Error budgets and the nines table

The error budget is 1 minus the SLO: the failure you are allowed. At 99.9 per cent over 30 days, that is 0.1 per cent of requests, or about 43 minutes of full outage. If the service handles 10 million requests a month, the budget is 10,000 failed requests.

Learn the nines table so you can convert percentages to minutes without a calculator: 99.9 per cent is about 8.8 hours a year, 99.99 about 53 minutes. The budget turns reliability into something you spend: on risky launches, migrations or experiments. A budget that is never touched suggests the SLO is too loose or the team is too cautious.