Count the small squares between two consecutive R waves.
Estimated Heart Rate
0 BPM
function calculateHeartRate() {
// Get input value
var boxesInput = document.getElementById('smallBoxes');
var boxes = parseFloat(boxesInput.value);
var resultDiv = document.getElementById('result');
var bpmDisplay = document.getElementById('bpmValue');
var statusDisplay = document.getElementById('hrStatus');
var detailDisplay = document.getElementById('calculationDetail');
// Validation
if (isNaN(boxes) || boxes <= 0) {
alert("Please enter a valid number of small boxes (greater than 0).");
return;
}
// Calculation: 1500 divided by number of small boxes
// Standard ECG speed is 25mm/s. 1 min = 1500 mm.
var heartRate = 1500 / boxes;
var roundedRate = Math.round(heartRate);
// Determine Status
var statusText = "";
var statusClass = "";
if (roundedRate = 60 && roundedRate 100) {
statusText = "Tachycardia (Fast)";
statusClass = "status-danger";
}
// Display Results
resultDiv.style.display = "block";
bpmDisplay.innerText = roundedRate + " BPM";
// Reset classes
statusDisplay.className = "status-badge " + statusClass;
statusDisplay.innerText = statusText;
detailDisplay.innerHTML = "Formula: 1500 ÷ " + boxes + " small boxes = " + heartRate.toFixed(1) + " BPM";
}
Understanding the Small Box Method for Heart Rate Calculation
The small box method, also known as the 1500 method, is widely considered the most accurate manual technique for calculating heart rate from an electrocardiogram (ECG) strip, specifically for regular heart rhythms. By counting the microscopic grid squares on the ECG paper, medical professionals can determine the precise beats per minute (BPM).
Quick Formula: Heart Rate = 1500 ÷ Number of Small Boxes between two R waves.
How the ECG Grid Works
To understand the math behind this calculator, you must understand standard ECG paper settings:
Paper Speed: Standard ECG paper runs at 25 mm/second.
Small Box: Each small square represents 0.04 seconds (1 mm).
Large Box: Each large square is made of 5 small boxes horizontally and represents 0.20 seconds.
Why "1500"?
The constant 1500 is derived from the amount of small boxes that pass in one minute.
There are 60 seconds in one minute.
Since each small box is 0.04 seconds: 60 ÷ 0.04 = 1500.
Therefore, there are 1,500 small boxes in one minute of ECG tracing. dividing this total by the number of boxes between beats gives you the beats per minute.
Step-by-Step Calculation Guide
Locate an R Wave: Find a QRS complex where the R wave (the tall spike) peaks on a grid line. This acts as your starting point.
Find the Next R Wave: Look at the very next beat and locate its R wave.
Count the Small Boxes: Count the number of small squares between the peak of the first R wave and the peak of the second R wave (the R-R interval).
Divide: Divide 1500 by the count you obtained.
Example Calculation
If you count 20 small boxes between two consecutive R waves:
Calculation: 1500 ÷ 20 = 75 BPM.
This result falls within the normal range of 60-100 beats per minute.
Interpreting the Results
Once you have calculated the heart rate, you can categorize the rhythm (assuming a sinus rhythm):
Bradycardia: Less than 60 BPM. This indicates a slow heart rate.
Normal: 60 to 100 BPM. This is the standard resting heart rate for adults.
Tachycardia: Greater than 100 BPM. This indicates a fast heart rate.
When to Use This Method
The Small Box Method is best used for regular rhythms. If the patient has an irregular rhythm (such as Atrial Fibrillation), the R-R intervals will vary, making this calculation inaccurate. For irregular rhythms, the "6-Second Method" is typically preferred, where you count the number of QRS complexes in a 6-second strip and multiply by 10.