Corrected QT Interval Calculator
The formula
What the qtc tells you
Normalizes the measured QT to a standard heart rate so repolarization risk is comparable across beats, using one of four published corrections.
Calculation: QTc by selected formula (Bazett/Fridericia/Hodges/Framingham); normal <450 M / <460 F. Enter your inputs and every substitution is shown below the result.
Reading the result
For drug-monitoring decisions, the consistency of using one method beats the elegance of switching — Fridericia is the safest default at extremes.
Rules of thumb: Normal: ≤450 ms men, ≤460 ms women; >500 = actionable risk
Limitations and best practice
Bazett over-corrects at fast rates (false alarms in tachycardia) and under-corrects at slow ones — the FDA QT-validation guidance prefers linear models for that reason.
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
Which QTc formula is best?
Do ectopics count?
Source & methodology
Rautaharju Am Heart J 2004.
Corrected QT Interval: what this tool answers
This page was written for one job: turning your own inputs into a defensible figure, with the working shown. Corrected QT Interval is the kind of number people argue about from memory. This page ends that argument with arithmetic you can watch happen: enter your figures, get QTc (selected) in real time, and see every substitution printed below.
There is a habit that separates people who get useful answers from Clinical Calculators numbers: they decide what question the figure answers before they compute it. Corrected QT Interval answers “QTc (selected)?” — and nothing else, which is precisely why the result is trustworthy.
Trends beat snapshots. Corrected QT Interval measured once tells you where you stand today; measured monthly against Measured QT (ms) tells you which way the needle is moving — and direction is the actionable information in Clinical Calculators.
Every figure in the prose is generated by the engine rather than written beside it — the worked example resolves to qTc (selected) = 410 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.
How Corrected QT Interval is computed, line by line
The engine implements a single stated model: QTc by selected formula (Bazett/Fridericia/Hodges/Framingham); normal <450 M / <460 F. There is no hidden coefficient and no login-gated precision — the formula printed is the formula executed, which is also what makes the result citable.
Measured QT (ms) (example value 413) is a plain measured quantity — the unit shown in the label is the unit the formula expects. This input feeds the formula directly; a defensible figure here is worth more than false precision anywhere else.
RR interval (ms) (example value 1,013) is a plain measured quantity — the unit shown in the label is the unit the formula expects. 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 58) 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.
Formula (example value Bazett) is a choice rather than a quantity — pick the option that matches your situation, because the calculation path branches here. Replace the example with the figure from your own records — the tool recomputes instantly and the printed steps follow your numbers.
Sex (example value Male) is a choice rather than a quantity — pick the option that matches your situation, because the calculation path branches here. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
What each “Formula” choice actually does: Bazett → 410 ms · Fridericia → 411 ms · Hodges → 410 ms · Framingham → 411 ms These figures are computed from this engine, not estimated — switch the dropdown above and you will land on exactly these numbers.
Corrected QT Interval on the example inputs: the complete substitution
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), RR interval (ms), Heart rate (bpm), Formula. The endpoint of that chain is 410 ms of qTc (selected), sitting in the “A QTc of 410 ms is within normal limits for this method” band.
- RR = 1,013 ms = 1.01 s
- Chosen method gives 410 ms; Bazett would give 410 ms
- Automated ECG QTc is unreliable at extremes — measure manually and average leads II/V5
Bazett QTc — 410 ms. Compare it against the main result of 410 ms before drawing conclusions.
Fridericia QTc — 411 ms. Read next to the headline, this is the figure that shows the shape of the trade-off.
Thresholds — 450 ms upper limit. If your inputs change, this is usually the first companion figure to move.
Rate-dependence warning — methods agree near this rate. Read next to the headline, this is the figure that shows the shape of the trade-off.
Keep this worked example open while you enter your own figures: matching the shape of the substitution is the fastest way to spot a unit or convention mistake.
How far Corrected QT Interval can reasonably move
Read this table as the practical error bar on the headline figure. It answers the question people actually have after seeing a single number: how wrong could this be if the main input is off by ten or twenty percent?
| Measured QT setting | QTc (selected) |
|---|---|
| 80 % of the example value | 328 ms |
| 90 % of the example value | 369 ms |
| example inputs | 410 ms |
| 110 % of the example value | 451 ms |
| 120 % of the example value | 492 ms |
Across 5 settings of Measured QT, the result spans 328 ms to 492 ms. A decision that only works at one row of this table is a decision waiting for an input to move. Reproduce it in the form above by editing Measured QT alone; the substitution steps will follow your figures exactly.
The levers behind Corrected QT Interval, ranked by effect
Nobody should have to guess which input matters. The engine was re-run once per field below — ten percent up, ten percent down, everything else frozen — and the results ranked by how far they moved the qTc (selected). Read it as a map of where Corrected QT Interval 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 451 ms, ten percent down gives 369 ms — ≈10.0 % of the qTc (selected) either way
- RR interval +10 % → Corrected QT Interval = 391 ms · −10 % → 433 ms (the qTc (selected) shifts ≈5.4 %)
Measured QT is the dominant lever here: ten percent either way moves the result by about 10.0 %, from 369 ms to 451 ms. That is where a careless figure does the most damage, so check it against a source, and treat RR interval as the second priority (≈5.4 % 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.
Is your Corrected QT Interval good — reading the result bands
Nobody acts on a bare figure — they act on a figure with a comparison attached. The verdict attached to the default calculation — “A QTc of 410 ms is within normal limits for this method” — is one band on a ladder your own figure may sit somewhere else on. Before treating the band as fact, note its width: ten percent either way on Measured QT already spans 369 ms to 451 ms. The practical reading rules: ask what would have to change to move you one band. If the answer is “a small, plausible input difference”, the band itself is not the finding — the uncertainty is.
Where Corrected QT Interval sits next to similar measures: this tool answers “how much is the Corrected QT Interval”, while the related calculators answer “what do I do about it”. Reading the result alone tends to over-weight a single snapshot; pairing it with the companion tools above turns the number into a decision.
Common mistakes with Corrected QT Interval (and the fix)
Bazett over-corrects at fast rates (false alarms in tachycardia) and under-corrects at slow ones — the FDA QT-validation guidance prefers linear models for that reason. 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.
Reporting the headline and quietly discarding Bazett QTc. The companions are where the shape of the trade-off survives the rounding.
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.
The subtlest error is answering the wrong question precisely. Confirm that qTc (selected) is actually the quantity your decision needs — the arithmetic is flawless either way, which is what makes the mistake invisible.
Corrected QT Interval verification checklist
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Check that every field the formula needs actually arrived from a document, not from recollection.
- Put Measured QT under a magnifier; the sensitivity table above is the reason why.
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Confirm the example values are plausible for your own situation before replacing them.
- Run the ±10 % stress on the top two levers and check that your conclusion survives both.
A routine for decision-grade Corrected QT Interval
Using a calculator well sounds trivial, but the difference between a decision-grade figure and a throwaway one lives entirely in the routine. The routine for Corrected QT Interval takes a minute to learn and applies to every input above.
Step 1 — collect, then type. Everything this tool asks for is listed above (measured qt (ms), rr interval (ms), heart rate (bpm), formula, sex). 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. Rounding early is the quietest way to introduce a permanent error.
Step 2 — baseline before exploration. Get one defensible QTc (selected) on screen before you start nudging numbers. Without a baseline, every variation looks interesting and none of them means anything. Rounding early is the quietest way to introduce a permanent error.
Step 3 — measure what deserves measuring. The ranked levers above tell you where a careful figure pays off. Everything else can stay approximate, which is a licence to stop hunting for data you do not need. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.
Step 4 — stress before you commit. A decision that survives a ten-percent squeeze on its two biggest inputs needs no luck to be sound. One that does not survive is a decision about hope, not about arithmetic. Most surprises in numbers are surprises about inputs, not about formulas.
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. The input you reach for first is usually the one you measured longest ago.
From calculation to decision: where Corrected QT Interval earns its keep
In Clinical Calculators the figure computed here is usually the first one somebody asks to see. Having it — with the formula behind it — changes your position in the conversation from participant to reference. Rounding early is the quietest way to introduce a permanent error.
The companion outputs — Bazett QTc and Fridericia QTc — 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. Precision is a promise you make about your data; make sure your inputs can keep it.
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. Write the assumptions in the same file as the result, and future arguments take minutes instead of meetings.
The questions that come up most about Corrected QT Interval
What affects Corrected QT Interval the most?
Corrected QT Interval is driven above all by Measured QT. Perturbing only that field by ten percent either way produced 451 ms against the 410 ms baseline — a ≈10.0 % response, while RR interval manages ≈5.4 %.
Which matters more: rr interval or measured qt?
Measured QT, by a factor of about 1.8. Concretely: ten percent on Measured QT is worth ≈10.0 % of Corrected QT Interval, the same nudge on RR interval only ≈5.4 %. That ranking is re-computed for each page from that page’s own defaults.
How much does the “Formula” choice change the answer?
Across every option this tool offers, the qTc (selected) ranges from 410 ms (Hodges) to 411 ms (Fridericia) with all other inputs held at their example values. The per-option table is printed in the inputs section — this is a decision that genuinely moves the number, not a cosmetic dropdown.
What is a typical Corrected QT Interval?
On the worked example above Corrected QT Interval comes out at 410 ms. Treat that as one point on a distribution rather than a norm: the bands, the companion outputs and the sensitivity ranking tell you how wide the plausible range really is.
How do I calculate Corrected QT Interval by hand?
QTc by selected formula (Bazett/Fridericia/Hodges/Framingham); normal <450 M / <460 F is the entire method. Work the inputs in that order — RR = 1,013 ms = 1.01 s — and you will reach 410 ms on the default figures, the same number this tool prints.
What happens to Corrected QT Interval if measured qt changes by 10 %?
Try it in the form above and the printed substitution will follow you; the pre-computed answer for the defaults is 369 ms to 451 ms across the ten-percent probe, i.e. ≈10.0 % sensitivity on the qTc (selected).
Is the Corrected QT Interval result of 410 ms good or bad?
The tool answers “what”, not “whether”: 410 ms carries the verdict “A QTc of 410 ms is within normal limits for this method” 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 often should I check Corrected QT Interval?
Whenever an input changes by more than a few percent, and otherwise on the rhythm of your decisions — before a purchase, a review, a renewal. Corrected QT Interval is a snapshot, not a subscription; stale snapshots are the most common silent error in Health & Medical planning.
When should I take Corrected QT Interval to a professional?
Whenever the verdict band is far from your everyday range, or the inputs include medication-influenced values. Corrected QT Interval is a screening lens — a clinician holds the diagnostic instruments.
Which QTc formula is best?
No consensus winner; Fridericia or Hodges for rate extremes, Framingham in population studies.
Do ectopics count?
Measure sinus beats — QT after a premature complex is unreliable; average 3–5 consecutive beats.
Can this tool see the numbers I type?
It cannot. There is no submission step: the calculation happens in your browser and the result disappears when you close the page. Treat the fields above as a scratchpad rather than a form.
The vocabulary of Corrected QT Interval
- Bazett QTc
- 410 ms for the Corrected QT Interval worked case; it sits next to the main result because it carries context the headline leaves out.
- Fridericia QTc
- 411 ms on the example inputs — a companion figure printed beside the headline of Corrected QT Interval so that no result is read in isolation.
- Thresholds
- 450 ms upper limit for the Corrected QT Interval worked case; it sits next to the main result because it carries context the headline leaves out.
- QTc (selected)
- The primary output of this tool: 410 ms for the default inputs, computed exactly from QTc by selected formula (Bazett/Fridericia/Hodges/Framingham); normal <450 M / <460 F.
- Worked example
- The step-by-step substitution printed above, so the arithmetic can be re-derived by hand or in a spreadsheet.
- Verdict band
- A named zone on the range of plausible results; the example inputs land in “A QTc of 410 ms is within normal limits for this method”, and your inputs may not.
- Sensitivity (±10 %)
- How far the Corrected QT Interval moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Where the Corrected QT Interval model comes from
There is no black box on this page — the engine evaluates QTc by selected formula (Bazett/Fridericia/Hodges/Framingham); normal <450 M / <460 F. Source of record: Rautaharju Am Heart J 2004. Specialist software layers extra coefficients on top of the same algebra; none are hidden here, and that transparency is what makes a disagreement traceable to a specific input instead of a black box.
This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. Nothing is smoothed, weighted or adjusted behind the scenes; where judgement enters, this page says so and names the input responsible.
Turning this Corrected QT Interval into a decision
Save the number together with its inputs and today’s date; a Corrected QT Interval figure without context is unusable the moment an input changes.
This page was written for people who need one trustworthy number before their next decision, and the promise is that nothing above required trust. Bookmark the tool, re-run it whenever an input moves by more than a few percent, and keep the date with the number.