Note: Calculated specifically for a paper speed of 50 mm/sec. Do not use for standard 25 mm/sec tracings.
Understanding ECG Calculation at 50 mm/sec
Electrocardiograms (ECGs) are most commonly recorded at a standard paper speed of 25 mm/sec. However, in certain clinical scenarios—such as analyzing complex arrhythmias, pediatric cases, or separating overlapping waveforms—the paper speed is doubled to 50 mm/sec. This expansion spreads out the complex, allowing for more detailed measurement of intervals, but it drastically changes the math required to calculate the heart rate.
WARNING: Applying the standard "300 rule" or "1500 rule" (meant for 25 mm/sec) to a 50 mm/sec tracing will result in a calculated heart rate that is exactly half of the actual rate. You must use the adjusted formulas below.
The Logic Behind the 50 mm/sec Formulas
To calculate heart rate, we determine how many beats occur in one minute (60 seconds). At a paper speed of 50 mm/sec, the paper travels 3000 mm in one minute (50 mm/sec × 60 sec).
Therefore, the fundamental constants change as follows:
1. The Small Square Method (Most Accurate)
On standard ECG paper, a small square is 1 mm. At 50 mm/sec speed, one small square represents 0.02 seconds (compared to 0.04s at standard speed).
Heart Rate (BPM) = 3000 ÷ (Number of Small Squares between R-R)
Example: If there are 30 small squares between two R waves, the HR is 3000 / 30 = 100 BPM.
2. The Large Square Method (Quick Estimate)
A large square is composed of 5 small squares (5 mm). At 50 mm/sec, one large square represents 0.10 seconds.
Heart Rate (BPM) = 600 ÷ (Number of Large Squares between R-R)
Example: If there are 6 large squares between two R waves, the HR is 600 / 6 = 100 BPM.
When is 50 mm/sec Paper Speed Used?
While 25 mm/sec is the industry standard, clinicians switch to 50 mm/sec for specific diagnostic purposes:
Tachyarrhythmias: When the heart rate is extremely fast, P waves may be buried in T waves. Spreading the tracing out helps identify P waves to distinguish between Sinus Tachycardia, SVT, or Atrial Flutter.
Interval Measurement: Precise measurement of the QT interval or QRS duration is easier when the waveform is horizontally expanded.
Pediatric Cardiology: Infants often have much higher heart rates than adults, necessitating a faster paper speed to separate the beats for analysis.
Interpreting the Results
Once you have calculated the rate using the adjusted 50 mm/sec formula, standard interpretation ranges generally apply for adults:
Sinus Bradycardia: Heart Rate < 60 BPM
Normal Sinus Rhythm: Heart Rate 60 – 100 BPM
Sinus Tachycardia: Heart Rate > 100 BPM
Note: Pediatric ranges differ significantly by age group.
function updateInputLabel() {
var method = document.getElementById('calcMethod').value;
var label = document.getElementById('inputLabel');
var input = document.getElementById('rrInputValue');
if (method === 'small') {
label.textContent = "Number of Small Squares (1mm) between R waves";
input.placeholder = "e.g., 40";
} else {
label.textContent = "Number of Large Squares (5mm) between R waves";
input.placeholder = "e.g., 8";
}
}
function calculateHeartRate() {
// 1. Get input values
var method = document.getElementById('calcMethod').value;
var inputValue = document.getElementById('rrInputValue').value;
var resultContainer = document.getElementById('result-container');
// 2. Validate input
if (inputValue === "" || isNaN(inputValue) || parseFloat(inputValue) squares * 0.02
rrIntervalSec = squares * 0.02;
} else {
// Formula: 600 / large squares
bpm = 600 / squares;
// Time calculation: squares * (5mm / 50mm/s) -> squares * 0.1
rrIntervalSec = squares * 0.1;
}
// 4. Determine Status
var statusText = "";
var statusClass = "";
// Round BPM to nearest integer for display standard
var displayBPM = Math.round(bpm);
if (displayBPM 100) {
statusText = "Tachycardia";
statusClass = "status-tachy";
} else {
statusText = "Normal Rate";
statusClass = "status-normal";
}
// 5. Output Results
document.getElementById('bpmResult').textContent = displayBPM + " BPM";
document.getElementById('timeResult').textContent = rrIntervalSec.toFixed(3) + " sec";
var statusEl = document.getElementById('statusResult');
statusEl.textContent = statusText;
statusEl.className = "highlight-status " + statusClass;
resultContainer.style.display = "block";
}