ECG Boxes to Seconds Calculator
Enter the number of boxes measured on an ECG strip and this calculator converts them to a duration in seconds and milliseconds. Choose between small boxes (1 mm) or large boxes (5 mm) and select the paper speed - 25 mm/s is the international standard, while 50 mm/s is common in some European countries. Results update as you type, with a show-your-work panel and a reference table of normal ECG intervals.
How ECG paper grid boxes work
Standard ECG paper is printed with a grid of 1 mm small squares. Five small squares form one large square (5 mm x 5 mm). The horizontal axis always represents time, and the vertical axis represents voltage. At the most common paper speed of 25 mm/s, each small 1 mm box equals 0.04 seconds (40 ms), and each large 5 mm box equals 0.2 seconds (200 ms). At 50 mm/s, which is used in many European countries and for some paediatric studies, all time values are halved: each small box = 0.02 s and each large box = 0.1 s. The paper speed is usually printed in a corner of the ECG strip.
Measuring ECG intervals with this calculator
Clinical ECG reading involves measuring the width of waves, intervals and segments between landmarks. Common measurements include: the PR interval (start of P wave to start of QRS), the QRS complex width, the QT interval (start of QRS to end of T wave), and the RR interval between consecutive R peaks. To use this calculator, count the boxes from the start to the end of the feature you are measuring using ECG calipers or a ruler, then enter the count. Select the box type (small or large) and confirm the paper speed from the ECG header. The calculator converts that count to seconds and milliseconds and compares it with the reference table of normal ranges.
Paper speed and why it matters
Paper speed changes how wide waveforms appear on the strip without changing their actual duration. A 10-box QRS complex at 50 mm/s represents only 0.2 s (200 ms), while the same 10 boxes at 25 mm/s represent 0.4 s. Misidentifying the paper speed is a common source of error when interpreting ECGs from unfamiliar machines or scanned records. Always verify the speed marking before measuring. Some modern recorders print the speed alongside the calibration pulse in the top-left corner.
Key ECG intervals and their clinical importance
The PR interval reflects the time from the start of atrial depolarisation to the onset of ventricular depolarisation. It is normally 120-200 ms (3-5 small boxes at 25 mm/s). A PR interval greater than 200 ms indicates first-degree atrioventricular block, while a very short PR may suggest pre-excitation syndromes such as Wolff-Parkinson-White. The QRS complex represents ventricular depolarisation and is normally under 100-120 ms; widening suggests bundle branch block, ventricular ectopy, or hyperkalaemia. The QT interval covers ventricular depolarisation and repolarisation together; corrected QT (QTc) greater than 440-470 ms (depending on sex) is associated with increased risk of life-threatening arrhythmias.
Normal ECG interval and segment durations
| ECG Feature | Normal Duration | Small Boxes (25 mm/s) | Clinical Significance |
|---|---|---|---|
| P wave | < 120 ms | < 3 boxes | Atrial depolarisation |
| PR interval | 120-200 ms | 3-5 boxes | > 200 ms = first-degree heart block |
| QRS complex | 80-100 ms | 2-2.5 boxes | > 120 ms = bundle branch block |
| QT interval | 360-440 ms | 9-11 boxes | Prolonged QT increases arrhythmia risk |
| T wave | 100-250 ms | 2.5-6 boxes | Ventricular repolarisation |
| RR interval | 600-1000 ms | 15-25 boxes | Determines heart rate |
| ST segment | 80-120 ms | 2-3 boxes | Elevation or depression may indicate ischaemia |
At standard 25 mm/s paper speed. Values shown are for adults. Always interpret in the clinical context.
Frequently asked questions
How many seconds is one small ECG box?
At the standard paper speed of 25 mm/s, one small box (1 mm) equals 0.04 seconds (40 milliseconds). At the faster speed of 50 mm/s, one small box equals 0.02 seconds (20 milliseconds). This calculator handles both speeds automatically.
How many seconds is one large ECG box?
A large box contains 5 small boxes. At 25 mm/s, one large box equals 0.20 seconds (200 milliseconds). At 50 mm/s, one large box equals 0.10 seconds (100 milliseconds).
What is the normal PR interval in small boxes?
At 25 mm/s, the normal PR interval of 120-200 ms corresponds to 3-5 small boxes. A PR interval longer than 5 small boxes (> 200 ms) suggests first-degree heart block, which reflects slowed conduction through the atrioventricular node.
How do I calculate heart rate from an ECG?
Count the number of large boxes between two consecutive R peaks (the RR interval), then divide 300 by that number to get the heart rate in beats per minute. For example, 4 large boxes between R peaks gives 300 / 4 = 75 bpm. This method works at the standard 25 mm/s speed. At 50 mm/s, divide 600 by the number of large boxes instead.
What is a normal QRS duration in boxes?
A normal QRS complex is under 100-120 ms. At 25 mm/s, that is fewer than 2.5 to 3 small boxes. A QRS spanning 3 or more small boxes (> 120 ms) is considered wide and may indicate bundle branch block, ventricular ectopic beats, or metabolic abnormalities such as severe hyperkalaemia.
Does paper speed affect voltage readings?
No. Paper speed only affects the horizontal (time) axis. Voltage, measured on the vertical axis, is set by the gain or calibration of the machine (usually 10 mm per millivolt) and is independent of paper speed. Changing paper speed makes waveforms look wider or narrower but does not alter their height.
Can I use this for a 50 mm/s ECG?
Yes. Select 50 mm/s from the paper speed dropdown. The calculator will halve all durations accordingly. A feature that spans 5 large boxes at 25 mm/s (1.0 s) would span 10 large boxes at 50 mm/s for the same real duration, so make sure you are entering the correct box count from the correct strip.