This calculator allows healthcare professionals and students to quickly estimate heart rate based on the standard ECG paper speed of 25 mm/sec using the "300 Method" (also known as the sequence method).
Count the full large squares between two consecutive R waves.
Count any remaining small squares for higher precision.
Estimated Heart Rate:0 BPM
R-R Interval (Time):0 sec
Interpretation:Waiting…
function calculateHeartRate() {
var largeBoxesInput = document.getElementById("largeBoxes").value;
var smallBoxesInput = document.getElementById("smallBoxes").value;
var resultArea = document.getElementById("result-area");
var bpmDisplay = document.getElementById("bpmResult");
var timeDisplay = document.getElementById("timeResult");
var interpDisplay = document.getElementById("interpretationBadge");
// Validate Input
if (largeBoxesInput === "" || isNaN(largeBoxesInput) || largeBoxesInput <= 0) {
alert("Please enter a valid number of large boxes (at least 1).");
return;
}
var largeBoxes = parseFloat(largeBoxesInput);
var smallBoxes = smallBoxesInput === "" ? 0 : parseFloat(smallBoxesInput);
// Calculation Logic
// Standard ECG speed: 25mm/sec
// 1 Large Box = 5mm = 0.2 seconds
// 1 Small Box = 1mm = 0.04 seconds
// Formula A: 300 / Large Boxes (Approximation)
// Formula B: 1500 / Total Small Boxes (Precision)
var totalSmallBoxes = (largeBoxes * 5) + smallBoxes;
var heartRate = 1500 / totalSmallBoxes;
var rrIntervalSeconds = totalSmallBoxes * 0.04;
// Rounding
heartRate = Math.round(heartRate);
rrIntervalSeconds = rrIntervalSeconds.toFixed(2);
// Display Results
bpmDisplay.innerHTML = heartRate + " BPM";
timeDisplay.innerHTML = rrIntervalSeconds + " seconds";
// Interpretation Logic
var statusClass = "";
var statusText = "";
if (heartRate = 60 && heartRate 100 && heartRate < 160) {
statusText = "Tachycardia (Fast)";
statusClass = "status-danger";
} else {
statusText = "Severe Tachycardia";
statusClass = "status-danger";
}
interpDisplay.innerHTML = statusText;
interpDisplay.className = "interpretation " + statusClass;
resultArea.style.display = "block";
}
Understanding the 300 / 150 Method
The "300 Method" is a rapid calculation technique used by clinicians to estimate the heart rate on a standard 12-lead ECG strip without using a calculator. It relies on the fixed speed of ECG paper, which is 25 mm/second.
The Sequence
To use this method, you identify an R wave that falls on a thick vertical line (the edge of a large box). You then count the thick vertical lines following it until the next R wave appears. The rate corresponds to the following sequence:
Here is how the number of large squares translates to heart rate (Beats Per Minute):
Number of Large Squares
Heart Rate (BPM)
Calculation (300 ÷ N)
1
300
300 / 1
2
150
300 / 2
3
100
300 / 3
4
75
300 / 4
5
60
300 / 5
6
50
300 / 6
How to Calculate Manually
There are two primary variations of this calculation:
The 300 Rule (Large Boxes): Divide 300 by the number of large squares between two R-R intervals.
Formula: Rate = 300 / Large Squares
The 1500 Rule (Precision): For greater accuracy, specifically when the R waves do not fall exactly on the thick lines, count the number of small (1mm) squares and divide 1500 by that number.
Formula: Rate = 1500 / Small Squares
Why 300 and 1500?
Standard ECG paper moves at 25mm per second.
There are 60 seconds in a minute.
25 mm/sec × 60 sec/min = 1,500 mm/min.
This means 1,500 small (1mm) boxes pass through the machine every minute. Since one large box contains 5 small boxes, there are 300 large boxes in one minute (1500 ÷ 5 = 300). Therefore, dividing these constants by the interval gives the rate per minute.
Limitations
The 300/150 method is strictly valid only for regular rhythms. If the patient has an irregular rhythm (such as Atrial Fibrillation), the R-R intervals vary, making this method inaccurate. In such cases, the "6-Second Method" (counting QRS complexes in a 6-second strip and multiplying by 10) is preferred.