Rate

The 300 method, the 1500 method, and reading rate straight off an RR.

Understand the measurements

Rate by the ladder

The ladder is the 300 method drawn out. Rate = 300 / large boxes, so 1, 2, 3, 4, 5, 6 boxes read 300, 150, 100, 75, 60, 50. It is quick and coarse, which is fine for a regular rhythm.

Rate by the 1500 method

At 25 mm/s the paper covers 1500 mm in a minute, so there are 1500 small boxes per minute. Rate = 1500 / small boxes in one RR. Twenty boxes is 1500 / 20 = 75 bpm.

RR in milliseconds

Rate = 60000 / RR in ms, because there are 60000 ms in a minute. Monitors do exactly this arithmetic on every beat.