What does "three nines" mean?

99.9% uptime ("three nines") allows 8 hours 45 minutes and 57 seconds of downtime per year — roughly 43 minutes per month or 10 minutes per week.

How is monthly downtime calculated?

This calculator uses a standard 30-day month (720 hours). Some SLAs use calendar months; clarify with your vendor if precision matters.

What is the difference between availability and reliability?

Availability is the percentage of time a system is operational. Reliability (MTBF) measures how long it runs between failures. A system can be highly available with frequent short outages if each one is brief.

Can I enter downtime to get the uptime percentage?

Yes. Switch to the "Downtime → Uptime %" mode, enter a downtime value and time unit, and the calculator computes the equivalent uptime percentage.

Why does 100% uptime not appear in the table?

100% availability is theoretically impossible in production systems — hardware fails, software has bugs, and planned maintenance requires restarts. SLAs are written around realistic targets like 99.9%–99.999%.

Availability Calculator

Name: Availability Calculator
Availability: InStock
Author: Nham Vu

Enter an uptime percentage or a downtime duration to instantly see how much downtime your SLA allows across every time window.

Input

Uptime percentage

Enter a value between 0 and 100 (e.g. 99.95)

Quick presets

Result

99.9%

Per day

1m 26.4s

Per week

10m 4.8s

Per month (30d)

43m 49.7s

Per year (365d)

8h 45m 57.0s

Nines Reference Table

SLA	Uptime	Per month	Per year

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Summary

Enter an uptime percentage or a downtime duration to instantly see how much downtime your SLA allows across every time window.

How it works

Enter an uptime percentage (e.g. 99.9) or a downtime value with its unit.
The calculator instantly converts between percentage and downtime across all time windows.
Review the per-day, per-week, per-month, and per-year downtime allowances.
Use the nines reference table to compare common SLA tiers at a glance.
Copy any value directly from the results for use in your SLA document.

Use cases

Translate an SLA percentage into concrete downtime budgets for engineering teams.
Verify whether a vendor SLA meets your internal reliability targets.
Plan incident response and maintenance windows within downtime budgets.
Compare the cost of moving from three nines to four nines in allowed downtime.
Draft SLOs for new services with a clear, numeric downtime ceiling.
Educate stakeholders on what "five nines" actually means in minutes per year.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu