Failures per Hour
Failures per Million Hours (ppm)
FITs (Failures per Billion Hours)
MTTF (Hours)–
MTTF (Days)–
MTTF (Years)–
function calculateMTTF() {
// Get input values using var
var rateInput = document.getElementById("failureRateInput").value;
var unitMultiplier = parseFloat(document.getElementById("failureUnit").value);
var resultDiv = document.getElementById("mttfResult");
var errorDiv = document.getElementById("mttfErrorMessage");
// Basic validation
if (rateInput === "" || isNaN(rateInput)) {
errorDiv.style.display = "block";
errorDiv.innerHTML = "Please enter a valid numeric failure rate.";
resultDiv.style.display = "none";
return;
}
var lambda = parseFloat(rateInput);
// Logic Check: Failure rate cannot be zero (implies infinite life) or negative
if (lambda <= 0) {
errorDiv.style.display = "block";
errorDiv.innerHTML = "Failure rate must be greater than zero.";
resultDiv.style.display = "none";
return;
}
// Hide error if previously shown
errorDiv.style.display = "none";
// Step 1: Normalize Failure Rate to Failures per Hour
// Formula: Actual Rate = Input Value * Unit Multiplier
var failuresPerHour = lambda * unitMultiplier;
// Step 2: Calculate MTTF in Hours
// Formula: MTTF = 1 / λ (failures per hour)
var mttfHours = 1 / failuresPerHour;
// Step 3: Convert to other time units
var mttfDays = mttfHours / 24;
var mttfYears = mttfHours / (24 * 365.25); // Taking leap years into account
// Format numbers for display (add commas, limit decimals)
var displayHours = mttfHours.toLocaleString('en-US', { maximumFractionDigits: 0 });
var displayDays = mttfDays.toLocaleString('en-US', { maximumFractionDigits: 1 });
var displayYears = mttfYears.toLocaleString('en-US', { maximumFractionDigits: 2 });
// Update the result DOM
document.getElementById("resultHours").innerHTML = displayHours;
document.getElementById("resultDays").innerHTML = displayDays;
document.getElementById("resultYears").innerHTML = displayYears;
// Show result box
resultDiv.style.display = "block";
}
How to Calculate MTTF from Failure Rate
Mean Time To Failure (MTTF) is a critical reliability metric used primarily for non-repairable systems, components like resistors or integrated circuits, and devices intended to be replaced rather than fixed upon failure. Understanding how to convert Failure Rate (λ) to MTTF is essential for reliability engineering, warranty planning, and lifecycle analysis.
Understanding Failure Rate (λ)
The failure rate, denoted by the Greek letter lambda (λ), represents the frequency with which an engineered system or component fails. It is usually expressed in failures per unit of time.
In the electronics industry, failure rates are often extremely low, so they are expressed in smaller units to avoid excessive zeros:
Failures per Hour: The standard base unit.
FITs (Failures In Time): The number of failures per one billion ($10^9$) hours.
PPM (Parts Per Million): Sometimes used as failures per million hours.
The Calculation Formula
For the constant failure rate phase of a product's life (the flat bottom of the "Bathtub Curve"), the relationship between MTTF and the Failure Rate is an inverse reciprocity.
MTTF = 1 / λ
Where:
MTTF = Mean Time To Failure (in hours)
λ = Failure Rate (in failures per hour)
Example Calculation: Using FITs
Let's say a specific capacitor has a failure rate of 500 FITs. How long is the Mean Time To Failure?
To visualize this better, 2,000,000 hours is approximately 228 years. This demonstrates why FITs are the preferred unit for reliable electronic components; the raw hourly numbers are simply too small to work with comfortably.
MTTF vs. MTBF
While the math is often identical ($1/λ$), the terminology differs based on the system type:
MTTF (Mean Time To Failure): Used for non-repairable items (e.g., a lightbulb). Once it fails, the lifecycle ends.
MTBF (Mean Time Between Failures): Used for repairable systems (e.g., a car engine). It measures the time between one failure and the next, assuming the item is repaired and returned to service.
Why This Matters
Calculating MTTF helps engineers design redundancy. If a component has a low MTTF, engineers might place two in parallel to ensure the system continues working if one fails. It also assists in determining warranty periods; manufacturers generally want the warranty period to be significantly shorter than the MTTF to avoid high replacement costs.