How to Calculate Heart Rate from Rhythm Strip

ECG Heart Rate Calculator

Choose a method based on the rhythm's regularity

1500 Rule (Most Accurate for Regular Rhythms)

300 Rule (Quick for Regular Rhythms)

6-Second Method (Best for Irregular Rhythms)

function calculateBySmallSquares() { var val = document.getElementById("smallSquares").value; var resDiv = document.getElementById("resultSmall"); if (val && val > 0) { var bpm = 1500 / parseFloat(val); resDiv.innerHTML = "Heart Rate: " + Math.round(bpm) + " BPM"; resDiv.style.display = "block"; } else { alert("Please enter a valid number of small squares."); } } function calculateByLargeSquares() { var val = document.getElementById("largeSquares").value; var resDiv = document.getElementById("resultLarge"); if (val && val > 0) { var bpm = 300 / parseFloat(val); resDiv.innerHTML = "Heart Rate: " + Math.round(bpm) + " BPM"; resDiv.style.display = "block"; } else { alert("Please enter a valid number of large squares."); } } function calculateSixSecond() { var val = document.getElementById("qrsCount").value; var resDiv = document.getElementById("resultSix"); if (val && val > 0) { var bpm = parseFloat(val) * 10; resDiv.innerHTML = "Estimated Heart Rate: " + Math.round(bpm) + " BPM"; resDiv.style.display = "block"; } else { alert("Please enter the number of QRS complexes."); } }

Calculating the heart rate (HR) from an ECG rhythm strip is a fundamental skill for medical professionals and students. Standard ECG paper moves at a speed of 25 mm per second. Understanding this grid is the key to accurate calculation.

Standard ECG Grid Measurements

• 1 Small Square: 1mm (0.04 seconds)
• 1 Large Square: 5mm (0.20 seconds)
• 5 Large Squares: 1 second
• 15 Large Squares: 3 seconds
• 30 Large Squares: 6 seconds

1. The 1500 Rule (Most Accurate)

This method is preferred for regular rhythms because it provides the highest level of precision. To use this, count the number of small squares between two consecutive R waves (the R-R interval).

Formula: 1500 / Number of Small Squares = Heart Rate (BPM)

Example: If there are 15 small squares between R waves: 1500 / 15 = 100 BPM.

2. The 300 Rule (Sequence Method)

For a quick bedside estimate of a regular rhythm, you can count the large squares between R waves. This is often called the "Sequence Method" because you can memorize the sequence: 300, 150, 100, 75, 60, 50.

Formula: 300 / Number of Large Squares = Heart Rate (BPM)

Example: If there are 4 large squares between R waves: 300 / 4 = 75 BPM.

3. The 6-Second Strip Method (For Irregular Rhythms)

When the heart rhythm is irregular (such as in Atrial Fibrillation), the R-R interval varies, making the 1500 and 300 rules inaccurate. Instead, you calculate the average rate over a specific period of time.

On most ECG strips, there are markers at the top or bottom indicating 3-second or 6-second intervals. Count the number of QRS complexes (the spikes) within a 6-second window.

Formula: Number of QRS complexes in 6 seconds × 10 = Heart Rate (BPM)

Example: If you count 9 QRS complexes in a 6-second strip: 9 × 10 = 90 BPM.

Which Method Should You Use?

If the rhythm is regular (the distance between R waves is consistent), use the 1500 Rule for precision. If the rhythm is irregular, you must use the 6-Second Method to obtain an accurate average heart rate. Using the 1500 rule on an irregular rhythm will only give you the rate for that specific beat, which may not represent the overall clinical status of the patient.