Reference — Cardiac
ECG Measurements Reference
Accurate ECG interpretation depends on knowing normal measurement ranges and how to calculate them. This reference provides normal values for all major ECG intervals and explains the standard methods for calculating heart rate.
Educational use only. Reference ranges reflect standard adult values. Normal ranges may vary by age, sex, and clinical context. Always correlate ECG measurements with patient assessment and provider interpretation. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
ECG Paper Basics
Standard ECG paper runs at 25 mm/second. Each small box = 0.04 seconds (40 ms); each large box (5 small boxes) = 0.20 seconds (200 ms).
| Unit | Time | Notes |
|---|---|---|
| 1 small box | 0.04 sec (40 ms) | Amplitude = 0.1 mV |
| 1 large box (5 small) | 0.20 sec (200 ms) | Amplitude = 0.5 mV |
| 5 large boxes | 1.0 sec | Used for rate estimation |
| 30 large boxes | 6.0 sec | Standard strip length for 6-second count method |
Normal ECG Measurements
| Measurement | Normal Range | In Small Boxes | What It Represents |
|---|---|---|---|
| Heart Rate | 60–100 bpm | — | Ventricular rate |
| PR Interval | 0.12–0.20 sec | 3–5 small boxes | SA node → AV node → bundle of His conduction |
| QRS Duration | < 0.12 sec | < 3 small boxes | Ventricular depolarization |
| QT Interval | Rate-dependent | Varies with HR | Ventricular depolarization + repolarization |
| QTc (corrected) | ≤ 0.44 sec (M) / ≤ 0.46 sec (F) | ≤ 11 small boxes | Heart-rate corrected QT |
| ST Segment | Isoelectric | Flat at baseline | Early ventricular repolarization |
PR Interval
Measured from the beginning of the P wave to the beginning of the QRS complex. Represents conduction time through the AV node and bundle of His.
- Short (< 0.12 sec): Pre-excitation (WPW), junctional rhythm
- Normal: 0.12–0.20 sec
- Prolonged (> 0.20 sec): First-degree AV block
- Progressive lengthening: Mobitz I (Wenckebach)
- Constant but long, with dropped beats: Mobitz II
- Variable (no consistent relationship): Third-degree AV block
QRS Duration
Measured from the beginning of the Q wave (or R wave if no Q) to the end of the S wave. Represents ventricular depolarization through the His-Purkinje system.
- Normal: < 0.12 sec (narrow QRS — supraventricular origin)
- Wide: ≥ 0.12 sec — causes include: bundle branch block, ventricular rhythm, hyperkalemia, antiarrhythmic toxicity, aberrant conduction
- Wide QRS in tachycardia: Treat as VT until proven otherwise
QT Interval and QTc
The QT interval is measured from the beginning of the QRS to the end of the T wave. Because QT shortens at faster heart rates, the corrected QTc adjusts for rate using the Bazett formula: QTc = QT ÷ √RR interval.
| QTc Value | Interpretation | Clinical Action |
|---|---|---|
| ≤ 440 ms (M) / ≤ 460 ms (F) | Normal | Routine monitoring |
| 441–470 ms (M) / 461–480 ms (F) | Borderline prolonged | Review medications, monitor electrolytes |
| > 500 ms | Significantly prolonged | Notify provider; Torsades de Pointes risk |
Common causes of QT prolongation: Hypokalemia, hypomagnesemia, hypocalcemia, amiodarone, sotalol, quinidine, antipsychotics (haloperidol, ziprasidone), macrolide antibiotics, fluoroquinolones, methadone.
Heart Rate Calculation Methods
300 Method (Regular Rhythms)
Count the large boxes between two consecutive R waves. Divide 300 by that number.
1 box = 300 bpm | 2 = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50 | 7 = 43
1500 Method (Regular Rhythms — More Precise)
Count the small boxes between two consecutive R waves. Divide 1500 by that count.
6-Second Count Method (Irregular Rhythms)
Count the number of R waves (QRS complexes) in a 6-second strip (30 large boxes). Multiply by 10 to estimate the rate per minute.
Use this method for atrial fibrillation, irregular rhythms, or whenever R-R intervals vary.
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with American Heart Association (AHA) · American College of Cardiology (ACC) · AHA ACLS Guidelines. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →