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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<iframe src="https://a2z.tools/embed/w/uptime-sla-calculator" title="Uptime SLA Calculator by A2Z Tools" width="100%" height="780" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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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.

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

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