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 learning ECG interpretation or checking a strip at the bedside, it keeps the common conversions and reference values in one place.
ECG interval measurement depends on paper speed and box count. This converter keeps the common box-to-time conversions and reference tables together when you want a quick check.
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.
ECG paper speed changes the conversion factor, so confirm whether the strip is recorded at 25 mm/s or 50 mm/s before measuring intervals. Small box counts are best for short intervals like PR and QRS, while large box counts are often easier for quick heart-rate estimation.
The most common errors are forgetting the paper speed, mixing horizontal time boxes with vertical voltage boxes, or applying the 300 method to an irregular rhythm. For QTc, remember that correction formula choice matters as much as the raw QT measurement.
When the rhythm is irregular or the interval is borderline, use the box-to-time conversion as a check rather than the only measurement. That keeps the interval interpretation tied to the actual strip you are reading.
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This page converts ECG grid boxes to time using the selected recording speed and the standard millimeter-grid assumptions already used in the calculator logic. It also derives a quick heart-rate estimate from the RR interval using the familiar 300 method or the equivalent millisecond calculation. The result is an ECG measurement worksheet, not a rhythm diagnosis engine or a substitute for full strip interpretation.
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.
One large box contains 5 small boxes (5 mm), both horizontally (time) and vertically (voltage).
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).