What Is an Error Budget?
In Site Reliability Engineering (SRE), an error budget is the amount of unreliability a service is allowed to have while still meeting its Service Level Objective (SLO). If you promise 99.9% availability, you are implicitly allowing 0.1% downtime — that 0.1% is your error budget. This universal calculator converts any SLO target and time period into the maximum tolerable downtime in minutes, then subtracts the downtime you've already used to show how much budget remains.
How to Use It
Enter your target SLO as a percentage (for example 99.9), choose the budgeting period (week, month, quarter, or year), and enter how many minutes of downtime have already occurred in that period. The calculator returns your total error budget, the downtime consumed, and the remaining budget in both minutes and percent.
The Formula Explained
First the period is converted to minutes: \(\text{period\_minutes} = \text{days} \times 24 \times 60\). The error budget is \((1 - \text{SLO}) \times \text{period\_minutes}\), where SLO is expressed as a fraction (99.9% → 0.999). Finally, \(\text{budget\_remaining} = \text{error\_budget} - \text{downtime\_used}\). A negative remaining value means you have already burned through your budget and breached the SLO.
$$B_{\text{rem}} = \left(1 - \frac{\text{SLO (\%)}}{100}\right) \cdot P - \text{Downtime (min)}$$ $$\text{where}\quad P = \text{Period (days)} \times 24 \times 60$$
Worked Example
Suppose your SLO is 99.9% over a 30-day month. The period is \(30 \times 24 \times 60 = 43{,}200\) minutes. The error budget is:
$$(1 - 0.999) \times 43{,}200 = 43.2 \text{ minutes}$$If you've already had 10 minutes of downtime, your remaining budget is \(43.2 - 10 = 33.2\) minutes, or about 76.85% of the budget.
FAQ
What's a good SLO? It depends on the service. 99.9% ("three nines") is common for web apps; critical infrastructure may target 99.99% or higher. Higher SLOs leave a smaller error budget.
What does negative remaining mean? You've exceeded your allowed downtime and violated the SLO. SRE teams often freeze risky deployments until the budget recovers.
Does this include planned maintenance? Only if you count maintenance windows as downtime. Many teams exclude announced maintenance from the SLO calculation.
