Error budget
An error budget is the amount of unreliability an SLO allows — the gap between your target and 100%.
An error budget is the quantified amount of unreliability you are allowed to 'spend' under an SLO. If your SLO is 99.9%, your error budget is 0.1% of the window — the slice of downtime or failed requests you can absorb before you miss the objective. It reframes reliability from an absolute ('never break') into a budget you manage, just like money.
Why it matters: the error budget turns reliability into a decision tool that both engineering and product can use. When plenty of budget remains, teams can ship risky changes, run migrations, and experiment freely. When it is nearly spent, that is the signal to freeze risky work and stabilize. This makes the tension between velocity and reliability explicit instead of an argument.
A common misconception is that an unspent error budget is a success to celebrate. Consistently leaving the whole budget on the table usually means the SLO is too loose or the team is shipping too cautiously — reliability that users can't perceive is reliability you overpaid for. The budget is meant to be spent deliberately, not hoarded.
Error budgets also need a clear policy attached: what concretely happens when the budget is exhausted. Without an agreed response — a change freeze, a reliability sprint, a reprioritization — the budget is just a number on a dashboard rather than a lever that changes behavior.
Error budget = (1 − SLO) × time window
A 99.9% SLO over a 30-day month (43,200 minutes) gives a budget of 0.1% × 43,200 = 43.2 minutes; a single 20-minute incident spends about 46% of it.
Frequently asked questions
- How do you calculate an error budget?
- Subtract the SLO from 100% and multiply by the measurement window. A 99.9% SLO leaves 0.1%, which over 30 days (43,200 minutes) is 43.2 minutes of allowed downtime.
- What happens when the error budget runs out?
- Under a typical error-budget policy, the team pauses risky changes and focuses on reliability until the budget recovers over the rolling window. The exact response should be agreed in advance.
- Is it bad to have leftover error budget?
- Consistently unused budget can mean the SLO is too loose or the team is shipping too slowly. The budget is meant to be spent on change, so a healthy team uses most of it without breaching the objective.
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