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ECG strips are printed on standardized paper marked in small boxes (1mm) and large boxes (5mm). At the standard 25mm/s paper speed, one small box equals 0.04 seconds and one large box equals 0.2 seconds. This calculator converts a box count directly to time. This calculator is for general reference and educational purposes only — not a substitute for a qualified professional's ECG interpretation.
How it works
Select box type (small or large) and enter the number of boxes. The tool multiplies by the standard seconds-per-box value at 25mm/s paper speed.
- Enter box type.
- Enter number of boxes.
- Click Calculate to see your results.
Examples
5 large boxes
5 x 0.2s = 1.0 second (1000 ms).
Who should use it
- Converting a measured ECG interval (in boxes) to milliseconds for calculations like QTc.
- ECG interpretation training and practice.
Industry applications
- Cardiology and emergency medicine education
- Nursing and paramedic ECG training
Advantages
- Simple, standard unit conversion used throughout ECG interpretation training.
- Supports both small-box and large-box counting conventions.
Limitations
- Assumes standard 25mm/s paper speed unless manually adjusted.
Common mistakes to avoid
- Assuming a non-standard paper speed without adjusting the conversion accordingly.
- Confusing small boxes with large boxes when counting on the strip.
Best practices
- Confirm the ECG paper speed printed on the strip (usually 25mm/s) before applying standard box-to-time conversions.
Tips
- Use this alongside our ECG Heart Rate Calculator and QTc Calculator for a complete basic ECG interval workflow.