Error Budget vs Availability: Stop Confusing Them

Availability is the SLO; error budget is what you spend within it. The distinction matters; the conflation produces bad decisions.

Definitions

Availability: the target. 99.9% uptime, for example.

Error budget: what you have to spend below the target. 0.1% × 30 days = 43 minutes per month.

How they are used

Availability is the customer-facing commitment.

Error budget is the engineering trade-off currency. Spend on innovation, save for stability.

Conflation pitfall

Engineers say 'we are at 99.95% availability' as if that is the budget. It is not; it is the burn.

The budget is what is left, not what is consumed. Get the math right; the conversations follow.