SLA Uptime / Downtime / Error Budget Calculator

Allowed downtime quick reference

This table shows how much downtime the current target allows across common windows. It uses the same standard relation used in SRE error-budget planning: error budget percentage equals 100% - availability target.

What this calculator assumes

The calculator treats the selected measurement window as the full compliance period and compares all observed downtime to that same period. Allowed downtime is the time portion of the error budget. For example, a 99.9% target leaves 0.1% of the window available for unplanned unavailability.

Burn rate here is shown in a simple period-normalized way: observed downtime divided by allowed downtime for the same window. A burn rate above 1 means the budget for that window has already been exceeded. This is useful for quick planning and post-incident review, but alerting systems may calculate burn rate over shorter rolling windows with request-based SLIs rather than pure downtime.

Contractual SLAs often exclude maintenance, force majeure events, customer-caused incidents, or third-party dependencies. Some teams also track SLOs with request success rates instead of time-based uptime. Use this calculator as a transparent baseline, then apply your service's real policy and measurement method.

Formula

Error budget (%): 100 - availability target

Allowed downtime: window duration × (1 - availability target / 100)

Actual availability: (window duration - observed downtime) ÷ window duration × 100

Budget consumed: observed downtime ÷ allowed downtime × 100

Budget remaining: allowed downtime - observed downtime

Example calculation

With a 99.9% target over a 30-day window, the error budget is 0.1% of the period. 30 days × 0.001 = 0.03 days, which is 43 minutes 12 seconds of allowed downtime.

If the service actually experiences 30 minutes of downtime in that same 30-day window, actual availability is about 99.9306%. The budget consumed is about 69.44%, leaving 13 minutes 12 seconds before the target is breached.

FAQs

How is allowed downtime calculated?

Multiply the selected window by the unavailable fraction of the target. For 99.9%, that fraction is 0.001.

What is an error budget?

It is the portion of the window you can spend unavailable before missing the target.

Does a burn rate above 1 always mean a breach?

In this calculator, yes, because burn rate is based on downtime compared with the full-window budget.

Can I use this for request-based SLOs?

Only as a rough analogy. Request-based SLIs are usually measured by successful versus total events rather than raw downtime.

Is this private?

Yes. Inputs stay local to your browser.

How it works

Set a target, choose a compliance window, enter downtime for that same window, and compare actual performance against the budget.

Disclaimer

This calculator is for planning and communication. It does not interpret legal SLA language and should not replace your service contract, monitoring definitions, or incident review process.