💓 ECG Boxes to Seconds Converter

On a standard ECG running at 25 mm/s, one small box is 0.04 seconds and one large box (5 small) is 0.2 seconds. Five small boxes make 0.20 seconds; five large boxes make a full second. Enter any box count above for the exact time.

How to Use This Converter

ECG paper is a grid where distance is time. The horizontal axis is time, the vertical axis is voltage, and every measurement on a tracing starts as a box count.

Why the Numbers Are What They Are

The paper moves at 25 millimeters per second, so each millimeter of horizontal distance is 1/25th of a second. Small boxes are 1 mm; the bold large boxes are 5 mm. Doubling the paper speed to 50 mm/s stretches the tracing for easier measurement of fast rhythms, and every box then represents half the time. The vertical axis works the same way for voltage: 10 mm (two large boxes) equals 1 millivolt at standard calibration, which is why the calibration pulse at the start of the tracing matters.

Worked Example

A QRS complex spans 2.5 small boxes at 25 mm/s: 2.5 x 0.04 = 0.10 seconds, or 100 ms, comfortably narrow (under 120 ms). A PR interval of 4 small boxes: 4 x 0.04 = 0.16 seconds, inside the normal 0.12 to 0.20 range. An R-R interval of 20 large boxes: 20 x 0.2 = 4.0 seconds between beats, which is 15 beats per minute, profound bradycardia. The same 20 large boxes at 50 mm/s would be 2.0 seconds, or 30 bpm, which shows why confirming paper speed is not a formality.

Intervals Worth Knowing by Heart

Two of these earn their place in every clinician's memory. A QRS wider than 3 small boxes means the ventricles are not depolarizing through the normal fast pathway, which points at bundle branch block, a ventricular rhythm, or pre-excitation. A PR longer than 5 small boxes is first-degree AV block. Everything else on the tracing builds on converting boxes to seconds first, which is exactly what this tool does. To turn R-R box counts into a heart rate, use our ECG heart rate calculator, and for the heart rate numbers outside the ECG lab, our target heart rate calculator covers exercise zones.

Common Counting Mistakes

The classic error is counting the starting line as a box: an interval from one gridline to the next line is one box, so count the spaces, not the lines. Partial boxes are estimated to the nearest half small box, which is why reported intervals often end in .02 or .06. At 50 mm/s, people forget to halve and report every interval as twice its real length. And calibration matters: if the ECG was recorded at half standard calibration (5 mm/mV), voltage readings change but timing does not, so box-to-second conversion is unaffected. When in doubt about any tracing that belongs to a real patient, the overreading cardiologist's measurement stands, not yours.

Calipers or Counting: Getting a Precise Measurement

Eyeballing boxes to the nearest whole small box gives 0.04-second resolution, which is fine for most intervals. When precision matters, like borderline QT prolongation, clinicians use calipers or the digital measurement tools on the ECG machine, which resolve to the millisecond. A practical trick for hand measurement: line up the start of the interval on a bold gridline, because counting from a bold line reduces off-by-one errors. For the QRS, measure in the lead where it looks widest; the true duration is the earliest onset to the latest offset across all leads, which is why machines sometimes report a wider QRS than your single-lead count. Heart rate correction matters for the QT: divide the QT in seconds by the square root of the R-R interval in seconds (Bazett's formula) before judging it. An uncorrected QT of 0.44 seconds looks long at 60 bpm but is normal at 100 bpm, where the expected QT is shorter.

For informational purposes only, not medical advice. Consult a healthcare professional.