Uptime SLA Calculator Widget

Status pages, hosting plans and contract templates can show exactly what an availability promise means in minutes. Enter 99.9% and see the downtime budget for every period; enter an outage and see the availability it leaves. A table of the common nines is included.

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Under the widget on your page: Uptime SLA Calculator by A2Z Tools

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<div data-a2z-widget="uptime-sla-calculator" data-height="780"></div>
<p class="a2z-attribution" style="margin:6px 0 0;font:12px/1.4 system-ui,sans-serif;opacity:.8">Uptime SLA Calculator by <a href="https://a2z.tools/sla-uptime-calculator" target="_blank" rel="nofollow noopener">A2Z Tools</a></p>
<script async src="https://a2z.tools/embed.js"></script>

Recommended. A small script (what it does) sizes the widget to fit its content, loads it lazily and keeps it isolated in its own frame. Works in WordPress (Custom HTML block), Squarespace, Webflow, Shopify, Blogger, React / Next.js and plain HTML - anywhere a <script> tag is allowed.

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How it works

Time-based availability is (total time - downtime) / total time, the definition Google's Site Reliability Engineering book uses for service level objectives. The allowed downtime for a target is therefore the period length multiplied by (1 - availability). The widget measures each period on its own: a day is 86,400 seconds, a week 7 days, a month 30.44 days (the average Gregorian month), a quarter 91.25 days and a year 365 days. At 99.9% that is 1 min 26 s a day, 43 min 50 s a month and 8 h 45 min 36 s a year; at 99.99% the year allows 52 min 34 s, matching the SRE book's 52.56 minutes. In the reverse mode the downtime is divided by the chosen period: 30 minutes in a month leaves 99.9316%, or 3.16 nines. An outage longer than the period is refused. The table lists 99% to 99.999% for a day, a month and a year.

Calculation method

  • Availability = (period - downtime) / period
  • Allowed downtime = period x (1 - availability % / 100)
  • Periods: day 86,400 s; week 604,800 s; month 30.44 d; quarter 91.25 d; year 365 d
  • Nines = -log10(1 - availability % / 100)

Worked examples

Four nines

Inputs: Availability 99.99%

Result: 8.64 s a day; 4 min 23 s a month; 52 min 34 s a year

31,536,000 s x 0.0001 = 3,153.6 s = 52.56 min.

A 30-minute outage

Inputs: 30 minutes in a month

Result: 99.9316% availability (3.16 nines)

1 - 1,800 / 2,630,016 = 0.999316.

Limitations

  • Time-based availability only; request-based SLOs (successful requests / total requests) need traffic data.
  • Contract-specific exclusions, measurement windows and service credits are not modelled.

Where publishers use it

  • A web host's plan page translating its 99.9% guarantee into minutes
  • A SaaS company's public status page explaining its SLO
  • A managed-service provider's contract template appendix
  • An SRE or DevOps training course on error budgets
  • A procurement checklist comparing vendors' availability commitments

Questions

How much downtime does 99.9% allow?

About 1 min 26 s a day, 10 min 5 s a week, 43 min 50 s a month (30.44 days), 2 h 11 min a quarter and 8 h 45 min 36 s a year.

How much downtime is four nines?

99.99% allows 8.64 seconds a day, about 4 min 23 s a month and 52 min 34 s a year (52.56 minutes, the figure in Google's SRE book).

Why 30.44 days for a month?

It is the average Gregorian month (365.2425 / 12). Contracts often define a month as the calendar month, so a 31-day month allows about 3% more downtime than a 30-day one at the same percentage - check the wording.

Can a 99.9% monthly SLA lose 8 hours in one month?

No. A monthly SLA is assessed month by month, so each month may lose only about 44 minutes. The yearly figure of 8.76 hours only applies to an SLA measured over a whole year.

What is an error budget?

The allowed downtime treated as a resource. A team with a 99.9% monthly objective has about 44 minutes a month to spend on risky releases, maintenance and incidents; once it is used up, reliability work takes priority.

Do planned maintenance windows count?

That depends on the agreement. Many SLAs exclude announced maintenance and outages caused by the customer; the calculator counts every minute you enter.

Sources

  1. Site Reliability Engineering, Chapter 3 - Embracing Risk - Google . Defines time-based availability as the share of total time a service is up, and works out that a 99.99% annual target allows 52.56 minutes of downtime. Checked 2026-10-01.

Cite or recommend this tool

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A2Z Tools Uptime SLA Calculator
https://a2z.tools/sla-uptime-calculator

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