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RMSSD
0
milliseconds
SDNN
0
milliseconds
Mean RR Interval
0
milliseconds
Calculated Avg HR
0
beats / min
How to Calculate HRV from Heart Rate Data
Heart Rate Variability (HRV) is a physiological phenomenon where the time interval between consecutive heartbeats varies. Unlike your average Heart Rate (measured in beats per minute), HRV measures the specific changes in time (milliseconds) between successive beats. This variation is controlled by the autonomic nervous system and is a key indicator of recovery, stress, and overall health.
Understanding the Input Data
To accurately calculate HRV, you cannot simply use a single "average" heart rate number. You need a sequence of data points representing the timing of individual heartbeats. This data usually comes in two forms:
RR Intervals (ms): The exact time in milliseconds between R-waves (the peak of the heartbeat) on an ECG. This is the rawest and most accurate format.
Sequential BPM: The instantaneous heart rate for each beat. While less common as raw data, this can be converted to RR intervals using the formula: RR = 60,000 / BPM.
Key HRV Metrics Explained
This calculator processes your heartbeat data to derive the two most common time-domain metrics used in HRV analysis:
1. RMSSD (Root Mean Square of Successive Differences)
RMSSD is the gold standard for measuring short-term HRV and is primarily associated with parasympathetic nervous system activity (rest and digest). It is less influenced by breathing patterns than other metrics.
RMSSD = √ [ (Sum of squared differences between successive RR intervals) / (Number of intervals – 1) ]
A higher RMSSD generally indicates better recovery and lower stress levels.
2. SDNN (Standard Deviation of NN Intervals)
SDNN represents the overall variability of the heart rate over the recording period. It reflects both sympathetic (fight or flight) and parasympathetic activity.
SDNN = √ [ (Sum of squared differences from the Mean RR) / (Total number of intervals) ]
SDNN is highly dependent on the length of the recording. Longer recordings (like 24-hour Holter monitors) naturally produce higher SDNN values than short 5-minute readings.
Why You Can't Calculate HRV from Average Heart Rate
It is a common misconception that one can derive HRV from a single resting heart rate number (e.g., "My HR is 60"). This is mathematically impossible. A heart rate of 60 BPM could mean every beat is exactly 1.0 second apart (0 HRV), or it could mean intervals fluctuate between 0.8s and 1.2s (High HRV). The "Variability" in HRV specifically refers to the fluctuation, which requires a dataset of multiple beats to calculate.
Interpretation of Results
High HRV: Generally signals that your body is responsive to inputs, recovered, and ready for strain. It is associated with high parasympathetic activity.
Low HRV: May indicate accumulated stress, overtraining, illness, or dehydration. It suggests the sympathetic nervous system is dominant.
function togglePlaceholder() {
var type = document.getElementById('inputType').value;
var placeholder = "";
if (type === 'rr') {
placeholder = "Example (ms): 800, 815, 790, 820, 805…";
} else {
placeholder = "Example (bpm): 75, 73, 76, 72, 74…";
}
document.getElementById('heartData').placeholder = placeholder;
}
function calculateHRV() {
// 1. Get Inputs
var rawInput = document.getElementById('heartData').value;
var inputType = document.getElementById('inputType').value;
var errorMsg = document.getElementById('errorMsg');
var resultsSection = document.getElementById('resultsSection');
// 2. Parse Data
// Split by comma, space, tab, or newline and filter out empty strings
var tokens = rawInput.split(/[\s,]+/);
var dataPoints = [];
for (var i = 0; i 0) {
dataPoints.push(val);
}
}
// 3. Validation
if (dataPoints.length < 3) {
errorMsg.style.display = 'block';
resultsSection.style.display = 'none';
return;
} else {
errorMsg.style.display = 'none';
}
// 4. Convert BPM to RR if necessary
var rrIntervals = [];
if (inputType === 'bpm') {
for (var j = 0; j 0) {
rrIntervals.push(60000 / dataPoints[j]);
}
}
} else {
rrIntervals = dataPoints;
}
// 5. Calculate Metrics
// A. Calculate Mean RR
var sumRR = 0;
for (var k = 0; k < rrIntervals.length; k++) {
sumRR += rrIntervals[k];
}
var meanRR = sumRR / rrIntervals.length;
// B. Calculate Mean HR (from Mean RR)
var meanHR = 60000 / meanRR;
// C. Calculate SDNN (Standard Deviation)
var sumSquaredDeviations = 0;
for (var l = 0; l < rrIntervals.length; l++) {
var deviation = rrIntervals[l] – meanRR;
sumSquaredDeviations += (deviation * deviation);
}
var sdnn = Math.sqrt(sumSquaredDeviations / rrIntervals.length);
// D. Calculate RMSSD (Root Mean Square of Successive Differences)
var sumSquaredDiffs = 0;
var countDiffs = 0;
for (var m = 0; m < rrIntervals.length – 1; m++) {
var diff = rrIntervals[m+1] – rrIntervals[m];
sumSquaredDiffs += (diff * diff);
countDiffs++;
}
var rmssd = Math.sqrt(sumSquaredDiffs / countDiffs);
// 6. Display Results
document.getElementById('rmssdResult').innerText = rmssd.toFixed(1);
document.getElementById('sdnnResult').innerText = sdnn.toFixed(1);
document.getElementById('meanRRResult').innerText = meanRR.toFixed(0);
document.getElementById('meanHRResult').innerText = meanHR.toFixed(0);
// 7. Generate Interpretation
var interpretation = "";
if (rmssd < 20) {
interpretation = "Low HRV Score: An RMSSD below 20ms usually indicates high stress, fatigue, or significant sympathetic nervous system activation. Ensure you are resting enough.";
} else if (rmssd >= 20 && rmssd < 50) {
interpretation = "Moderate HRV Score: This is a common range for healthy adults, though it varies heavily by age. It suggests a balance between stress and recovery.";
} else {
interpretation = "High HRV Score: An RMSSD above 50ms is generally associated with good fitness, high parasympathetic tone, and a well-recovered state.";
}
document.getElementById('interpretationText').innerHTML = interpretation;
resultsSection.style.display = 'block';
}