How this instrument works
Standard ECG paper runs at 25 millimetres per second, which fixes the size of every square on the grid to a known slice of time: each large box, five millimetres wide, takes exactly 0.2 seconds to pass under the pen. Sixty seconds therefore contains exactly 300 large boxes, which is where the shortcut gets its name — divide 300 by the number of large boxes separating two consecutive R waves, the tall spike marking each heartbeat, and the result is heart rate in beats per minute, with no unit conversion required.
The trick only holds cleanly for a regular rhythm, where the spacing between beats stays roughly constant strip to strip. LITFL's rate-interpretation reference lays out the same shortcut clinicians memorise as a running ladder — one box apart is 300 bpm, two boxes is 150, three is 100, four is 75, five is 60, six is 50 — letting an experienced reader estimate rate almost by eye. An irregular rhythm, where R-to-R spacing keeps shifting, defeats the method entirely, since there is no single box count to divide into 300.
This calculator performs only the division; it does not read a rhythm strip, locate a P wave, or diagnose an arrhythmia. Counting boxes correctly still depends on finding consecutive R waves accurately on real ECG paper or a monitor display, and a single misread box changes the answer by a wide margin at fast rates. Treat the output as the bedside estimate it has always been — one input among the many a full ECG interpretation actually requires.
- Locate two consecutive R waves — the tall spikes marking each heartbeat — on the rhythm strip.
- Count the large boxes between them, including fractions of a box if an R wave falls partway across one.
- Enter that figure into Large boxes between R waves.
- Read Heart rate (bpm) for the resulting estimate.
Worked example — 4 large boxes between R waves
Two consecutive R waves sit 4 large boxes apart on a standard strip. Dividing 300 by 4 gives exactly 75, so Heart rate (bpm) reads 75 — squarely inside the normal resting band of roughly 60 to 100 bpm for an adult.
Widen the gap to 5 boxes and the same division gives 300 ÷ 5 = 60 bpm, a slower pace consistent with a well-conditioned heart or simple bradycardia depending on context. Narrow it instead to just 2 boxes, the beats crowding much closer together, and 300 ÷ 2 = 150 bpm, comfortably into tachycardia — the kind of jump the 300 rule is built to surface at a glance, using nothing more than a single division.
Questions
Why does dividing 300 by the box count give heart rate?
Because standard ECG paper moves at 25 mm/s, which makes each large box exactly 0.2 seconds and packs exactly 300 of them into one full minute. If two heartbeats sit a fixed number of boxes apart, that same spacing repeated across 300 boxes tells you how many beats fit into 60 seconds — which is exactly 300 divided by the box count.
Does the 300 rule work on an irregular rhythm?
No, and this is its main limitation. The method assumes the gap between R waves stays constant, so one box count can stand in for the whole minute. An irregular rhythm has R-to-R intervals that keep changing, so any single measurement only describes that one interval, not a stable, repeatable rate.
What if the R waves don't land on a whole number of boxes?
Estimate the fraction — half a box, a quarter box — and enter it directly; the field accepts decimals. Clinicians commonly eyeball fractional boxes on real strips rather than rounding to the nearest whole one, since rounding at fast rates can shift the estimate by several beats per minute.
Is this the same idea as the 1500 rule?
It is the same paper-speed fact expressed with small boxes instead of large ones. Since a large box holds 5 small boxes, and 300 large boxes make a minute, 1500 small boxes make that same minute — so 1500 divided by the small-box count between R waves gives an equivalent, and often more precise, heart rate.
Can I use this on a strip printed at a different paper speed?
Not without adjusting the constant first. The figure of 300 assumes the standard 25 mm/s paper speed; a strip run at 50 mm/s packs twice as many boxes into a minute, so applying the 300 rule unchanged would understate the true heart rate unless the box count is halved beforehand.
Does a normal 300-rule result mean the whole ECG is normal?
No — rate is only one piece of an ECG reading. A strip can show a textbook 75 bpm rate while still carrying a dangerous rhythm, a widened QRS, or an abnormal interval elsewhere on the same beat. This calculator answers the rate question alone, not the full interpretation.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.