECG QT values are frequently wrong in the presence of a broad T wave, a prominent U wave, atrial fibrillation or artefact, and manual measurement between observers routinely varies by 20 ms. Bazett was derived from 39 healthy young subjects in 1920 and systematically over-corrects at fast heart rates, so a tachycardic Bazett QTc above 500 ms should always be re-read with Fridericia before anyone stops a drug. None of these corrections is valid in bundle branch block, ventricular pacing or a wide QRS without a further adjustment, and in atrial fibrillation you should average several beats. The 450/460/500 ms cut-offs are conventions, not physiology.
What is the QTc Calculator?
QTc corrects the measured QT interval for heart rate. Bazett divides QT by the square root of the RR interval in seconds, Fridericia divides by the cube root, Framingham adds 154 × (1 − RR), and Hodges adds 1.75 × (heart rate − 60).
- Bazett, Fridericia, Framingham and Hodges computed simultaneously
- Rate entered as bpm or as an RR interval, converted automatically
- Editable sex-specific borderline, prolonged and high-risk thresholds
- Shows how far Bazett diverges from Fridericia at the entered rate
- Rejects impossible inputs, such as a QT longer than the RR interval
- Fully client-side — no ECG data is uploaded anywhere
How to use the QTc Calculator
- 1
Enter the measured QT interval in milliseconds, taken from lead II or V5 using the tangent method.
- 2
Choose whether to enter the rate as heart rate in beats per minute or as an RR interval in milliseconds, then enter it.
- 3
Pick which correction you want as the headline figure and banded result.
- 4
Adjust the sex-specific borderline and prolonged thresholds, and the markedly-prolonged cut-off, to match your protocol.
- 5
Read all four corrected values in the comparison table, each with its working and a note on where it behaves well.
About the QTc Calculator
The ByteTools QTc Calculator takes a measured QT interval and a heart rate and returns all four commonly used rate corrections at once — Bazett, Fridericia, Framingham and Hodges — so you can see immediately how much of a borderline result is real and how much is an artefact of which formula was chosen. You can enter the rate as beats per minute or as an RR interval in milliseconds.
The prolongation thresholds are editable fields, because societies and drug-safety protocols do not agree on them. The tool uses whichever correction you nominate as the headline figure, bands it against your own cut-offs, and shows the exact substitution it performed, along with how far Bazett sits from Fridericia at the rate you entered.
Everything runs locally in your browser and nothing is uploaded. These are estimates for education and cross-checking only, not medical advice — a QTc is only as trustworthy as the QT measurement it was built from, and a real tracing should always be read by someone qualified to read it.
Frequently asked questions
What is a normal QTc?
The most commonly quoted limits are 450 ms for men and 460 ms for women, with 500 ms treated as the point of substantially raised torsades risk regardless of sex. These are conventions rather than physiology, which is why this calculator makes each threshold an editable field.
Which QTc formula should I use?
Bazett is the most widely reported but over-corrects at fast heart rates and under-corrects at slow ones. Fridericia is flatter across rate and is preferred in regulatory thorough-QT studies and most modern drug-safety work. Seeing all four together is the quickest way to tell whether a borderline result is real.
Why does Bazett give a longer QTc when the heart rate is high?
Bazett's square-root correction was fitted to 39 healthy young subjects in 1920 and assumes a relationship that does not hold at the extremes. Above roughly 90 beats per minute it systematically inflates the result, which is why tachycardic patients so often produce an alarming Bazett QTc that Fridericia brings back into range.
How do I convert heart rate to an RR interval?
The RR interval in seconds is 60 divided by the heart rate in beats per minute, so 80 bpm gives 0.75 seconds or 750 milliseconds. This calculator does the conversion for you in either direction, and shows the RR value it used.
Can I use QTc correction in atrial fibrillation or bundle branch block?
Not straightforwardly. In atrial fibrillation the RR interval changes beat to beat, so several beats need averaging. In bundle branch block or ventricular pacing the QRS is wide and inflates the QT, so a further adjustment is required before any of these formulas mean much.
How accurate is a machine-measured QT interval?
Less accurate than people assume. Automated QT measurement is frequently wrong with broad T waves, prominent U waves, artefact or atrial fibrillation, and manual measurement between two experienced readers routinely differs by around 20 ms. Always confirm the QT by eye before acting on a QTc.
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