Count the number of large (5mm) grid squares between two consecutive R-waves.
Calculated Heart Rate
— BPM
Mastering ECG Heart Rate Calculation Methods
Interpreting an Electrocardiogram (ECG or EKG) is a fundamental skill in cardiology and emergency medicine. While modern ECG machines provide automated readings, understanding how to manually calculate heart rate is crucial for verification, especially when artifacts are present or when the machine's algorithm fails to track irregular rhythms. This guide explores the three primary methods used by clinicians to calculate heart rate from an ECG strip.
1. The 300 Method (Big Box Method)
The 300 method, also known as the Big Box method, is the quickest way to estimate heart rate for regular rhythms. Standard ECG paper speed is 25 mm/sec. This means that 300 large squares (5mm each) pass through the machine every minute.
Formula: Heart Rate = 300 / Number of Large Squares between R-R intervals
How to use it:
Find an R-wave that lands on a thick line (start of a large square).
Count the number of large squares until the next R-wave.
Divide 300 by this number.
Example: If there are 4 large squares between two R-waves, the heart rate is 300 ÷ 4 = 75 BPM.
2. The 1500 Method (Small Box Method)
For a more precise calculation, or when the R-waves do not fall exactly on the thick grid lines, the 1500 method is preferred. Since there are 5 small squares in every large square, there are 1500 small squares in a one-minute strip (300 large squares × 5).
Formula: Heart Rate = 1500 / Number of Small Squares between R-R intervals
How to use it:
Count the total number of small (1mm) squares between two consecutive R-waves.
Divide 1500 by this count.
Example: If there are 20 small squares between R-waves, the heart rate is 1500 ÷ 20 = 75 BPM. If there are 23 small squares, the rate is 1500 ÷ 23 ≈ 65 BPM.
3. The 6-Second Method
The previous two methods assume a regular rhythm (where the distance between heartbeats is constant). However, in cases of Atrial Fibrillation or other arrhythmias, the R-R intervals vary. The 6-second method is the gold standard for irregular rhythms.
Formula: Heart Rate = Number of R-waves in a 6-second strip × 10
How to use it:
Obtain a 6-second strip of the ECG (usually marked by 30 large squares or 3-second hash marks at the top/bottom of the paper).
Count the number of complete QRS complexes (R-waves) within this 6-second period.
Multiply the count by 10 to get the beats per minute.
Example: If you count 8 QRS complexes in the 6-second strip, the heart rate is 8 × 10 = 80 BPM.
Interpreting the Results
Once you have calculated the heart rate, clinical interpretation follows these general guidelines for resting adults:
Normal Sinus Rhythm: 60 to 100 BPM.
Bradycardia: Less than 60 BPM. This can be normal in athletes or during sleep, but may indicate pathology in others.
Tachycardia: Greater than 100 BPM. This can be caused by stress, exercise, fever, or cardiac arrhythmias.
// Function to update input labels based on selected method
function updateLabels() {
var method = document.getElementById("calculationMethod").value;
var label = document.getElementById("dynamicLabel");
var helpText = document.getElementById("inputHelp");
var inputField = document.getElementById("inputValue");
// Reset input value when switching methods for clarity
inputField.value = "";
document.getElementById("resultDisplay").style.display = "none";
if (method === "300") {
label.innerText = "Number of Large Squares between R-R:";
helpText.innerText = "Count the large (5mm) grid squares between two consecutive R-waves. Decimals (e.g., 3.5) are allowed.";
inputField.placeholder = "e.g., 4";
} else if (method === "1500") {
label.innerText = "Number of Small Squares between R-R:";
helpText.innerText = "Count the small (1mm) grid squares between two consecutive R-waves for higher precision.";
inputField.placeholder = "e.g., 20";
} else if (method === "6sec") {
label.innerText = "Number of R-Waves in 6 Seconds:";
helpText.innerText = "Count the total number of QRS complexes found within a 6-second strip (30 large squares).";
inputField.placeholder = "e.g., 7";
}
}
// Main calculation logic
function calculateHeartRate() {
var method = document.getElementById("calculationMethod").value;
var inputValue = parseFloat(document.getElementById("inputValue").value);
var resultBox = document.getElementById("resultDisplay");
var bpmDisplay = document.getElementById("bpmResult");
var interpDisplay = document.getElementById("interpretation");
var noteDisplay = document.getElementById("methodUsedNote");
// Validation
if (isNaN(inputValue) || inputValue <= 0) {
alert("Please enter a valid positive number.");
return;
}
var heartRate = 0;
var methodText = "";
// Calculation Logic
if (method === "300") {
heartRate = 300 / inputValue;
methodText = "Calculation: 300 / " + inputValue + " large squares";
} else if (method === "1500") {
heartRate = 1500 / inputValue;
methodText = "Calculation: 1500 / " + inputValue + " small squares";
} else if (method === "6sec") {
heartRate = inputValue * 10;
methodText = "Calculation: " + inputValue + " complexes × 10";
}
// Rounding
heartRate = Math.round(heartRate);
// Interpretation Logic
var statusClass = "";
var statusText = "";
if (heartRate < 60) {
statusText = "Bradycardia ( 100) {
statusText = "Tachycardia (>100 BPM)";
statusClass = "status-tachy";
} else {
statusText = "Normal Resting Rate (60-100 BPM)";
statusClass = "status-normal";
}
// DOM Updates
bpmDisplay.innerText = heartRate + " BPM";
interpDisplay.innerText = statusText;
interpDisplay.className = "result-interpretation " + statusClass;
noteDisplay.innerText = methodText;
resultBox.style.display = "block";
}