Interpreting an Electrocardiogram (ECG) is a fundamental skill for medical professionals. While modern ECG machines provide automated readouts, manual calculation remains a critical skill for verification and for situations where automated interpretation may be erroneous due to artifacts or arrhythmias. This guide covers the three primary methods used in our calculator.
1. The 1500 Method (Small Squares)
This is considered the most precise method for calculating heart rate on a regular rhythm. It relies on the standard speed of ECG paper, which is 25 mm/second.
Heart Rate = 1500 / Number of Small Squares between R-R
How to use it: Count the number of small millimeter squares between two consecutive R waves (the peaks of the QRS complex). Because there are 1500 small squares in one minute (25mm/sec × 60 sec), dividing 1500 by your count gives the beats per minute (BPM).
Example: If there are 20 small squares between two R waves, the heart rate is 1500 / 20 = 75 BPM.
2. The 300 Method (Large Squares)
The 300 method is a quick estimation technique, useful for rapid assessment of regular rhythms. An ECG grid consists of large squares, each containing 5 small squares (5mm).
Heart Rate = 300 / Number of Large Squares between R-R
How to use it: Count the number of large (5mm) squares between two consecutive R waves. Since there are 300 large squares in a minute (1500 / 5), this formula provides a quick BPM estimate.
Example: If there are 4 large squares between R waves, the heart rate is 300 / 4 = 75 BPM.
3. The 6-Second Method
This is the only reliable method for calculating heart rate in irregular rhythms (such as Atrial Fibrillation). It does not rely on the consistency of the R-R interval.
Heart Rate = Number of R Waves in 6 Seconds × 10
How to use it: Obtain a 6-second strip of the ECG (usually marked by 30 large squares or 3-second hash marks). Count the total number of QRS complexes (R waves) within that 6-second period and multiply by 10 to estimate the rate for 60 seconds.
Clinical Interpretation Guidelines
Once you have calculated the heart rate, interpretation follows standard clinical ranges:
Sinus Bradycardia: Heart Rate < 60 BPM.
Normal Sinus Rhythm: Heart Rate 60 – 100 BPM.
Sinus Tachycardia: Heart Rate > 100 BPM.
Note: Always correlate the calculated rate with the patient's clinical status. Athletes may have resting rates below 60 BPM which is normal for them.
function updateInputLabels() {
var method = document.getElementById("calcMethod").value;
var label = document.getElementById("inputLabel");
var input = document.getElementById("ecgInput");
if (method === "1500") {
label.innerText = "Number of Small Squares between R-R";
input.placeholder = "e.g., 20";
} else if (method === "300") {
label.innerText = "Number of Large Squares between R-R";
input.placeholder = "e.g., 4";
} else if (method === "6sec") {
label.innerText = "Number of QRS Complexes (in 6 seconds)";
input.placeholder = "e.g., 7";
}
// Clear previous results when method changes
document.getElementById("result").style.display = "none";
document.getElementById("ecgInput").value = "";
}
function calculateHeartRate() {
var method = document.getElementById("calcMethod").value;
var inputValue = parseFloat(document.getElementById("ecgInput").value);
var bpmDisplay = document.getElementById("bpmDisplay");
var interpretationDiv = document.getElementById("interpretation");
var methodExp = document.getElementById("methodExplanation");
var resultDiv = document.getElementById("result");
// Validation
if (isNaN(inputValue) || inputValue <= 0) {
alert("Please enter a valid positive number.");
return;
}
var hr = 0;
var formulaText = "";
// Calculation Logic
if (method === "1500") {
// Formula: 1500 / small squares
hr = 1500 / inputValue;
formulaText = "Formula used: 1500 ÷ " + inputValue + " small squares";
} else if (method === "300") {
// Formula: 300 / large squares
hr = 300 / inputValue;
formulaText = "Formula used: 300 ÷ " + inputValue + " large squares";
} else if (method === "6sec") {
// Formula: count * 10
hr = inputValue * 10;
formulaText = "Formula used: " + inputValue + " QRS complexes × 10";
}
// Round to nearest integer
hr = Math.round(hr);
// Interpretation Logic
var status = "";
var statusClass = "";
if (hr = 60 && hr <= 100) {
status = "Normal Resting Rate";
statusClass = "status-normal";
} else {
status = "Tachycardia (Fast)";
statusClass = "status-fast";
}
// Output
bpmDisplay.innerText = hr + " BPM";
interpretationDiv.innerText = status;
interpretationDiv.className = "interpretation " + statusClass;
methodExp.innerText = formulaText;
resultDiv.style.display = "block";
}
function generateQuizScenario() {
var method = document.getElementById("calcMethod").value;
var inputField = document.getElementById("ecgInput");
var randomVal = 0;
// Generate realistic random numbers based on method
if (method === "1500") {
// Random small squares between 10 (150bpm) and 50 (30bpm)
randomVal = Math.floor(Math.random() * (50 – 10 + 1)) + 10;
} else if (method === "300") {
// Random large squares between 1 (300bpm) and 10 (30bpm)
randomVal = (Math.random() * (10 – 1) + 1).toFixed(1);
} else if (method === "6sec") {
// Random QRS count between 3 (30bpm) and 18 (180bpm)
randomVal = Math.floor(Math.random() * (18 – 3 + 1)) + 3;
}
inputField.value = randomVal;
// Hide result initially so user can guess first
document.getElementById("result").style.display = "none";
// Optional: Alert or visual cue
alert("Quiz Scenario Generated!\nMethod: " + document.getElementById("calcMethod").options[document.getElementById("calcMethod").selectedIndex].text + "\nValue: " + randomVal + "\n\nTry to calculate the BPM mentally, then click 'Calculate Heart Rate' to check.");
}