So basically, ECG is an electrical representation of heart activity on ECG paper.
Here are some important postulates of ECG paper:
The needle of galvanometer travels 300 big squares in 1 minute or 60 seconds.
That makes 0.2 seconds for each big square.
In turn each big square is divided into 5 small boxes /squares.
So we get each small box counting about
So let's review all calculations:
1 Big square = 0.2 seconds.
1 small square = 0.04 seconds.
Each P wave travels about 2 and 1/2 small squares making upto 0.1 second.
The segment PQ (Sometimes we use PR) again takes 2 and 1/2 small squares making upto 0.1 second.
QRS complex used another 2 and 1/2 small squares making another 0.1 second.
While S-T segments uses 10 small squares forming using 0.4 seconds.
Note: We normally consider 2 and 1/2 small squares but there can be 2 or 3 small squares.