# EKG Calculator

> Free EKG/ECG interval calculator computing QTc (Bazett and Fridericia), heart rate from RR, plus PR and QRS interpretation against clinical cutoffs.

**URL:** https://calculators.im/ekg-calculator  
**Category:** health  
**Last updated:** 2026-07-04  

Use our free EKG calculator to compute QTc using both Bazett and Fridericia formulas, derive heart rate from RR interval, and classify PR and QRS durations against clinical cutoffs.

ECG interval analysis is the foundation of cardiac rhythm interpretation. Our EKG calculator handles the four most common interval calculations: heart rate from RR interval (or vice versa), QTc using both Bazett (QT/√RR) and Fridericia (QT/∛RR) corrections, PR interval categorization, and QRS duration assessment. Bazett is the historical standard but tends to overcorrect at high heart rates and undercorrect at low heart rates — Fridericia is generally more accurate. Each interval is classified against standard clinical cutoffs.

## Formula

```
QTc(Bazett) = QT/√RR  •  QTc(Fridericia) = QT/∛RR  •  HR = 60000/RR
```

**Variables:**

- **QT** — QT interval (ms) — measured on ECG
- **RR** — RR interval (ms) — beat-to-beat distance
- **QTc** — Heart-rate corrected QT interval
- **PR** — PR interval (ms) — atrial to ventricular conduction
- **QRS** — QRS duration (ms) — ventricular depolarization

The QT interval lengthens at slow heart rates and shortens at fast ones, so it must be corrected before comparing values across patients. Bazett's formula divides by the square root of RR; Fridericia uses the cube root, which is more accurate at extreme heart rates. PR > 200 ms = first-degree AV block; QRS > 120 ms = bundle branch block.

## Worked Example

### Example: HR 75, QT 400, PR 160, QRS 90

RR = 60000/75 = 800 ms. QTc (Bazett) = 400/√0.8 = 447 ms — borderline (440-460). PR 160 normal, QRS 90 normal.

## How to Use the EKG Calculator

1. **Enter heart rate or RR** — Provide either HR or RR interval — the other is calculated.
2. **Enter QT** — Input the measured QT interval in ms.
3. **Optional: PR and QRS** — Add PR and QRS for complete interval interpretation.
4. **Read QTc** — Review Bazett and Fridericia QTc plus interval categories.

## Pro Tips

- QTc normal: <440 ms men, <460 ms women
- QTc >500 ms = high risk for torsades de pointes
- PR interval normal: 120-200 ms; >200 = first-degree AV block
- QRS normal: <120 ms; >120 = bundle branch block
- Bazett overcorrects at high HR; Fridericia is more reliable at extreme rates
- Many medications prolong QTc — always check before prescribing in patients with borderline QTc

## Frequently Asked Questions

### What is an EKG calculator?

An EKG calculator computes corrected ECG intervals (especially QTc), classifies PR and QRS durations, and helps interpret rhythm strips against clinical cutoffs. It's commonly used by clinicians and cardiology students.

### How is QTc calculated?

Bazett formula: QTc = QT / √RR (in seconds). Fridericia formula: QTc = QT / ∛RR. Both correct for heart rate. Fridericia is generally more accurate, especially at high or low heart rates.

### What is a normal QTc?

Normal QTc is under 440 ms for men and 460 ms for women. 440-460 is borderline. >460 (men) or >480 (women) is prolonged. >500 ms is severely prolonged and carries high risk for torsades de pointes arrhythmia.

### What causes QT prolongation?

Many drugs (antiarrhythmics, antibiotics, antipsychotics, antifungals), electrolyte abnormalities (low K, Mg, Ca), hypothermia, and congenital long QT syndrome. Always check current medications before prescribing in patients with borderline QTc.

### What is a normal PR and QRS interval?

PR: 120-200 ms (>200 = first degree AV block, >300 = severe). QRS: <120 ms (>120 = bundle branch block, >150 = wide QRS often suggesting LBBB).

## Deep Dive

Electrocardiogram (ECG/EKG) interpretation is a fundamental clinical skill required of every physician, nurse, and paramedic. Among the most critical measurements on a 12-lead ECG are the QT interval and its rate-corrected value, QTc. The QT interval represents the total time for ventricular depolarization and repolarization, and when prolonged beyond 500 milliseconds (corrected), it significantly increases the risk of torsades de pointes — a life-threatening ventricular arrhythmia. Over 100 commonly prescribed medications can prolong the QT interval, including certain antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, quetiapine), and antiarrhythmics (amiodarone, sotalol). The Bazett formula (QTc = QT divided by the square root of RR) has been the clinical standard since 1920, though the Fridericia correction (QTc = QT divided by the cube root of RR) is increasingly preferred for its accuracy at extreme heart rates. This EKG calculator computes QTc using both Bazett and Fridericia formulas simultaneously, derives heart rate from the RR interval, classifies the QTc as normal or prolonged based on sex-specific thresholds, and flags clinically significant prolongation requiring immediate attention.

### Why correct QT for heart rate?

The QT interval physiologically lengthens at slow heart rates and shortens at fast ones. Without correction, comparing QT values across different heart rates would be meaningless. Bazett's 1920 formula is the most commonly cited but tends to overcorrect at high HR. Modern guidelines often prefer Fridericia or Framingham corrections for more reliable results.

### What Do the QT, PR, and QRS Intervals Measure on an EKG?

**Each EKG interval times a specific electrical event in the heart**, measured horizontally across the tracing in milliseconds. The **PR interval** captures the time from the start of atrial depolarization to the start of ventricular depolarization — essentially how long the signal takes to cross the AV node. The **QRS duration** measures ventricular depolarization, and the **QT interval** spans the full cycle of ventricular depolarization plus repolarization.

Each interval maps to a labeled segment of the waveform:

- PR interval: from the beginning of the P wave to the beginning of the QRS complex
- QRS duration: from the start of the Q wave to the end of the S wave
- QT interval: from the start of the Q wave to the end of the T wave

Standardized definitions for these measurements come from the **American Heart Association (AHA)**, the **American College of Cardiology (ACC)**, and the **Heart Rhythm Society (HRS)**, whose joint recommendations govern how ECG intervals are recorded and reported.

### How to Calculate QTc Step by Step From an EKG Strip

**To calculate QTc, measure the QT interval and the RR interval in seconds, then apply a correction formula.** The RR interval is the distance between two consecutive R waves; QTc adjusts the raw QT so it can be compared regardless of heart rate.

Work through it in order:

- Measure QT from the start of the Q wave to the end of the T wave (e.g., 400 ms = 0.40 s)
- Measure RR between two R waves, or derive it as 60,000 divided by heart rate (at 75 bpm, RR = 800 ms = 0.80 s)
- Apply Bazett: QTc = QT / √RR = 0.40 / √0.80 = 0.447 s = **447 ms**
- Apply Fridericia: QTc = QT / ∛RR = 0.40 / ∛0.80 = **431 ms**

A QTc near 447 ms falls in the borderline range. This calculator runs both formulas automatically, but understanding the arithmetic helps you sanity-check any tool. The **American Heart Association** provides the reference standards behind these calculations.

### Bazett vs Fridericia: Which QTc Formula Should You Use?

**Fridericia is generally more accurate than Bazett at very fast or very slow heart rates**, which is why many modern protocols favor it. Bazett divides QT by the square root of RR, while Fridericia divides by the cube root, producing a flatter, more stable correction curve.

The practical differences matter most at the extremes:

- At high heart rates, **Bazett tends to overcorrect**, reporting a longer QTc than reality and potentially triggering false alarms
- At low heart rates, **Bazett tends to undercorrect**, understating true prolongation
- **Fridericia** (and the Framingham linear correction) reduces this rate dependence

Despite its limitations, Bazett remains the historical clinical standard cited in most references and drug labeling. Guidance from the **American College of Cardiology** and the **Heart Rhythm Society** recognizes both; when heart rate is far from 60 bpm, cross-check Bazett against Fridericia rather than relying on a single value.

### What Is a Normal QTc for Men Versus Women?

**A normal QTc is generally under 440 ms in men and under 460 ms in women**, with women having a physiologically longer baseline. Values between these cutoffs and roughly 460–480 ms are considered borderline, and higher values are prolonged.

The widely used thresholds break down as:

- Normal: below 440 ms (men) / below 460 ms (women)
- Borderline: approximately 440–460 ms
- Prolonged: above 460 ms (men) / above 480 ms (women)
- Severely prolonged: above **500 ms**, regardless of sex

A QTc above 500 ms carries the highest risk for **torsades de pointes**, a dangerous ventricular arrhythmia. These sex-specific cutoffs are drawn from consensus documents by the **American Heart Association**, **American College of Cardiology**, and **Heart Rhythm Society**. Always interpret a single number alongside the patient's symptoms, medications, and prior tracings rather than in isolation.

### How to Read the PR Interval and Detect AV Block

**A normal PR interval runs from 120 to 200 milliseconds; anything longer than 200 ms indicates first-degree AV block.** The PR interval reflects how long the electrical impulse takes to travel from the atria through the AV node to the ventricles, so a prolonged value signals delayed conduction.

Read the PR interval against these landmarks:

- 120–200 ms: normal AV conduction
- Greater than 200 ms: **first-degree AV block** (delayed but every impulse conducts)
- Progressively lengthening or dropped beats: higher-degree (second- or third-degree) block
- Shorter than 120 ms: possible pre-excitation, as seen in Wolff-Parkinson-White

First-degree block is often benign, but higher-degree blocks can cause symptomatic bradycardia. The conduction-system framework and these cutoffs follow **American Heart Association** and **Heart Rhythm Society** standards. This calculator classifies your PR value automatically, but a full rhythm strip is needed to distinguish first-degree block from more serious types.

### What QRS Duration Tells You About Bundle Branch Block

**A QRS duration of 120 milliseconds or more indicates a bundle branch block or another intraventricular conduction delay.** The QRS represents how quickly electrical activation spreads through the ventricles; a widened complex means the signal is taking an abnormal, slower path.

Interpret QRS width in tiers:

- Under 120 ms: normal ventricular conduction
- 120 ms or greater: **bundle branch block** (right or left) or nonspecific delay
- Markedly wide (often above 150 ms): frequently associated with **left bundle branch block**

Distinguishing left from right bundle branch block requires examining QRS morphology in specific leads, not just the width. A newly wide QRS with chest symptoms can also signal acute ischemia and warrants urgent evaluation. These duration thresholds come from **American Heart Association** and **American College of Cardiology** ECG standardization recommendations. The calculator flags a wide QRS, but the underlying cause must be confirmed on the full 12-lead tracing.

### Which Medications and Conditions Prolong the QT Interval?

**Dozens of common medications and several electrolyte disturbances can prolong the QT interval**, raising the risk of dangerous arrhythmia. Because effects are additive, patients on multiple QT-prolonging drugs or with abnormal electrolytes face the highest risk.

Frequently implicated contributors include:

- Certain **antibiotics** (macrolides such as azithromycin, and fluoroquinolones)
- Some **antipsychotics** and **antiemetics**
- **Antiarrhythmics** such as sotalol and amiodarone
- **Low potassium, magnesium, or calcium** (hypokalemia, hypomagnesemia, hypocalcemia)
- **Congenital long QT syndrome** and hypothermia

The **U.S. Food and Drug Administration (FDA)** requires QT-liability testing for new drugs, and the **National Institutes of Health (NIH)** maintains resources on inherited long QT syndromes. Before prescribing a QT-prolonging medication to a patient with a borderline QTc, review the full medication list and correct electrolytes. This tool measures QTc but cannot assess drug interactions — always consult current clinical references.

### How to Calculate Heart Rate From the RR Interval

**Heart rate equals 60,000 divided by the RR interval in milliseconds**, because there are 60,000 milliseconds in one minute. This lets you convert a measured beat-to-beat distance into beats per minute, or work backward from a known heart rate to the RR interval.

The relationship works both directions:

- Heart rate (bpm) = 60,000 / RR (ms) — an 800 ms RR gives 75 bpm
- RR (ms) = 60,000 / heart rate — a 60 bpm rate gives a 1,000 ms RR

A quick bedside shortcut on standard paper is to divide 300 by the number of large boxes between R waves. Normal resting adult heart rate is roughly **60 to 100 bpm** per the **American Heart Association**; below 60 is bradycardia and above 100 is tachycardia. Accurate RR measurement matters because QTc correction depends directly on it — an error in RR propagates straight into the QTc result.

### Common Mistakes When Measuring QT and Calculating QTc

**The most common QTc errors come from mismeasuring the QT interval or applying the wrong units in the formula.** Small measurement slips are amplified by the correction math, so precision at the source is essential.

Watch for these frequent pitfalls:

- **Mixing milliseconds and seconds** — Bazett and Fridericia expect RR in seconds; feeding raw milliseconds gives nonsense results
- **Ending the QT at the wrong point** — the interval ends where the T wave returns to baseline, not at its peak; a prominent U wave is easily mistaken for the T-wave end
- **Using Bazett at extreme heart rates** without cross-checking Fridericia
- **Measuring in a single lead** — the longest QT across several leads is more reliable
- **Ignoring sex-specific cutoffs**, applying the male threshold to a female patient or vice versa

Guidance from the **American Heart Association** and **Heart Rhythm Society** stresses consistent lead selection and clear T-wave endpoints. When a QTc looks surprisingly high or low, re-measure before acting on it, and treat the tool's output as a check on your own reading, not a replacement for it.

### When Does a Prolonged QTc Require Urgent Clinical Action?

**A QTc above 500 milliseconds, or an increase of more than about 60 ms from a patient's baseline, is a red flag that warrants prompt clinical attention.** At this level the risk of torsades de pointes rises sharply, and immediate steps may be needed to lower the danger.

High-risk situations to escalate include:

- QTc consistently **above 500 ms** on repeat tracings
- A large jump from a prior baseline QTc
- Prolonged QTc plus symptoms such as **syncope, palpitations, or seizures**
- Prolongation combined with low potassium or magnesium

Typical urgent measures — stopping offending drugs and correcting electrolytes — should be directed by a qualified clinician, not by a calculator. The **American Heart Association**, **American College of Cardiology**, and **Heart Rhythm Society** publish the consensus behind these risk thresholds. This EKG calculator is an educational and workflow aid for trained users; it does not diagnose, and any concerning result must be confirmed and managed by a licensed healthcare professional.

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## Authoritative Sources

- [AHA/ACCF/HRS - ECG Standards](https://www.heart.org/) — Recommendations for the standardization of ECG
- [American College of Cardiology](https://www.acc.org/) — Clinical guidance and educational resources on ECG interpretation, QT interval, and conduction abnormalities.
- [Heart Rhythm Society](https://www.hrsonline.org/) — Professional standards on cardiac arrhythmias, QT prolongation, and torsades de pointes risk.
- [U.S. Food and Drug Administration (FDA)](https://www.fda.gov/) — Regulatory guidance on drug-induced QT prolongation and cardiac safety testing of medications.
- [National Institutes of Health (NIH)](https://www.nih.gov/) — Research and reference material on long QT syndrome and inherited arrhythmia disorders.

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