ECG Boxes to Seconds Calculator — Time Intervals on ECG
Count the small or large boxes on an ECG strip and instantly convert them to seconds and milliseconds. Also calculates heart rate if the boxes represent the RR interval between two consecutive R waves.
Box size
Paper speed
Time represented by the selected ECG boxes
- 1
Box width
5Large box = 5 mm - 2
Time per box
5 ÷ 25 = 0.2 - 3
Total duration
5 × 0.2 = 1
How does this calculator work?
At 25 mm/s (standard ECG): 1 small box = 0.04 s (40 ms); 1 large box = 0.20 s (200 ms). Multiply the box count by the per-box duration to get the time interval. For heart rate from the RR interval: HR = 60 ÷ RR(s), or approximately 300 ÷ (number of large boxes) at 25 mm/s.
Formula
How this is calculated
ECG paper is printed with a grid of small (1 mm) and large (5 mm) boxes. The horizontal axis represents time. At the universal standard speed of 25 mm/s, each small box (1 mm) spans 0.04 seconds (40 ms), and each large box (5 mm) spans 0.20 seconds (200 ms). Some centres use 50 mm/s paper (common in paediatrics and exercise stress testing) — at that speed, durations halve: small box = 0.02 s, large box = 0.10 s.
To convert boxes to time, multiply the box count by the duration per box for the chosen paper speed. The calculator accepts fractional boxes for partial intervals. Durations are reported in both seconds and milliseconds, as both units are used in clinical practice (QRS duration and PR interval in ms; QT interval often in ms; RR interval sometimes in ms for QTc formulas).
If the boxes represent the RR interval (peak to peak between consecutive R waves), the heart rate in beats per minute is 60 ÷ RR(s). This is the basis of the "300 rule" taught to medical students: at 25 mm/s, HR ≈ 300 ÷ (number of large boxes in RR). The rule gives a quick bedside estimate; the exact formula (60/RR) is always more accurate.
Frequently asked questions
At the standard paper speed of 25 mm/s, one large box (5 mm) = 0.20 seconds = 200 ms. At 50 mm/s (fast speed), one large box = 0.10 seconds = 100 ms. Always confirm the paper speed (printed on the ECG) before measuring intervals.
Count the large boxes between two consecutive R wave peaks (the RR interval). At 25 mm/s, HR ≈ 300 ÷ large boxes. For a regular rhythm: 1 box = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50. For irregular rhythms, count beats over 6 seconds (30 large boxes) and multiply by 10.
Normal ranges (at 25 mm/s): PR interval 120–200 ms (3–5 small boxes); QRS duration < 120 ms (< 3 small boxes); QT interval < 440 ms (men) / < 460 ms (women), corrected for heart rate; P wave < 120 ms. These are approximate adult values — consult current clinical guidelines for paediatric ranges.
TG we-Calculate Editorial Team. (2026). ECG Boxes to Seconds Calculator — Time Intervals on ECG [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/ecg-boxes-seconds-calculator
TG we-Calculate Editorial Team. "ECG Boxes to Seconds Calculator — Time Intervals on ECG." TG we-Calculate. 2026. https://we-calculate.com/calculator/ecg-boxes-seconds-calculator.
TG we-Calculate Editorial Team, "ECG Boxes to Seconds Calculator — Time Intervals on ECG," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/ecg-boxes-seconds-calculator
@misc{wecalculate_ecg_boxes_seconds_calculator, title = {ECG Boxes to Seconds Calculator — Time Intervals on ECG}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/ecg-boxes-seconds-calculator}}, year = {2026}, note = {TG we-Calculate} }
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