How to Calculate Heart Rate on an ECG with the 6 Second Method

Amanda Thomas May 22, 2024
Digital EKG-ECG Wave

If you’ve ever watched a medical drama (any “Grey’s Anatomy” fans here?), you’ve probably seen a doctor or nurse look at an electrocardiogram (ECG) monitor and confidently announce the client’s heart rate. 

But how do they do it?

And more importantly, how can you calculate heart rate on an ECG (EKG) accurately?

In this article, we’ll discuss the 6 second ECG method for calculating heart rate and other methods.

Jump to Section


  1. What Is the 300 Rule on an ECG?
  2. The 6 Second Method on an ECG
  3. Other Ways to Calculate Heart Rate on an ECG

What Is the 300 Rule on an ECG?

Before we explore the 6-second method, it’s essential to understand the 300 rule.

Here’s a quick reminder of the PQRST wave:

This rule is a quick approximation for determining heart rate from the number of big squares between two consecutive R waves on an ECG. By dividing 300 by the number of big squares, you can estimate the heart rate in beats per minute (bpm).

For example, if there are three big squares between R waves, the heart rate is approximately 100 bpm (300/3 = 100).

The 6 Second Method on an ECG

The 6 second method is another way to calculate heart rate.

It’s especially useful for irregular rhythms like: 

To use the 6 second method, you must familiarize yourself with the squares on an ECG strip. Each small square represents 0.04 seconds, and each big square represents 0.2 seconds. 

Thirty big squares are equal to six seconds. Here’s how to calculate the atrial and ventricular rates using the 6 second method.

For Atrial Rate

  1. Find the P waves, which represent atrial rhythm.
  2. Count the number of P waves between 30 big squares.
  3. Multiply that number by 10 to get the atrial rate in bpm. 

For example, if there are 10 P waves between 30 big squares, you calculate the rate as:

10 x 10 = 100 bpm

For Ventricular Rate

  1. Find the R waves, which represent ventricular rhythm.
  2. Count the number of R waves between 30 big squares.
  3. Multiply that number by 10 to get the ventricular rate in beats per minute. 

For example, if you count 6 R waves between 30 big squares, you calculate the rate as: 

6 x 10 = 60 beats per minute.

Other Ways to Calculate Heart Rate on an ECG 

In addition to the 6 second and 300 methods, there are a few other ways to calculate heart rate on an ECG.

For Regular Rhythms

The big square method is a quick way to calculate heart rate for regular rhythms.

Count the number of big squares between two consecutive R waves and divide it by 300. The resulting number is the heart rate in beats per minute.

For example, if there are three squares between two R waves, you can calculate the heart rate as:

300 / 3 = 100 bpm

For Irregular Rhythms

You can use the R wave counting method for irregular rhythms where the rate equals the number of R waves in a rhythm strip multiplied by 6.

The number of waves on the strip gives the average rate over 10 seconds. You multiply this average by 6 (10 seconds x 6 = 1 minute) to get the rate in bpm. 

For example, if there are 11 R waves in a 10-second strip, you calculate the rate as: 

11 x 6 = 66 bpm

For Fast Rhythms

The small square method is another way to calculate heart rate for fast rhythms.

Count the small squares between two consecutive R waves and divide them by 1500. The resulting number is the heart rate in beats per minute.

For example, if there are 15 small squares between two R waves, you can calculate the heart rate as:

1500 / 15 = 100 bpm

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