Convert ECG small and large boxes to milliseconds and seconds. Estimate heart rate from RR interval with support for 25 mm/s and 50 mm/s paper speeds.
Reading an electrocardiogram (ECG or EKG) requires understanding the relationship between the grid boxes on ECG paper and time. Standard ECG paper runs at 25 mm/s, creating a grid where each small box (1 mm) represents 40 milliseconds and each large box (5 mm) represents 200 milliseconds. In pediatric or high-detail recordings at 50 mm/s, each small box represents 20 ms and each large box 100 ms.
Accurately converting between boxes and time is essential for measuring PR intervals, QRS duration, QT intervals, and estimating heart rate. A normal PR interval spans 3–5 small boxes (120–200 ms), a normal QRS complex is less than 3 small boxes (< 120 ms), and the QT interval varies with heart rate but is typically 9–11 small boxes at normal rates. Heart rate can be quickly estimated using the "300 method" — dividing 300 by the number of large boxes between consecutive R waves.
This converter supports both standard (25 mm/s) and double-speed (50 mm/s) paper settings. It converts between small boxes, large boxes, and time in either direction, and includes an RR interval heart rate calculator. The built-in reference tables list normal ECG interval durations and the quick heart rate estimation chart used by clinicians worldwide. Whether you are a medical student learning ECG interpretation or a clinician verifying measurements, this tool provides instant, accurate conversions.
Precise ECG interval measurement is fundamental to cardiac diagnosis. This converter eliminates mental arithmetic errors and provides instant conversions with built-in reference tables for all major ECG intervals. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
At 25 mm/s: 1 small box = 40 ms, 1 large box = 200 ms. At 50 mm/s: 1 small box = 20 ms, 1 large box = 100 ms. Time (ms) = boxes × (box_width_mm / paper_speed_mm_s) × 1000. Heart Rate = 300 / (number of large boxes between R waves) or 60000 / (RR interval in ms).
Result: 200 ms
Five small boxes at 25 mm/s paper speed equals 5 × 40 ms = 200 ms (1 large box), which is the upper limit of a normal PR interval.
Use consistent units, verify assumptions, and document conversion standards for repeatable outcomes.
Most mistakes come from mixed standards, rounding too early, or misread labels. Recheck final values before use. ## Practical Notes
Use this for repeatability, keep assumptions explicit. ## Practical Notes
Track units and conversion paths before applying the result. ## Practical Notes
Use this note as a quick practical validation checkpoint. ## Practical Notes
Keep this guidance aligned to expected inputs. ## Practical Notes
Use as a sanity check against edge-case outputs. ## Practical Notes
Capture likely mistakes before publishing this value. ## Practical Notes
Document expected ranges when sharing results.
At standard 25 mm/s paper speed, one small box (1 mm) = 40 ms. At 50 mm/s, one small box = 20 ms.
Count the number of large boxes between two consecutive R waves and divide 300 by that number. For example, 4 large boxes = 300/4 = 75 bpm.
A normal PR interval is 3–5 small boxes (120–200 ms) at 25 mm/s. More than 5 small boxes suggests first-degree AV block.
50 mm/s doubles the horizontal resolution and is used in pediatric ECGs, when analyzing fine details of wide QRS complexes, or studying specific arrhythmia mechanisms. Use this as a practical reminder before finalizing the result.
One large box contains 5 small boxes (5 mm), both horizontally (time) and vertically (voltage). Keep this note short and outcome-focused for reuse.
Measure the QT interval from the beginning of the QRS to the end of the T wave, then correct for heart rate using Bazett formula: QTc = QT / √(RR interval in seconds). Apply this check where your workflow is most sensitive.