What this calculator does
On an ECG trace, heart rate can be read directly from the spacing between two consecutive R waves, the tall spike that marks each heartbeat. That spacing, called the R-R interval, is the time between one beat and the next, and heart rate in beats per minute is simply 60 divided by that interval in seconds.
ECG paper is normally recorded at a standard speed of 25 millimetres per second, which is why the same calculation is often done by counting squares on the paper instead of timing directly: each small square represents 0.04 seconds and each large square represents 0.2 seconds at that standard speed. This calculator accepts the R-R interval in seconds, or a square count, and converts between them.
The formula
Heart rate (bpm) equals 60 divided by the R-R interval measured in seconds. If counting squares on standard 25 mm/s ECG paper instead, each small square is 0.04 seconds and each large square is 0.2 seconds, so the square count is first converted to seconds before applying the same 60 ÷ interval formula.
| Term | Meaning |
|---|---|
| R-R interval | The time between two consecutive R waves on an ECG trace, corresponding to one full heartbeat cycle. |
| Small square | The smallest grid division on standard ECG paper, equal to 0.04 seconds at the standard 25 mm/s paper speed. |
| Large square | A larger grid division made of five small squares, equal to 0.2 seconds at the standard 25 mm/s paper speed. |
The inputs explained
| Field | What to enter |
|---|---|
| What you are entering | Choose whether you are entering a timed interval in seconds, or a square count read off standard ECG paper. |
| R-R interval (seconds) | The R-R interval in seconds, if that is what you measured directly. |
| Small squares between two R waves | The number of small squares counted between two consecutive R waves. |
| Large squares between two R waves | The number of large squares counted between two consecutive R waves. |
When to use it
Reading a regular rhythm strip
For a rhythm with evenly spaced beats, one R-R interval measurement is enough to estimate the overall rate, since every interval should be roughly the same length.
Working from a paper printout without a stopwatch
Counting large squares between two R waves and dividing 300 by that count, which is the same as this calculator's 60-÷-seconds formula applied to 0.2-second squares, is the classic bedside shortcut for standard-speed ECG paper.
Cross-checking a monitor reading
A displayed heart rate can be checked by measuring the R-R interval independently on the same strip and confirming the two figures line up.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How estimated heart rate changes with R-R interval
A range of R-R intervals converted to an estimated heart rate.
Questions
Is this an official diagnosis of heart rhythm?
No. This is a straightforward arithmetic conversion from a measured interval to a rate, intended as an informational and educational aid. Any concerns about a real ECG reading should go to a qualified clinician.
Why does the calculation assume 25 mm/s paper speed?
That is the standard, most widely used ECG paper speed, which is why the small-square and large-square timings of 0.04 and 0.2 seconds are fixed at that speed. A trace recorded at a different speed would need different square timings.
Does this method work for an irregular rhythm?
A single R-R interval only describes the rate for that one gap between beats. An irregular rhythm has R-R intervals that vary from beat to beat, so a single measurement will not represent the overall rate as reliably as it does for a regular rhythm.
What is the difference between using small squares and large squares?
They are just two ways of counting the same distance on the paper: five small squares make one large square, so the two counts always describe the same time interval and produce the same heart rate once converted to seconds.
For heart rate training zones based on resting and maximum heart rate, see the heart rate calculator.