1500 Method (Small Boxes) – Most Precise
300 Method (Large Boxes) – Quick Estimate
6-Second Method – For Irregular Rhythms
Count the number of 1mm small squares between two consecutive R waves.
0 BPM
How to Calculate Rate on an ECG Strip
Interpreting an Electrocardiogram (ECG) begins with determining the heart rate. The heart rate dictates how we interpret the rhythm and is a critical vital sign. While modern ECG machines provide automated calculations, knowing how to manually calculate the rate from the strip is a fundamental skill for medical professionals, especially when verifying machine data or analyzing rhythm strips.
An ECG prints on grid paper where time is measured on the horizontal axis. At the standard paper speed of 25mm per second, specific methods can be applied to calculate the heart rate in beats per minute (BPM).
Method 1: The 1500 Method (Small Box Method)
This is considered the most precise method for calculating heart rate, provided the rhythm is regular (the distance between heartbeats is consistent).
The Math: There are 1,500 small (1mm) boxes in one minute (60 seconds) of ECG paper. Calculation: 1500 ÷ Number of small boxes between two R waves.
Steps to Calculate:
Identify two consecutive R waves (the tall spikes representing ventricular contraction).
Count the number of tiny 1mm squares between the peak of the first R wave and the peak of the second.
Divide 1,500 by this count.
Example: If there are 20 small boxes between R waves, the heart rate is 1500 / 20 = 75 BPM.
Method 2: The 300 Method (Large Box Method)
This method is a quicker way to estimate heart rate without counting tiny squares. It relies on the larger, heavy-lined 5mm grids.
The Math: There are 300 large (5mm) boxes in one minute. Calculation: 300 ÷ Number of large boxes between two R waves.
The Sequence:
Many clinicians memorize the sequence of rates corresponding to the large grid lines:
1 large box = 300 BPM
2 large boxes = 150 BPM
3 large boxes = 100 BPM
4 large boxes = 75 BPM
5 large boxes = 60 BPM
6 large boxes = 50 BPM
Method 3: The 6-Second Method
This is the only valid method for calculating heart rate when the rhythm is irregular (e.g., Atrial Fibrillation). The other methods rely on a consistent R-R interval, which irregular rhythms lack.
Steps to Calculate:
Obtain a 6-second strip of the ECG. Most ECG paper has markers at the top or bottom indicating 3-second intervals. Alternatively, count 30 large boxes (5mm x 30 = 150mm = 6 seconds).
Count the number of complete QRS complexes (R waves) within that 6-second window.
Multiply that number by 10 to get the rate for 60 seconds.
Example: If you count 8 QRS complexes in a 6-second strip, the rate is 8 x 10 = 80 BPM.
Interpreting the Results
Once you have calculated the numerical rate, classify the clinical status:
Bradycardia: Heart rate is slower than normal (< 60 BPM).
Normal Sinus Rate: Heart rate is between 60 and 100 BPM.
Tachycardia: Heart rate is faster than normal (> 100 BPM).
function updateCalculatorInputs() {
var method = document.getElementById('calcMethod').value;
var label = document.getElementById('dynamicLabel');
var hint = document.getElementById('dynamicHint');
var inputField = document.getElementById('metricInput');
// Reset input value for clarity when switching methods
inputField.value = ";
document.getElementById('result-area').style.display = 'none';
if (method === '1500') {
label.innerHTML = 'Number of Small Boxes (R-R Interval)';
hint.innerHTML = 'Count the tiny 1mm squares between two consecutive R waves.';
inputField.placeholder = "e.g., 20";
} else if (method === '300') {
label.innerHTML = 'Number of Large Boxes (R-R Interval)';
hint.innerHTML = 'Count the heavy-line 5mm squares between two consecutive R waves.';
inputField.placeholder = "e.g., 4";
} else if (method === '6sec') {
label.innerHTML = 'Number of QRS Complexes';
hint.innerHTML = 'Count the total number of R waves found in a 6-second strip (30 large boxes).';
inputField.placeholder = "e.g., 8″;
}
}
function calculateHeartRate() {
var method = document.getElementById('calcMethod').value;
var inputVal = document.getElementById('metricInput').value;
var resultArea = document.getElementById('result-area');
var bpmDisplay = document.getElementById('bpmResult');
var interpretationDisplay = document.getElementById('rhythmInterpretation');
var summaryDisplay = document.getElementById('calculationSummary');
// Basic Validation
if (inputVal === " || isNaN(inputVal)) {
alert("Please enter a valid number.");
return;
}
var numericVal = parseFloat(inputVal);
if (numericVal <= 0) {
alert("Please enter a value greater than zero.");
return;
}
var heartRate = 0;
var formulaText = "";
// Calculation Logic based on method
if (method === '1500') {
// Formula: 1500 / small boxes
heartRate = 1500 / numericVal;
formulaText = "Calculation: 1500 ÷ " + numericVal + " small boxes";
} else if (method === '300') {
// Formula: 300 / large boxes
heartRate = 300 / numericVal;
formulaText = "Calculation: 300 ÷ " + numericVal + " large boxes";
} else if (method === '6sec') {
// Formula: Count * 10
heartRate = numericVal * 10;
formulaText = "Calculation: " + numericVal + " complexes × 10";
}
// Rounding
heartRate = Math.round(heartRate);
// Display Logic
resultArea.style.display = 'block';
bpmDisplay.innerHTML = heartRate + ' BPM';
summaryDisplay.innerHTML = formulaText;
// Interpretation Logic
interpretationDisplay.className = 'interpretation'; // reset classes
if (heartRate 100) {
interpretationDisplay.innerHTML = "Tachycardia";
interpretationDisplay.classList.add('status-tachy');
} else {
interpretationDisplay.innerHTML = "Normal Resting Rate";
interpretationDisplay.classList.add('status-normal');
}
}