Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
QTc (Bazett)
447
QTc (Fridericia)
431
Prolongation threshold (ms)
450
Bazett interpretation
Normal
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How to use this
- 1Enter measured qt interval (ms).
- 2Enter heart rate (bpm).
- 3Enter sex.
- 4Read your qtc (bazett) on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
The QT interval on an EKG shortens as heart rate increases, so raw QT cannot be compared across different heart rates without correction. Bazett's formula (QT ÷ √RR) is the most widely taught and used in practice, but it overcorrects at high heart rates and undercorrects at low heart rates, which is why Fridericia's formula (QT ÷ RR^(1/3)) is often preferred in research and by cardiology for tachycardic or bradycardic patients. This calculator reports both so you can see where they diverge. QTc matters because prolongation increases risk of torsades de pointes, particularly from QT-prolonging medications (many antiarrhythmics, antipsychotics, antiemetics, and antibiotics), electrolyte derangements (low potassium, magnesium, or calcium), and congenital long QT syndromes. A QTc over 450 ms in men or 460 ms in women is generally considered prolonged, and values over 500 ms carry meaningfully elevated arrhythmia risk. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.
Worked example
Using the values the calculator loads with:
Inputs
- Measured QT interval: 400 ms
- Heart rate: 75 bpm
- Sex: Male
Results
- QTc (Bazett): 447
- QTc (Fridericia): 431
- Prolongation threshold (ms): 450
- Bazett interpretation: Normal
What each field means
Inputs
- Measured QT interval (ms)
- The measured qt interval used in the calculation, measured in ms. Starts at 400 ms so you have a working example on load. Accepted range: 200–700 ms.
- Heart rate (bpm)
- The heart rate used in the calculation, measured in bpm. Starts at 75 bpm so you have a working example on load. Accepted range: 20–220 bpm.
- Sex
- Pick the option that matches your situation — the maths changes per option. Choices: Male, Female.
Results
- QTc (Bazett)
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- QTc (Fridericia)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Prolongation threshold (ms)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Bazett interpretation
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Which formula should I trust more?
Fridericia performs better at heart rates outside the 60-100 range because it corrects less aggressively than Bazett. If the two formulas disagree significantly, particularly at a fast or slow heart rate, lean toward Fridericia or consult a cardiologist rather than picking whichever number looks more reassuring.
What heart rate range does Bazett distort most?
Bazett overcorrects (reports falsely high QTc) at heart rates above about 100 bpm and undercorrects (reports falsely low QTc) at heart rates below about 60 bpm, since it assumes a fixed square-root relationship that does not hold at the extremes.
What's a dangerous QTc value?
Above 500 ms is generally considered high risk for torsades de pointes regardless of formula, and an increase of more than 60 ms from a patient's own baseline is also concerning even if the absolute number looks moderate. Any QTc-prolonging medication should be reviewed against a fresh EKG in that range.
Do electrolytes matter as much as medications?
Yes. Hypokalemia, hypomagnesemia, and hypocalcemia all prolong QT independently and also amplify the effect of QT-prolonging drugs. Checking and correcting electrolytes is often the fastest, safest intervention before deciding whether a medication needs to be held.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). QTc Calculator (Bazett & Fridericia). Retrieved from https://www.revenuelab.fyi/toolbox/qtc-interval
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/qtc-interval" target="_blank" rel="noopener">QTc Calculator (Bazett & Fridericia) — RevenueLab</a> (2026).</p>
Source: [QTc Calculator (Bazett & Fridericia) — RevenueLab](https://www.revenuelab.fyi/toolbox/qtc-interval) (2026).