Big Box Method (300 Method)
Small Box Method (1500 Method)
R-R Interval (Seconds)
Count the large squares (5mm) between two consecutive R wave peaks.
// Inline script to handle logic updates
function updateInputLabel() {
var method = document.getElementById('methodSelect').value;
var label = document.getElementById('dynamicLabel');
var helper = document.getElementById('helperText');
var input = document.getElementById('inputValue');
if (method === 'large') {
label.innerText = "Number of Large Boxes between R waves:";
helper.innerText = "Count the large squares (5mm) between two consecutive R wave peaks. Standard paper speed 25mm/s.";
input.placeholder = "e.g., 4";
} else if (method === 'small') {
label.innerText = "Number of Small Boxes between R waves:";
helper.innerText = "Count the small squares (1mm) between two consecutive R wave peaks. This is more precise.";
input.placeholder = "e.g., 20";
} else if (method === 'time') {
label.innerText = "R-R Interval Duration (seconds):";
helper.innerText = "Measure the exact time in seconds between two consecutive R waves.";
input.placeholder = "e.g., 0.8″;
}
// Clear previous result when method changes
document.getElementById('result').style.display = 'none';
input.value = ";
}
function calculateHeartRate() {
var method = document.getElementById('methodSelect').value;
var val = parseFloat(document.getElementById('inputValue').value);
var resultDiv = document.getElementById('result');
// Validation
if (isNaN(val) || val <= 0) {
resultDiv.style.display = 'block';
resultDiv.innerHTML = "Please enter a valid positive number greater than zero.";
return;
}
var heartRate = 0;
// Logic based on standard paper speed of 25mm/s
if (method === 'large') {
// Formula: 300 / number of large boxes
heartRate = 300 / val;
} else if (method === 'small') {
// Formula: 1500 / number of small boxes
heartRate = 1500 / val;
} else if (method === 'time') {
// Formula: 60 / time in seconds
heartRate = 60 / val;
}
// Interpretation
var statusClass = "";
var statusText = "";
if (heartRate = 60 && heartRate <= 100) {
statusClass = "status-normal";
statusText = "Normal Sinus Rhythm";
} else {
statusClass = "status-alert";
statusText = "Tachycardia (Fast Heart Rate)";
}
// Display Result
resultDiv.style.display = 'block';
resultDiv.innerHTML = `
Interpreting an Electrocardiogram (EKG or ECG) is a fundamental skill in cardiology and emergency medicine. While modern EKG machines provide automated readings, understanding how to manually calculate heart rate is essential for verifying machine accuracy and interpreting strips where the machine fails due to artifact or arrhythmia.
This calculator assumes the standard paper speed of 25 mm/second. There are three primary methods used to calculate rate based on the regularity of the rhythm.
1. The 300 Method (Large Box Method)
This is the quickest method for estimating heart rate on regular rhythms. The standard EKG paper grid consists of large boxes (5mm wide), representing 0.20 seconds of time.
Formula: Heart Rate = 300 / Number of Large Boxes between R-R
Example: If there are 4 large boxes between two R waves, the heart rate is 300 ÷ 4 = 75 BPM.
The sequence for large boxes is easy to memorize: 300, 150, 100, 75, 60, 50.
2. The 1500 Method (Small Box Method)
The 1500 method is the most precise way to calculate heart rate for regular rhythms. Each large box contains 5 small boxes (1mm each), representing 0.04 seconds. Since there are 1500 small boxes in one minute (25mm/sec × 60 sec), we use this constant.
Formula: Heart Rate = 1500 / Number of Small Boxes between R-R
Example: If there are 20 small boxes between R waves, the heart rate is 1500 ÷ 20 = 75 BPM. If there are 22 small boxes, the rate is 1500 ÷ 22 ≈ 68 BPM.
3. The Time Interval Method
If you have measured the exact duration in seconds between the peaks of the QRS complex (the R-R interval), you can calculate the rate mathematically.
Formula: Heart Rate = 60 / R-R Interval (in seconds)
This method is often used by digital analysis tools. An R-R interval of 0.8 seconds results in a heart rate of 75 BPM.
Interpreting the Results
Normal Sinus Rhythm: 60 to 100 BPM.
Sinus Bradycardia: Less than 60 BPM (Slow). Common in athletes or during sleep.
Sinus Tachycardia: Greater than 100 BPM (Fast). Common during exercise, stress, or fever.
Note: These calculation methods are most accurate for regular rhythms (Sinus Rhythm). For irregular rhythms (like Atrial Fibrillation), the "6-Second Strip Method" (counting QRS complexes in 30 large boxes and multiplying by 10) is generally preferred over R-R interval calculation.