QTc Interval Calculator (Bazett & Fridericia)
Educational tool, not a medical device. This calculator and the Caliper Lab game below are learning aids for understanding how QT rate-correction arithmetic works. They are not diagnostic software, not a medical device, and not clinical advice. QT and QTc interpretation belongs with a qualified clinician reading a real 12-lead ECG alongside the rhythm, symptoms, medicines and electrolytes. Nothing here should be used to make a decision about anyone's care.
Introduction: why a raw QT interval cannot be compared across heart rates
The QT interval is the time from the start of ventricular depolarization to the end of ventricular repolarization on an ECG—classically measured from the beginning of the QRS complex (often the Q wave) to the end of the T wave. Because QT duration changes with heart rate, clinicians often use a corrected QT (QTc) to compare QT values across different heart rates.
Two ECGs with an identical measured QT of 400 ms can carry very different meanings if one was recorded at 50 bpm and the other at 120 bpm. Rate correction is an attempt to express both on a common scale: the QT the same heart would have shown at 60 bpm. Every correction formula on this page is a different guess at that relationship, and this page exists to make the size of the disagreement between those guesses visible.
This QTc calculator estimates corrected QT using four established rate-correction methods:
- Bazett (most commonly reported, but biased at very fast/slow heart rates)
- Fridericia (often more reliable than Bazett at higher heart rates)
- Framingham (a linear correction based on the RR interval)
- Hodges (a linear correction based on heart rate)
Before calculating QTc: what to enter
Measured QT interval (ms)
For a QTc calculation, enter the measured QT interval in milliseconds. On a standard ECG, QT is typically measured from the beginning of QRS to the end of the T wave (avoid including U waves unless clinically indicated). Measurement approaches vary by rhythm, lead selection, and guideline.
RR interval for QT correction (seconds)
For the RR-based QTc formulas, enter the RR interval in seconds (time between two consecutive R peaks), or enter heart rate and let the calculator convert it:
- RR (seconds) = 60 / HR
QTc correction formulas and units
These QTc formulas use RR in seconds. This tool accepts RR in seconds or converts from heart rate using RR seconds = 60 / heartRateBpm.
Bazett QTc correction
Formula: QTc = QT / sqrt(RR)
Where QT is in milliseconds and RR is in seconds (after conversion). Conceptually, this is:
Plain-text Bazett formula: QTc = QT / sqrt(RR)
Fridericia QTc correction
Formula: QTc = QT / (root(RR, 3))
Conceptually:
Plain-text Fridericia formula: QTc = QT / RR^(1/3)
Framingham QTc correction
The Framingham QTc equation uses a linear RR relationship rather than a power of RR, so its behavior differs from Bazett and Fridericia at fast and slow rates:
Formula: QTc = QT + 154 · (1 − RR)
Conceptually:
Plain-text Framingham formula: QTc = QT + 154 * (1 - RR)
Hodges QTc correction
The Hodges QTc equation corrects against heart rate directly rather than against the RR interval:
Formula: QTc = QT + 1.75 · (HR − 60)
Conceptually:
Plain-text Hodges formula: QTc = QT + 1.75 * (HR - 60)
Each QTc equation returns the measured QT at a heart rate of 60 bpm, where second. Away from that rate, the correction methods can produce different estimates, which is why the calculator displays all four rather than treating one result as interchangeable with another.
Comparing the Bazett and Fridericia corrections
Bazett and Fridericia both estimate QTc from the same QT and RR inputs, but they divide by different powers of RR. The table also places their outputs alongside the Framingham and Hodges corrections:
| Feature | Bazett (QT / RR1/2) | Fridericia (QT / RR1/3) | Framingham (linear in RR) | Hodges (linear in HR) |
|---|---|---|---|---|
| Correction at HR 60 (RR 1 s) | QTc = QT | QTc = QT | QTc = QT | QTc = QT |
| At fast heart rates | Overestimates QTc (over-corrects) | Closer to true rate-independent value | Modest, well-behaved correction | Modest, well-behaved correction |
| At slow heart rates | Underestimates QTc | More stable | Stable | Stable |
| Reporting | Most widely reported; default on many ECG machines | Preferred in drug-safety and research settings | Common in epidemiology | Easy to compute at the bedside |
| Example (QT 420 ms, RR 0.8 s, HR 75) | 469.6 ms | 452.4 ms | 450.8 ms | 446.3 ms |
For the QT 420 ms and RR 0.8 s example, this calculator returns the values shown in the final row. Bazett and Fridericia differ by about 17 ms for that beat, and the gap can widen as the heart rate moves farther from 60 bpm. A Bazett QTc near a clinical threshold at a high heart rate merits careful measurement and clinical interpretation.
How to interpret QTc results in context
QTc interpretation depends on the correction formula, age, sex, clinical setting, ECG measurement method, and medication status. The following adult ranges are a general educational overview, not diagnostic or treatment thresholds:
| QTc category | Typical adult thresholds (approx.) | What it may mean |
|---|---|---|
| Normal | Men: < 450 ms; Women: < 460 ms | Usually low concern in isolation; interpret with symptoms and clinical context. |
| Borderline | Men: ~450–470 ms; Women: ~460–480 ms | May warrant review of medications, electrolytes, and repeat ECG depending on context. |
| Prolonged | ≥ 470 ms (men) or ≥ 480 ms (women) (varies) | Higher concern; consider causes (drugs, electrolytes, congenital, ischemia, etc.). |
| Markedly prolonged | ≥ 500 ms (commonly used high-risk flag) | Associated with increased risk of torsades de pointes, especially with triggers. |
Bazett versus Fridericia: Bazett is widely reported but can overestimate QTc at high heart rates and underestimate at low heart rates. Fridericia may perform better at higher heart rates, but no correction is perfect—especially in irregular rhythms.
Worked example: calculating QTc from QT 420 ms and RR 0.8 seconds
Scenario: For an ECG with QT = 420 ms and RR = 800 ms (equivalent to 0.8 s and approximately 75 bpm), the four QTc equations use the same measured interval but apply different rate corrections.
- Convert RR to seconds: RR = 800 ms = 0.8 s
- Bazett: QTc = 420 / √0.8 = 420 / 0.894 ≈ 470 ms
- Fridericia: QTc = 420 / (0.8)^(1/3) = 420 / 0.928 ≈ 453 ms
- Framingham: QTc = 420 + 154 × (1 − 0.8) = 420 + 30.8 = 450.8 ms
- Hodges: QTc = 420 + 1.75 × (75 − 60) = 420 + 26.25 = 446.3 ms
In this QTc example, the highest and lowest correction differ by roughly 23 ms even though the measured QT and RR are identical. Formula choice therefore matters most when a result is close to a decision threshold or the heart rate is notably different from 60 bpm.
The calculator’s side-by-side QTc results make that rate-correction spread explicit; they do not replace review of the ECG tracing, rhythm, symptoms, medicines, or electrolytes.
Practical tips for QT interval measurement
Reliable QTc output begins with a defensible QT and RR measurement from the ECG.
- Lead selection matters: QT can differ slightly by lead; many clinicians measure in leads where the T-wave end is clearest.
- Use a representative beat: Avoid ectopic beats and measure in a stable segment.
- Confirm rhythm regularity: QT correction is less reliable with irregular RR intervals (e.g., atrial fibrillation).
- Consider clinical context: Symptoms (syncope, palpitations), family history, and meds/electrolytes are critical.
Reading intervals off ECG graph paper at 25 mm/s
Standard ECG recording in most of the world runs the paper at 25 mm per second with a gain of 10 mm per millivolt. On that paper, one small square is 1 mm wide and one large square is 5 mm wide, so the horizontal grid converts directly into time:
Formula: interval (ms) = (width (mm) × 1000) / 25 = width (mm) × 40
One small square is therefore 40 ms and one large square is 200 ms. A QT interval spanning two large squares plus one small square measures 2 × 200 + 40 = 440 ms. The same arithmetic gives the RR interval, and heart rate follows from it as 60 divided by RR in seconds. The Caliper Lab game further down the page drills exactly this step, because a caliper misplaced by a single small square shifts QTc by roughly 40 ms — more than the entire gap between two correction formulas at a normal heart rate.
QTc calculation assumptions and limitations
These QTc estimates depend on the entered measurements and on rate-correction equations that cannot account for every clinical circumstance.
- Educational use: This calculator provides estimates and is not a diagnosis or treatment tool.
- RR unit conversion: The formulas assume RR in seconds. Entering RR in the wrong unit will produce incorrect results.
- Irregular rhythms: In atrial fibrillation or frequent ectopy, a single RR interval may not represent typical cycle length; corrections can be unreliable.
- Heart-rate extremes: Bazett is notably biased at very fast or very slow heart rates; Fridericia may be preferable at higher HR, but neither is perfect.
- Measurement variability: Manual vs automated QT, U waves, T-wave morphology, and baseline artifact can change QT by tens of milliseconds.
- Population differences: Pediatrics, pregnancy, bundle branch block, pacing, and certain cardiomyopathies may require specialized interpretation.
- Threshold variability: “Normal” and “prolonged” cutoffs vary among guidelines and clinical settings.
When a QTc result needs medical advice
If an ECG or this QTc estimate suggests a markedly prolonged result (often cited as ≥ 500 ms), or if you have fainting, seizures, or sustained palpitations—especially while taking QT-prolonging medications—seek urgent medical evaluation. Discuss QT and QTc results with a qualified clinician.
Sources. Every QTc correction implemented here is a published formula, reproduced in full above so any result can be checked by hand. Clinical thresholds vary between guidelines and are given as approximate educational ranges rather than decision rules.
- QT measurement convention and rate correction, primary reference: Rautaharju PM, Surawicz B, Gettes LS, et al. "AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part IV: The ST Segment, T and U Waves, and the QT Interval", Circulation 2009;119:e241–e250 (PMID 19228821). This scientific statement defines QT as running from the earliest onset of the QRS complex to the end of the T wave, and reviews the rate-correction problem including Bazett's known bias.
- Recording convention (25 mm/s paper speed, 10 mm/mV gain) used by the graph paper in the Caliper Lab game: Kligfield P, Gettes LS, Bailey JJ, et al. "AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part I", Circulation 2007;115:1306–1324.
- Bazett (QT / RR1/2) and Fridericia (QT / RR1/3) are the two original 1920 rate corrections and remain the most widely reported.
- Framingham linear correction, QTc = QT + 154 × (1 − RR): Sagie and colleagues, "An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)", Am J Cardiol 1992, fitted to 5,018 subjects. Published in seconds as QTc = QT + 0.154 × (1 − RR), which is the same equation once both QT terms are expressed in milliseconds.
- Hodges linear correction, QTc = QT + 1.75 × (HR − 60), published in 1983 and computed against heart rate rather than cycle length.
- A practical comparison of the four corrections and their rate-dependent bias: American College of Emergency Physicians Toxicology Section, "QTc: So many formulae, but which one to use?".
Not a medical device. This page performs published arithmetic on numbers you type in. It does not read an ECG, does not diagnose, and does not produce a clinical recommendation.
How to use this QTc interval calculator
- Enter the QT interval in milliseconds, measured from the start of the QRS complex to the end of the T wave.
- Enter the RR interval in seconds (the time between two R peaks). If you only know the heart rate, leave RR blank and fill in the heart-rate field instead — the calculator applies RR seconds = 60 / heart rate.
- Press Calculate QTc to see the Bazett, Fridericia, Framingham, and Hodges estimates together, along with the derived heart rate and a plain-language flag when a value falls in a prolonged range.
- Compare the QTc corrections: when they differ meaningfully and the heart rate is far from 60 bpm, recheck the ECG measurement and use clinical judgment about the appropriate formula.
- Watch the two guard rails. The calculator refuses a cycle length implying a heart rate below 20 or above 250 bpm, which is almost always RR typed in milliseconds instead of seconds, and it refuses a QT that is longer than the whole RR cycle, which is physiologically impossible and usually means the T-wave end was placed past the next QRS.
QTc interval: frequently asked questions
What Is the QT Interval?
On an ECG, the QT interval runs from the beginning of ventricular depolarization to the completion of ventricular repolarization, conventionally from the beginning of the QRS complex or Q wave to the end of the T wave. It is measured in milliseconds. A long QT interval can be associated with susceptibility to arrhythmias such as torsades de pointes, and medicines, electrolyte disturbances, and congenital conditions can affect it. The measurement should be interpreted by a clinician in its clinical setting.
Which correction formula should I use?
Bazett is commonly printed by ECG machines, but its rate-dependent bias tends to over-correct at fast heart rates and under-correct at slow ones. Fridericia often gives a less extreme correction when the heart rate is elevated. This calculator also displays the linear Framingham and Hodges estimates, so the effect of formula choice is visible. A clinician should decide which reported value is appropriate for the setting.
Why do the four formulas disagree by 20 ms or more?
The four QTc equations use different relationships between QT duration and cycle length. They all return the measured QT at 60 beats per minute, when RR is one second, but separate as the heart rate moves away from 60. The size and direction of the difference depend on the measured QT and RR interval.
Does QT correction work in atrial fibrillation?
A QTc based on one beat is less reliable in atrial fibrillation because consecutive RR intervals vary. The selected beat and its preceding cycle length can substantially affect the result. Measurements over multiple beats or beats with comparable preceding intervals may be used clinically, but interpretation remains more difficult than in a regular rhythm.
How many milliseconds is one small square on ECG paper?
At the standard paper speed of 25 mm per second, one small 1 mm square is 40 ms and one large 5 mm square is 200 ms. An interval in milliseconds is its width in millimetres multiplied by 40. Because a single small square is 40 ms, a caliper placed one square off changes the QT by 40 ms, which is larger than the disagreement between most correction formulas at a normal heart rate.
Why Correct the QT Interval?
QT duration normally changes with heart rate, becoming shorter when the rate is faster. QT correction formulas use the RR interval or heart rate to express the measurement as QTc, an estimate intended to make values from different rates more comparable. QTc is not a diagnosis, and reference ranges and clinical thresholds vary by guideline, patient characteristics, ECG method, and clinical context.
Caliper Lab: measure QT and RR on a scrolling rhythm strip
A synthetic rhythm strip scrolls across standard pink ECG paper at 25 mm per second and settles. Each round you place a pair of calipers twice: first from the onset of the QRS complex to the end of the T wave to read the QT interval, then across two consecutive R peaks to read the RR interval. The lab then feeds your measurement through Bazett, Fridericia, Framingham and Hodges and lines the four answers up on one bar, so you can watch them separate as the heart rate moves away from 60 bpm. Points come only from how accurately the calipers land — this is a measuring drill, never an assessment of a person.
Keyboard, with the strip focused: ← and → slide the active caliper leg by 8 ms per press; ↑ and ↓ switch between the left leg and the right leg; Space or Enter sets the caliper and moves on; R restarts the current strip. Pointer or touch: press on the strip to grab the nearer caliper leg, drag left or right to position it, and release. The leg you touched stays active, so a second drag anywhere on the strip keeps moving it. The buttons above do the same job as the keys.
Grid reminder: at 25 mm/s one small square is 40 ms and one large square is 200 ms, so the live caliper readout shows both milliseconds and squares. Scoring: up to 600 points for the QT caliper and 400 for the RR caliper on each of five strips, scaled by the average placement error of the two legs. Land both legs within one small square and you keep almost everything; miss by 50 ms or more and that caliper scores zero.
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Press Start Caliper Lab, then focus the strip and move the calipers with the arrow keys, a mouse or a finger.