Reliability · SRE
Error budget calculator.
Error budget is 100% minus the SLO. See it as minutes of downtime and as a number of failed requests, and how much you have already spent.
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How to calculate an error budget
Google's SRE Workbook defines it directly: the SLO is a target percentage and the error budget is 100% minus the SLO. Multiply the budget by the volume to get a count. Its example is a 99.9% success SLO over 3 million requests in four weeks: a 0.1% budget, or 3,000 failed requests.
For time, multiply the window by the budget: 99.9% over 30 days leaves 43,200 minutes x 0.001 = 43.2 minutes. What you do when the budget runs out is a policy your team sets.
Frequently asked questions
What is an error budget?
The amount of unreliability an SLO allows: 100% minus the SLO, over a chosen window.
What is the error budget for 99.9%?
0.1%: 43.2 minutes per 30 days, or 3,000 failed requests per 3 million.
Should I use time or requests?
Request-based is more accurate for partial outages and uneven traffic; Google defines availability as the request success rate. Time-based is easier to explain.
What happens when the budget is gone?
That is a policy decision for your team; the budget only tells you how much unreliability is left.
Sources
Checked
- Google SRE Workbook: Implementing SLOs
- Google SRE Book: Embracing Risk
- Google SRE Book: Appendix A, Availability Table
Formulas follow the sources above. Periods use a 30-day month and a 365-day year. Runs entirely in your browser; nothing you type is sent or stored.
Scope
Planning estimates only. Your SLA contract defines its own measurement window, exclusions and remedies, and those override these generic figures. Theoretical availability assumes independent failures.