QTc Bazett Calculator
The formula
What the bazett qtc tells you
The original 1920s square-root correction still printed by every ECG machine, and still the standard for QT-prolongation safety thresholds.
Calculation: QTc = QT ÷ √(RR), RR = 60/HR. Enter your inputs and every substitution is shown below the result.
Reading the result
Use its number for the 500 ms alarm, and Fridericia when the rate is not roughly normal.
Rules of thumb: >450/460 ms prolonged; >500 ms high risk
Limitations and best practice
Sinus tachycardia inflates Bazett falsely — the classic cause of “stop the drug” reactions to a normal heart.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
Why is Bazett still standard?
Is Bazett dangerous?
Source & methodology
Bazett 1920; AHA/ACCF series 2009.
QTc Bazett: the number, the method and the caveats
You are here because you need a specific number, and you want to see how it was produced before you rely on it. Searchers land on a QTc Bazett page for three reasons — to plan, to compare options, and to check that somebody else’s arithmetic is honest. This tool serves all three, and everything after the calculator explains the model instead of hiding it.
People rarely disagree about arithmetic; they disagree about assumptions that were never written down. Printing the substitution and the sensitivity ranking removes that category of disagreement for QTc Bazett.
In Health & Medical the same quantity is often quoted under two or three different conventions. The formula printed above fixes one convention and applies it consistently, which is what makes results from this page comparable with each other.
Every figure in the prose is generated by the engine rather than written beside it — the worked example resolves to qTc (Bazett) = 439 ms on the default inputs, and the sensitivity section ranks the inputs that move the result — Measured QT first, at ≈10.0 % of the headline for a ten-percent error either way in that input — which is the thread the rest of this guide pulls on.
The QTc Bazett formula, explained
One formula, published in full: QTc = QT ÷ √(RR), RR = 60/HR. Anything you can read, you can reproduce in a spreadsheet, and that reproducibility is the whole point of printing it rather than hiding it behind a button.
Measured QT (ms) (example value 406) is a quantity rather than a category — precision here is cheap and worth having. If you are unsure what belongs in this field, look at the printed steps: the substitution shows exactly how the engine uses it.
Heart rate (bpm) (example value 70) is a quantity rather than a category — precision here is cheap and worth having. Rates are the most mis-transcribed input on any page like this; copy it from the document rather than retyping from memory.
QTc Bazett with real numbers, step by step
Abstract formulas are hard to trust, so here is the complete substitution using the default inputs, written the way an examiner would want it. Starting figures: Measured QT (ms), Heart rate (bpm). The endpoint of that chain is 439 ms of qTc (Bazett), sitting in the “Bazett QTc 439 ms at 70 bpm — the classic formula is famous for over-correcting above 100 bpm and under-correcting below 60” band.
- RR = 60/70 = 0.857 s
- QTc = 406 ÷ 0.926 = 439 ms
- Sanity-check with Fridericia: 427 ms
Underlying RR — 857 ms. Compare it against the main result of 439 ms before drawing conclusions.
Fridericia cross-check — 427 ms. Compare it against the main result of 439 ms before drawing conclusions.
Risk band — normal. Compare it against the main result of 439 ms before drawing conclusions.
Rate-bias warning — minimal. If your inputs change, this is usually the first companion figure to move.
Re-running with your own figures is instant; re-running by hand once is the best way to know the tool is telling you the truth.
How far QTc Bazett can reasonably move
The honest version of any estimate is a corridor, and this one is cheap to produce: QTc Bazett was recomputed at 5 points along the dominant input, with every other field left untouched.
| Measured QT setting | QTc (Bazett) |
|---|---|
| 80 % of the example value | 351 ms |
| 90 % of the example value | 395 ms |
| example inputs | 439 ms |
| 110 % of the example value | 482 ms |
| 120 % of the example value | 526 ms |
The corridor runs from 351 ms to 526 ms across that band — which is the range worth planning inside rather than the single 439 ms point estimate. Reproduce it in the form above by editing Measured QT alone; the substitution steps will follow your figures exactly.
The levers behind QTc Bazett, ranked by effect
Generic advice says “it depends on your inputs”; this section says exactly how much. Each field was perturbed by ±10 % in isolation and the qTc (Bazett) recomputed, so the ranking below is measured rather than asserted. Read it as a map of where QTc Bazett is fragile — drawn for the example inputs, which is the map you want when deciding what to measure carefully.
- Measured QT: ten percent up gives 482 ms, ten percent down gives 395 ms — ≈10.0 % of the qTc (Bazett) either way
- Heart rate: ten percent up gives 460 ms, ten percent down gives 416 ms — ≈5.1 % of the qTc (Bazett) either way
Measured QT is the dominant lever here: ten percent either way moves the result by about 10.0 %, from 395 ms to 482 ms. Measure that input from a document rather than from memory, and treat Heart rate as the second priority (≈5.1 % per ten percent). The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
What a QTc Bazett of 439 ms means in practice
A number on its own is trivia; a number against a range is intelligence. The default case lands in the “Bazett QTc 439 ms at 70 bpm — the classic formula is famous for over-correcting above 100 bpm and under-correcting below 60” band, and bands are deliberately wide because real situations overlap. How much of that band is luck? On Measured QT alone ten percent either way covers 395 ms to 482 ms. The practical reading rules: compare against your own history before comparing against other people; treat band boundaries as zones, not walls — one point on the wrong side of a threshold rarely changes a real outcome; and when a companion output contradicts the headline, believe the pair.
In Health & Medical, QTc Bazett is one instrument in an orchestra: on its own it is a number, next to Underlying RR and Fridericia cross-check it becomes a diagnosis. The tools linked above exist so the headline never has to carry that weight alone.
Common mistakes with QTc Bazett (and the fix)
Sinus tachycardia inflates Bazett falsely — the classic cause of “stop the drug” reactions to a normal heart. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Reading QTc Bazett as a promise rather than a conditional. Every number here is of the form “if these inputs, then this result”; when an input moves, the honest response is to recompute rather than to defend the old figure.
This model has its own particular trap: Measured QT carries ≈10.0 % of the response per ten percent of error, so effort spent elsewhere is largely decorative.
A final trap is presentation: reporting QTc Bazett to more decimal places than the inputs support. The tool prints what the arithmetic gives; you should quote only what your least precise field justifies.
A quick check for any QTc Bazett figure
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Record the date and the inputs alongside the number so future-you knows what was assumed.
- Put Measured QT under a magnifier; the sensitivity table above is the reason why.
- Verify each input against its original source document, not memory.
- Compare the result with one independent method or published benchmark.
- Repeat the ±10 % test on the inputs that matter and see whether the decision — not just the number — still holds.
Five steps from raw inputs to a figure you can defend
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — collect, then type. Everything this tool asks for is listed above (measured qt (ms), heart rate (bpm)). Pull each figure from something you can point at — paperwork, a dashboard, a label — before touching a field. Guesswork at the input stage cannot be repaired at the interpretation stage. Re-running a number costs seconds; retracting a decision costs considerably more.
Step 2 — baseline before exploration. Get one defensible QTc (Bazett) on screen before you start nudging numbers. Without a baseline, every variation looks interesting and none of them means anything. Numbers are cheap to produce and expensive to retract; the extra minute is the cheapest insurance available.
Step 3 — play with the inputs on purpose. Ten percent up, ten percent down, one field at a time. Within a few minutes you will know this calculation better than most people who quote its results. The input you reach for first is usually the one you measured longest ago.
Step 4 — compute a corridor, not a point. Three runs — worst, likely, best — turn QTc Bazett from a snapshot into a range. Plan against the pessimistic end and treat the optimistic end as the upside story. A figure with a date beside it becomes evidence; without one it stays an anecdote.
Step 5 — schedule the next run. In Health & Medical the inputs drift quietly. Put a reminder a quarter out, re-enter the same fields, and read the change rather than the level. Write the figure down before moving on; memory flatters whichever number you saw last.
The real-world conversations QTc Bazett settles
Numbers earn their keep in conversations, and QTc Bazett is no exception. In Health & Medical discussions around Clinical Calculators, this figure decides whether an option stays on the table: it converts an opinionated debate into an arithmetic one, and arithmetic is the only referee both sides accept. The test of a good estimate is not that it feels right but that it changes what you do.
The companion outputs — Underlying RR and Fridericia cross-check — exist for the second stage of that screen: the headline sizes the prize, while the companions describe the trade-off you are accepting to get it. Estimates improve most when the worst case is written down in ink.
Re-running this page costs nothing and catches the slow drifts that never announce themselves. In Health & Medical the expensive mistakes are rarely sudden; they are small changes in Measured QT (ms) that nobody recalculated for a year. A figure with a date beside it becomes evidence; without one it stays an anecdote.
People also ask about QTc Bazett
What affects QTc Bazett the most?
The short answer: Measured QT. The longer answer is the sensitivity table on this page, where every numeric input was probed by ten percent and the qTc (Bazett) recalculated — Measured QT moved it furthest, by roughly 10.0 % (395 ms to 482 ms).
Which matters more: heart rate or measured qt?
For QTc Bazett, Measured QT wins the head-to-head (≈10.0 % against ≈5.1 % per ten-percent change). Knowing which input dominates is the difference between careful work and merely slow work.
What is a typical QTc Bazett?
There is no honest universal average for QTc Bazett, but there is a reference point: the defaults on this page produce 439 ms (Bazett QTc 439 ms at 70 bpm — the classic formula is famous for over-correcting above 100 bpm and under-correcting below 60). Replace the inputs with your own and compare — that comparison is the useful meaning of “typical”.
How do I calculate QTc Bazett by hand?
Use the stated model — QTc = QT ÷ √(RR), RR = 60/HR — and substitute in the order the tool prints (it starts from RR = 60/70 = 0.857 s). On the example inputs the hand route lands on 439 ms, exactly what the calculator shows. A spreadsheet reproduces it in two or three cells; doing it once by hand is what makes the automated answer trustworthy.
What happens to QTc Bazett if measured qt changes by 10 %?
On these inputs a ±10 % change in measured qt takes QTc Bazett to 482 ms against the 439 ms example — around 10.0 % of the headline. The same test on every other field is in the sensitivity section.
Is the QTc Bazett result of 439 ms good or bad?
The tool answers “what”, not “whether”: 439 ms carries the verdict “Bazett QTc 439 ms at 70 bpm — the classic formula is famous for over-correcting above 100 bpm and under-correcting below 60” for the default case, which is a starting interpretation rather than a verdict on your situation. Re-run with your own figures and compare the bands.
How accurate is QTc Bazett for me personally?
The formula encodes averages; body-composition and measurement error typically shifts results by a few percent in either direction. Use ranges and trends, and treat any single 439 ms as approximate.
How do I calculate QTc Bazett manually?
Write down the inputs, then follow the printed steps in order — QTc = QT ÷ √(RR), RR = 60/HR. A spreadsheet replicates it with two or three cells. The manual route is worth doing once to build intuition; after that the calculator saves time and eliminates transcription slips.
Why is Bazett still standard?
Regulatory thresholds were built on it — switching changes your population’s baseline numbers.
Is Bazett dangerous?
Only when trusted blindly at rate extremes; its false-positive QTc at tachycardia can drive unnecessary drug changes.
Is my data private on this page?
Yes — computation happens locally in your browser, with no account, no tracking of your inputs and nothing sent to a server. The printed formula is the whole story of what happens to your figures.
Glossary for QTc Bazett
- Underlying RR
- 857 ms in the default QTc Bazett calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Fridericia cross-check
- 427 ms in the default QTc Bazett calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Risk band
- normal on the example inputs — a companion figure printed beside the headline of QTc Bazett so that no result is read in isolation.
- QTc (Bazett)
- The primary output of this tool: 439 ms for the default inputs, computed exactly from QTc = QT ÷ √(RR), RR = 60/HR.
- Worked example
- Every substitution in order, from the example inputs to 439 ms — the audit trail for the headline figure.
- Verdict band
- A named zone on the range of plausible results; the example inputs land in “Bazett QTc 439 ms at 70 bpm — the classic formula is famous for over-correcting above 100 bpm and under-correcting below 60”, and your inputs may not.
- Sensitivity (±10 %)
- How far the QTc Bazett moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Method, sources and what this tool cannot tell you
The model is published arithmetic implemented exactly as written: QTc = QT ÷ √(RR), RR = 60/HR. Source of record: Bazett 1920; AHA/ACCF series 2009. Where a professional version of this calculation differs, it differs in inputs and jurisdiction-specific coefficients, not in the algebra — which is why re-checking your inputs beats hunting for a second calculator.
Treat the output as a precise answer to the question you typed: exact arithmetic, transparent formula, published source, no hidden rounding. The promise to the reader is narrow and testable: same inputs, same output, no silent rounding, and no figure presented without the formula that produced it.
After the number: what to do with it
Save the number together with its inputs and today’s date; a QTc Bazett figure without context is unusable the moment an input changes.
Whoever you are (people who need one trustworthy number before their next decision), the next step is the same: replace the example inputs with your own, note the date, and re-run whenever a figure moves. In Health & Medical, a dated figure is a trend, and an undated one is a rumour.