Guides 6 min read
Error Budgets: What They Are and How to Use One
How to calculate an error budget from an SLO, what spends it, how burn rate works, and how to write an error budget policy your team will actually follow.
ReadGlossary · Reliability metrics
What is an error budget?
An error budget is the amount of unreliability a service level objective (SLO) allows over a period: 100% minus the target. A 99.9% SLO over 30 days leaves an error budget of 43 minutes and 12 seconds of downtime, or one failed request in a thousand.
How it works and why it matters
The budget turns an SLO into a rule for decisions. While budget is left, the team ships changes and takes planned risks; when it is spent, reliability work comes first until the window recovers. Agreeing on that rule in advance, in an error budget policy, is what makes the number useful.
It can be counted in time, as minutes of downtime measured by checks from outside, or in requests, as the share of failed requests in load balancer or application logs. Time-based budgets match how most SLAs are written; request-based budgets weigh busy hours more, which is closer to what users feel.
Decide in advance what spends it. Confirmed outages, errors, responses slower than the SLI’s limit and outages at providers you depend on usually do; false positives should not. How fast it is being spent is the burn rate.
Formula
Error budget = (100% − SLO) × window; budget left = error budget − downtime so far
Example: A 99.9% SLO over 30 days gives 43 minutes and 12 seconds. After one outage of 25 minutes, 58% of it is spent and 18 minutes and 12 seconds are left.
How Uptimecheck helps
Uptimecheck measures a time-based budget from outside your network: checks as often as every minute, each failure confirmed by a second check before it counts, and maintenance windows left out. Reports show uptime, downtime and incidents per monitor for this month, last month or the last 7, 30 or 90 days against an SLA target from 99% to 99.99%, also as CSV. The budget left is the downtime the target allows minus the downtime in the report; the app doesn’t show it as a separate number.
14-day free trial · No credit card required
Read more in the guides
Guides 7 min read
SLA vs SLO vs SLI: Definitions, Examples and How to Set Them
An SLI measures how a service behaves, an SLO is the target for that measurement and an SLA is the promise to customers. Definitions, downtime tables and how to set each one.
ReadGuides 6 min read
What Does 99.9% Uptime Mean? SLA Downtime Explained
99.9% uptime allows 43 minutes and 49 seconds of downtime in an average month. What each level of nines allows per day, month and year, how SLAs measure uptime, how service credits work and how to choose a target.
ReadQuestions
What happens when the error budget runs out?
Whatever your error budget policy says. A common rule: only bug fixes and reliability work ship until the window recovers, and risky changes wait.
Should the window be 30 days or a calendar month?
A rolling 30-day window remembers a bad week for a month, which is stricter. A calendar month starts fresh on the 1st and matches how most SLAs and invoices are counted. Pick one and keep it.
Does planned maintenance spend the error budget?
Only if the SLO says so. Many teams leave announced maintenance out, with a cap per month, so it can’t hide outages. In Uptimecheck, maintenance windows never count as downtime.
More from Reliability metrics
All terms-
Uptime
The time a service is available and working, usually as a percentage of a period. Read the definition : Uptime -
Downtime
The time a service is unavailable or too broken to use. Read the definition : Downtime -
The nines (99.9% uptime)
Nines of availability Availability targets counted in nines, from 99% to 99.999%, and the downtime each allows. Read the definition : The nines (99.9% uptime)
Downtime is obsolete.
Set up your first monitor in under a minute, or look around the live demo first.
14-day free trial · No credit card required