FIT (Failures In Time) is a standard metric used in reliability engineering to report the failure rate of a component or system. One FIT equals one failure per one billion ($10^9$) device-operating hours. Use the calculator below to determine the FIT rate and Mean Time Between Failures (MTBF) based on your test data.
Reliability Calculator (FIT & MTBF)
The total sample size of units in the test.
How long each unit was stressed or operated.
Total confirmed failures during the test period.
Enter 1.0 for standard usage. Use higher values for accelerated life testing (e.g., high temp).
Total Device Hours:–
FIT Rate (Failures per Billion Hours)–
MTBF (Hours):–
MTBF (Years):–
function calculateFIT() {
// 1. Get input values
var numDevices = document.getElementById("numDevices").value;
var testDuration = document.getElementById("testDuration").value;
var numFailures = document.getElementById("numFailures").value;
var accelFactor = document.getElementById("accelFactor").value;
// 2. Validate inputs
if (numDevices === "" || testDuration === "" || numFailures === "" || accelFactor === "") {
alert("Please fill in all fields to calculate the FIT rate.");
return;
}
var N = parseFloat(numDevices);
var H = parseFloat(testDuration);
var F = parseFloat(numFailures);
var AF = parseFloat(accelFactor);
if (N <= 0 || H <= 0 || AF <= 0) {
alert("Devices, Duration, and Acceleration Factor must be greater than zero.");
return;
}
if (F < 0) {
alert("Number of failures cannot be negative.");
return;
}
// 3. Perform Calculations
// Total Device Hours = Units * Hours per Unit * Acceleration Factor
var totalDeviceHours = N * H * AF;
// FIT Rate Formula: (Failures / Total Device Hours) * 10^9
// 10^9 = 1,000,000,000
var fitRate = (F / totalDeviceHours) * 1000000000;
// MTBF (Mean Time Between Failures)
// MTBF (Hours) = 1 / Failure Rate = 10^9 / FIT
var mtbfHours = 0;
var mtbfYears = 0;
var mtbfDisplayHours = "";
var mtbfDisplayYears = "";
if (fitRate === 0) {
// If 0 failures, FIT is 0 (observed), MTBF is theoretically infinite based on observed data
mtbfDisplayHours = "∞ (See Note)";
mtbfDisplayYears = "∞";
} else {
mtbfHours = 1000000000 / fitRate;
mtbfYears = mtbfHours / (24 * 365); // 8760 hours per year
// Formatting numbers
mtbfDisplayHours = mtbfHours.toLocaleString("en-US", {maximumFractionDigits: 0});
mtbfDisplayYears = mtbfYears.toLocaleString("en-US", {maximumFractionDigits: 1});
}
// Format Large Numbers
var deviceHoursFormatted = totalDeviceHours.toLocaleString("en-US");
var fitRateFormatted = fitRate.toLocaleString("en-US", {maximumFractionDigits: 2});
// 4. Update the DOM
document.getElementById("resDeviceHours").innerText = deviceHoursFormatted;
document.getElementById("resFIT").innerText = fitRateFormatted;
document.getElementById("resMTBFHours").innerText = mtbfDisplayHours;
document.getElementById("resMTBFYears").innerText = mtbfDisplayYears;
// Show results div
document.getElementById("results").style.display = "block";
}
What is FIT Rate?
FIT stands for Failures In Time. It is a standard unit used in the electronics and manufacturing industries to express the reliability of a component.
One FIT is defined as one failure per one billion ($10^9$) device operating hours.
This metric allows engineers to predict the failure rates of semiconductor components and other high-reliability hardware. Because modern electronic components are extremely reliable, using percentages (like % failure per year) results in tiny, unmanageable numbers. FIT scales these numbers up to a billion hours to make them easier to compare.
FIT Rate Formula
The basic formula for calculating the observed FIT rate is:
FIT = (Number of Failures $\times$ 1,000,000,000) / (Total Device Hours)
Where:
Number of Failures: The count of units that failed during testing.
Total Device Hours: The number of devices tested multiplied by the number of hours they were tested. (e.g., 100 devices tested for 10 hours = 1,000 device hours).
What is the Acceleration Factor (AF)?
Reliability testing often takes too long at normal operating conditions. Engineers perform "Accelerated Life Testing" (ALT) by increasing temperature or voltage to stress components faster. The Acceleration Factor (AF) normalizes this data back to standard usage conditions.
If you are testing at standard conditions, the AF is 1. If you test at high stress where failures occur 10 times faster, the AF is 10.
Fit Rate Calculation Example
Let's look at a practical scenario to understand how the calculation works.
Scenario: A manufacturer tests a new batch of capacitors.
Step 2: Apply the FIT Formula
$$ \text{FIT} = \frac{3 \times 1,000,000,000}{2,000,000} $$
$$ \text{FIT} = \frac{3,000,000,000}{2,000,000} $$
$$ \text{FIT} = 1,500 $$
Result: The component has a failure rate of 1,500 FIT.
Converting FIT to MTBF
MTBF (Mean Time Between Failures) is the inverse of the failure rate. While FIT tells you how many failures to expect in a billion hours, MTBF tells you the average number of hours between failures.