ECG Heart Rate Calculator (1500, 300 and 6-second methods)

Understanding the ECG heart rate calculator — concept and principle

This calculator works out heart rate from a printed ECG using the 1500 rule, the 300 rule, a QRS count over 6 or 10 seconds, or an R–R interval in milliseconds. It is a quick check for students and clinicians reading a tracing by hand.

1500 rule at 25 mm/s: with the default inputs the result is 75 bpm (1,500 ÷ 20).

How it is calculated

1500 rule: rate = 60 ÷ (small boxes ÷ paper speed), which at 25 mm/s equals 1,500 ÷ small boxes.
300 rule: rate = 60 ÷ (large boxes × 5 ÷ paper speed), which at 25 mm/s equals 300 ÷ large boxes.
6-second strip: rate = QRS complexes × 10.
10-second strip: rate = QRS complexes × 6.
R–R interval: rate = 60,000 ÷ R–R (ms).

The average R–R interval shown is 60,000 ÷ rate. At 50 mm/s the box constants double to 3,000 and 600.

The principle

At 25 mm/s one minute of paper is 25 × 60 = 1,500 mm, which is 1,500 small boxes or 300 large boxes. If one beat spans 20 small boxes, 1,500 ÷ 20 = 75 beats fit in a minute. The box methods assume each beat is the same length, so they suit regular rhythms. When the rhythm is irregular, as in atrial fibrillation, counting complexes over a fixed time gives a fairer average. A standard 12-lead printout covers 10 seconds, which is why "count × 6" is common.

Worked example

Default inputs: 1500 rule, 20 small boxes, 25 mm/s.

R–R time = 20 ÷ 25 = 0.8 s.
Rate = 60 ÷ 0.8 = 75 bpm (the same as 1,500 ÷ 20).
Average R–R interval = 60,000 ÷ 75 = 800 ms.
75 bpm is between 50 and 100 → Usual range (50–100 bpm).

Each point is the calculator re-run with only “Your count or interval” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Your count or interval	Heart rate (bpm)
5	300
10	150
15	100
20	75
25	60
30	50
40	38

How to read the result

Where does your result fall?

The categories follow the ECG Library adult ranges: under 50 bpm slow, over 100 bpm fast. Many textbooks use 60 bpm as the lower limit. A slow rate can be normal in trained athletes or during sleep; a fast one can follow exercise, fever or anxiety. The rate is only one part of the rhythm analysis.

Common mistakes

Using box methods in an irregular rhythm.
Forgetting a 50 mm/s paper speed, which halves the true rate if read as 25 mm/s.
Counting partial complexes at the edges of a 6-second strip.
Entering seconds instead of milliseconds for the R–R method.

Keep in mind: This is a reading aid, not a diagnosis. An abnormal rate with symptoms such as fainting, chest pain or breathlessness needs prompt medical care.

Frequently asked questions

What is the 300 rule?

At 25 mm/s, divide 300 by the number of large boxes between two R waves. It is fast but rough.

Which method is most accurate?

For regular rhythms the small-box count; for irregular rhythms a count over 10 seconds.

Is 55 bpm too slow?

It may be normal for a fit, healthy adult at rest. Symptoms and the full ECG matter more than the number alone.

Sources: Life in the Fast Lane ECG Library, "ECG Rate Interpretation" (litfl.com)

Checked 9 Nov 2026. Life in the Fast Lane ECG Library, “ECG Rate Interpretation” (litfl.com): 300 and 1500 rules at 25 mm/s, R waves on a 10-second strip × 6, normal 50–100 bpm, bradycardia under 50, tachycardia over 100. 6-second × 10 method: standard.

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