Count the full large squares between two consecutive R waves.
For higher precision, count the remaining small (1mm) squares.
Estimated Heart Rate
— BPM
function calculateHeartRate() {
// Get input values
var largeSquares = document.getElementById('largeSquares').value;
var smallSquares = document.getElementById('smallSquares').value;
// Validate inputs
if (largeSquares === "" || largeSquares < 0) {
alert("Please enter a valid number of large squares.");
return;
}
// Convert to numbers
var large = parseFloat(largeSquares);
var small = smallSquares ? parseFloat(smallSquares) : 0;
// Logic:
// The "300 method" is an approximation where Rate = 300 / Large Squares.
// However, to accommodate the optional small squares for precision (which is scientifically Rate = 1500 / Small Squares total),
// we convert everything to small squares. 1 Large Square = 5 Small Squares.
// Formula: BPM = 1500 / ((Large * 5) + Small)
var totalSmallSquares = (large * 5) + small;
// Prevent division by zero
if (totalSmallSquares === 0) {
alert("R-R interval cannot be zero.");
return;
}
var bpm = 1500 / totalSmallSquares;
bpm = Math.round(bpm); // Round to nearest whole number for standard reporting
// Display Result
var resultBox = document.getElementById('ecgResult');
var bpmDisplay = document.getElementById('bpmOutput');
var interpretationDisplay = document.getElementById('rhythmInterpretation');
resultBox.style.display = "block";
bpmDisplay.innerHTML = bpm + " BPM";
// Interpretation Logic
var interpretationText = "";
var statusClass = "";
// Remove old classes
interpretationDisplay.className = "ecg-interpretation";
if (bpm = 60 && bpm 100 && bpm < 160) {
interpretationText = "Tachycardia (Fast Heart Rate)";
statusClass = "ecg-status-warning";
} else {
interpretationText = "Severe Tachycardia / Abnormal";
statusClass = "ecg-status-danger";
}
interpretationDisplay.innerHTML = interpretationText;
interpretationDisplay.classList.add(statusClass);
}
Understanding the 300-150-100-75-60-50 Method
The "300 Method" is one of the most popular and rapid techniques used by medical professionals to estimate heart rate from an electrocardiogram (ECG) strip. It relies on the standard paper speed of 25 mm/second and the grid layout of ECG paper.
300 → 150 → 100 → 75 → 60 → 50
This sequence represents the heart rate in beats per minute (BPM) based on the number of large grid squares (5mm boxes) found between two consecutive R-waves (the peak of the QRS complex).
How the Calculation Works
ECG paper is divided into small squares (1mm) and large squares (5mm). At a standard speed of 25mm/s:
1 Large Square represents 0.20 seconds.
1 Minute (60 seconds) contains 300 large squares.
Therefore, the formula is calculated as:
Heart Rate = 300 / Number of Large Squares between R-R interval
The Sequence Explained
If there is 1 large square between R waves: 300 / 1 = 300 BPM
If there are 2 large squares: 300 / 2 = 150 BPM
If there are 3 large squares: 300 / 3 = 100 BPM
If there are 4 large squares: 300 / 4 = 75 BPM
If there are 5 large squares: 300 / 5 = 60 BPM
If there are 6 large squares: 300 / 6 = 50 BPM
When to Use This Calculator
This method works best for regular rhythms. If the distance between R-waves (the R-R interval) varies significantly, as seen in Atrial Fibrillation, this method may provide inaccurate results. In such cases, the "6-second method" (counting the number of QRS complexes in a 6-second strip and multiplying by 10) is preferred.
Improving Precision
While the 300 method gives a quick estimate, falling between grid lines requires a more specific calculation. Our calculator above allows you to input "Small Squares" for this reason. The precise formula used when including small squares is:
Heart Rate = 1500 / Number of Small Squares
(Since there are 1500 small 1mm squares in a 60-second strip at standard speed).
Interpreting Your Results
For an average adult resting heart rate:
60 – 100 BPM: Normal Sinus Rhythm.
Below 60 BPM: Bradycardia (slow heart rate). Common in athletes but can indicate heart block or other issues in sedentary adults.
Above 100 BPM: Tachycardia (fast heart rate). Can be caused by exercise, stress, fever, or arrhythmias.