# Bathtub curve

Operations · `Bathtub curve` · verification: source checked

The bathtub curve is a plot of failure rate over time for a population of products. It shows three periods: an early failure period with a high but rapidly decreasing failure rate, an intrinsic failure period with a roughly constant failure rate, and a wearout failure period with an increasing failure rate. It is used in reliability evaluation to model failure behavior over the product lifecycle.

## Specification

| Property | Value |
|---|---|
| Kind | methodology |

## Study notes

- Early Failure Period (also called Infant Mortality Period) begins at time zero and has a high but rapidly decreasing failure rate, typically lasting several weeks to a few months.
- Intrinsic Failure Period (also called Stable Failure Period) has a roughly constant failure rate level called the Intrinsic Failure Rate; most systems spend most of their lifetimes in this flat portion.
- Wearout Failure Period occurs when materials wear out and degradation failures occur at an increasing rate.
- The Bathtub Curve also applies to repairable systems, where the vertical axis is the Repair Rate or the Rate of Occurrence of Failures (ROCOF).

## Sources

1. [8.1.2.4. "Bathtub" curve](https://www.itl.nist.gov/div898/handbook/apr/section1/apr124.htm) (accessed 2026-10-25)

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