How this calculator works
The horizontal ECG grid is a distance scale. Divide its width in millimetres by the recorded paper speed to obtain time. One small box is 1 mm wide and one large box spans five small boxes. Check the speed printed on the tracing before measuring. This conversion does not assess the rhythm. The result is an estimate for information only and is not medical advice.
Formula
Time (s) = boxes × box width (mm) / paper speed (mm/s). Boxes = time × speed / width.
Worked example
At 25 mm/s, five small boxes take 0.20 s. At 50 mm/s, five large boxes take 0.50 s.
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Frequently asked questions
Can boxes be fractional?
Yes. A measurement between grid lines may use a decimal count.
Does voltage calibration change time?
No. The vertical voltage scale is separate from horizontal paper speed.
Does the 300 rule work at 50 mm/s?
Its constant changes with speed. Use the heart-rate tool for that calculation.