How to Model Failure Rates with WEIBULL.DIST in Excel

Use WEIBULL.DIST to predict how many parts fail within warranty, survive to service, or fail soon after an inspection.

You will learn to

• Find the share of parts failing within a warranty period
• Find the share surviving to a service interval
• Find a conditional failure probability

How it works

The Weibull distribution is the workhorse of reliability engineering. Its shape parameter (alpha) says how failures behave over time — below 1 means early failures, 1 means random failures, above 1 means wear-out — and its scale parameter (beta) is the characteristic life, where about 63% of parts have failed.

WEIBULL.DIST with TRUE returns the share failed by time x; 1 minus that is the reliability. For a part that has already survived to time t, the chance it fails in the next d hours is (F(t+d) − F(t)) ÷ (1 − F(t)).

=WEIBULL.DIST(5000,1.8,12000,TRUE)