# What is the ECG heart rate?

Works out the heart rate from an ECG with the 300 rule (large squares), the 1500 rule (small squares), or a count of R-R intervals on a rhythm strip, at a paper speed of 25 or 50 mm/s.

- Page: https://www.acalculator.org/health/ecg-heart-rate-calculator
- JSON spec: https://www.acalculator.org/health/ecg-heart-rate-calculator.json
- Version: 5f40643e8e62

## Default answer

Example with the default inputs (Method Large squares, Paper speed 25 mm/s, Large squares between R waves 4): The heart rate is 75 beats per minute: 60 to 100.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| method | Method | Count large squares or small squares between two R waves, or R-R intervals on a strip. |
| speed | Paper speed | The ECG paper speed: 25 mm/s is the standard; some countries use 50 mm/s. |
| large | Large squares between R waves | Large (5 mm) squares between two consecutive R waves, 1 to 30; parts of a square are fine. |
| small | Small squares between R waves | Small (1 mm) squares between two consecutive R waves, 5 to 150. |
| n | R-R intervals on the strip | The number of spaces between QRS complexes on the strip, 1 to 100 (not the number of complexes). |
| s | Strip length (seconds) | How long the strip lasts, 1 to 60 seconds: 10 for a standard rhythm strip, or 6. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| rate | Heart rate (beats/min) | The heart rate in beats per minute, to a whole number. |
| rr | R-R interval (ms) | The time between the two R waves, in milliseconds, to a whole number. |
| reading | Reading | Bradycardia under 60, tachycardia over 100 beats per minute (BMJ 2002). |

## Method

Rate = 60 ÷ R-R interval in seconds. At 25 mm/s a large square is 0.2 s and a small square 0.04 s, so rate = 300 ÷ large squares = 1500 ÷ small squares (600 and 3000 at 50 mm/s). On a strip, rate = R-R intervals × 60 ÷ seconds. Rounded once to a whole number.

## Assumptions

- The square rules need a regular rhythm; for an irregular rhythm, count R-R intervals on a strip.
- The rate is worked from the exact typed counts and rounded once to a whole number, halves up; the reading uses the shown rate.
- The bradycardia and tachycardia cut-offs (under 60, over 100) are adult values; the BMJ article gives under 50 for sleep, and children have faster resting rates.

## Worked examples

1. method = large, speed = 25, large = 4 gives rate = 75, rr = 800, reading = 60 to 100. Source: Meek and Morris 2002 (BMJ): 300 ÷ 4 large squares = 75 beats/min.
2. method = large, speed = 25, large = 2 gives rate = 150, rr = 400, reading = Tachycardia (over 100). Source: Meek and Morris 2002 (BMJ), figure: an R-R interval of two large squares gives 300 ÷ 2 = 150 beats/min.
3. method = strip, n = 21, s = 10 gives rate = 126, reading = Tachycardia (over 100). Source: Meek and Morris 2002 (BMJ), figure: 21 intervals on a 10-second strip give 6 × 21 = 126 beats/min.
4. method = small, speed = 25, small = 22 gives rate = 68, rr = 880, reading = 60 to 100. Source: Meek and Morris 2002 (BMJ): 1500 ÷ small squares.
5. method = large, speed = 50, large = 10 gives rate = 60, rr = 1,000, reading = 60 to 100. Source: Meek and Morris 2002 (BMJ): at 50 mm/s, 600 ÷ 10 large squares = 60 beats/min ().
6. method = small, speed = 50, small = 40 gives rate = 75, rr = 800, reading = 60 to 100. Source: Meek and Morris 2002 (BMJ): at 50 mm/s, 3000 ÷ 40 small squares = 75 beats/min ().
7. method = strip, n = 7, s = 6 gives rate = 70, reading = 60 to 100.
8. method = large, speed = 25, large = 8 gives rate = 38, rr = 1,600, reading = Bradycardia (under 60). Source: Meek and Morris 2002 (BMJ): bradycardia is under 60 beats/min.
9. method = large, speed = 25, large = 3 gives rate = 100, rr = 600, reading = 60 to 100. Source: Meek and Morris 2002 (BMJ): tachycardia is over 100, so 300 ÷ 3 = 100 is not ().

## FAQ

### How do I work out the heart rate from an ECG?

With a regular rhythm at the standard paper speed of 25 mm/s, count the large squares between two R waves and divide 300 by that number. Two large squares give 300 ÷ 2 = 150 beats per minute. Or count the small squares and divide 1500 by that number.

### Why 300 and 1500?

At 25 mm/s a large square (5 mm) is 0.2 seconds and a small square (1 mm) is 0.04 seconds. A minute holds 60 ÷ 0.2 = 300 large squares and 60 ÷ 0.04 = 1500 small squares, so the rate is 300 ÷ large squares, or 1500 ÷ small squares, between beats.

### What if the rhythm is irregular?

The square rules need a regular rhythm. For an irregular one, count the R-R intervals (the spaces between QRS complexes) on a rhythm strip and scale to a minute. A standard strip is 10 seconds long, so the count × 6 is the rate: 21 intervals give 126 beats per minute. On a 6-second strip, the count × 10 is the rate.

### Should I count QRS complexes or the spaces between them?

This page counts the spaces (R-R intervals), as the BMJ article does. A strip that starts and ends on a QRS complex has one more complex than intervals, so counting complexes gives a higher rate: on a 6-second strip, 10 beats per minute more.

### What if the ECG was recorded at 50 mm/s?

Some countries use 50 mm/s as the standard. Each square then lasts half as long, so the rules become 600 ÷ large squares and 3000 ÷ small squares. Pick 50 mm/s on the page.

### What heart rate is too fast or too slow?

The BMJ article calls a heart rate over 100 beats per minute a tachycardia, and a rate under 60 a bradycardia (under 50 during sleep). These are adult values; children have faster resting rates.

### What is a rate ruler?

A printed scale that reads the rate from two or three R-R intervals and shows their average as a rate. The BMJ article says to use the scale for the right paper speed, count the right number of beats, and use it only for regular rhythms.

## Sources

- Meek S, Morris F. ABC of clinical electrocardiography: Introduction. I—Leads, rate, rhythm, and cardiac axis. BMJ. 2002;324(7334):415-418 (25 mm/s paper, 0.2 s and 0.04 s squares; 300, 1500, 600, and 3000 rules; R-R intervals in 10 seconds × 6; worked examples of 150 and 126 beats per minute; tachycardia over 100, bradycardia under 60). https://doi.org/10.1136/bmj.324.7334.415 (full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC1122339/, retrieved 2026-10-18)
- Goodacre S, McLeod K. ABC of clinical electrocardiography: Paediatric electrocardiography. BMJ. 2002;324(7350):1382-1385 (the resting heart rate falls from about 140 beats per minute at birth to adult values by 10 years; a rate over 100 may be normal in children). https://pmc.ncbi.nlm.nih.gov/articles/PMC1123332/ (retrieved 2026-10-18)