Large Squares (0.20s blocks)
Small Squares (0.04s blocks)
25 mm/sec (Standard)
50 mm/sec (High Speed)
Estimated Heart Rate
0 BPM
function updateLabel() {
var method = document.getElementById('calcMethod').value;
var label = document.getElementById('inputLabel');
var input = document.getElementById('squareCount');
if (method === 'large') {
label.innerText = "Number of Large Squares between R-Waves";
input.placeholder = "e.g., 4";
} else {
label.innerText = "Number of Small Squares between R-Waves";
input.placeholder = "e.g., 20";
}
}
function calculateHeartRate() {
// Get Input Values
var method = document.getElementById('calcMethod').value;
var count = parseFloat(document.getElementById('squareCount').value);
var speed = parseInt(document.getElementById('paperSpeed').value);
// Element to display results
var resultContainer = document.getElementById('result-container');
var bpmDisplay = document.getElementById('bpmResult');
var interpretationDisplay = document.getElementById('interpretation');
// Validation
if (isNaN(count) || count <= 0) {
resultContainer.style.display = 'block';
bpmDisplay.innerHTML = "–";
interpretationDisplay.innerHTML = "Please enter a valid number of squares greater than zero.";
return;
}
var bpm = 0;
// Logic based on Method and Paper Speed
// Standard Speed (25mm/s):
// – Large Square Method: 300 / large squares
// – Small Square Method: 1500 / small squares
// Universal Logic:
// BPM = (Speed in mm/sec * 60) / (Number of Squares * Size of Square in mm)
var squareSizeMM = (method === 'large') ? 5 : 1;
var distanceMM = count * squareSizeMM;
// BPM Formula: (Speed * 60) / Distance
bpm = (speed * 60) / distanceMM;
// Round to nearest integer
bpm = Math.round(bpm);
// Interpretation
var status = "";
if (bpm = 60 && bpm 100) {
status = "Tachycardia (Fast Heart Rate)";
}
// Display Results
resultContainer.style.display = 'block';
bpmDisplay.innerHTML = bpm + " BPM";
interpretationDisplay.innerHTML = "Interpretation: " + status + "Based on an R-R interval of " + (distanceMM/speed).toFixed(3) + " seconds.";
}
How to Calculate Heart Rate from an ECG Graph
Calculating heart rate from an electrocardiogram (ECG or EKG) strip is a fundamental skill for healthcare professionals and students. The grid on the ECG paper represents time on the horizontal axis and voltage on the vertical axis. By measuring the distance between specific waves (usually the R-R interval), you can mathematically determine the heart rate in Beats Per Minute (BPM).
The Standard Grid (at 25mm/sec):
1 Small Square = 1mm = 0.04 seconds
1 Large Square = 5mm = 0.20 seconds (contains 5 small squares)
5 Large Squares = 25mm = 1.00 second
Method 1: The 300 Method (Large Squares)
This method is quick and best used for regular rhythms where the heart rate falls on a heavy line. It relies on the number of large squares (5mm boxes) between two consecutive R-waves (the peak of the QRS complex).
Formula:Heart Rate = 300 / Number of Large Squares
The number 300 is derived from the fact that there are 300 large squares in one minute (60 seconds / 0.20 seconds per large square = 300).
Quick Reference Sequence:
Find an R-wave that lands on a thick line, then count the thick lines to the next R-wave:
1 square = 300 BPM
2 squares = 150 BPM
3 squares = 100 BPM
4 squares = 75 BPM
5 squares = 60 BPM
6 squares = 50 BPM
Method 2: The 1500 Method (Small Squares)
This is the most accurate method for calculating heart rate for regular rhythms. It involves counting the small (1mm) squares between two consecutive R-waves.
Formula:Heart Rate = 1500 / Number of Small Squares
The number 1500 is used because there are 1,500 small squares in one minute (25mm/sec * 60 seconds = 1500mm).
Example: If there are 20 small squares between R-waves, the heart rate is 1500 / 20 = 75 BPM.
Adjusting for Paper Speed
While 25mm/sec is the standard speed, some ECGs run at 50mm/sec to better visualize waveforms. If the paper speed is 50mm/sec, the time calibration changes (everything is stretched out by a factor of 2). To correct the calculation, you would multiply your result by 2, or use 600 (for large squares) and 3000 (for small squares) as your constant.
Interpreting the Results
Rate (BPM)
Classification
Potential Meaning
< 60
Bradycardia
Slow heart rate. Normal in athletes or during sleep, but can indicate heart block or sinus node dysfunction.
60 – 100
Normal
Normal resting heart rate for adults.
> 100
Tachycardia
Fast heart rate. Can be caused by exercise, stress, fever, or arrhythmias like atrial fibrillation.