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Uptime and SLA calculator: how much downtime is allowed?

Turn an uptime percentage into the downtime it allows per day, week, month and year, or work out the uptime you actually had from minutes of downtime.

Updated October 12, 20265 min read

Pick an uptime percentage and see how much downtime it actually allows. Use the second box to turn the downtime you had into an uptime figure.

Uptime percentage

Allowed downtime: 99.9%
Period	Allowed downtime at 99.9%
Per day	1 min 26 s
Per week	10 min 5 s
Per month	43 min 48 s
Per year	8 h 45 min 36 s

Month means one twelfth of a 365-day year.

How is allowed downtime calculated?

Allowed downtime = period length x (100 - uptime percentage) / 100

For 99.9 percent over a 30-day month: 43,200 minutes x 0.1 / 100 = 43.2 minutes. This calculator uses a month of one twelfth of a 365-day year, which is 43 minutes and 48 seconds at 99.9 percent. If a contract says "per month", check how it defines one.

What do the nines mean?

Uptime	Common name	Downtime per year
99%	Two nines	3 days 15 hours 36 minutes
99.5%		1 day 19 hours 48 minutes
99.9%	Three nines	8 hours 45 minutes 36 seconds
99.95%		4 hours 22 minutes 48 seconds
99.99%	Four nines	52 minutes 34 seconds
99.999%	Five nines	5 minutes 15 seconds

Each extra nine cuts the allowance to a tenth, and the cost of reaching it rises faster than the benefit for most small sites. See the pages for 99.9%, 99.99% and 99% for what each level means in practice.

What should I promise a client?

Promise only what you can measure and what your hosting can deliver. A single shared host rarely sustains more than three nines, and a promise is only worth something when you monitor it. Define the measurement window, exclude planned maintenance explicitly and keep the check history, so a monthly report can show the number instead of asserting it. Our guide on setting up uptime monitoring for client websites covers the setup.

What counts as downtime?

The definition matters as much as the percentage. Decide in advance whether a slow response counts, how many failed checks in a row make an outage and whether planned maintenance is excluded. Ambiguity here causes more arguments than the number does.

Where Datumboard fits

Datumboard shows the health of every client site next to deploys, renewals, search data and costs, so you can see at a glance which sites are failing before a client asks. It is in early access, and you can join the list below.

Frequently asked questions

How much downtime is 99.9 percent uptime?

About 43 minutes and 48 seconds per month, or about 8 hours 45 minutes and 36 seconds per year. The figure depends on how the month is defined, and this calculator uses a twelfth of a 365-day year.

How do I calculate uptime percentage?

Subtract the downtime from the total period, divide by the total period and multiply by 100. For example, 45 minutes of downtime in a 30-day month of 43,200 minutes is about 99.896 percent uptime.

Why do different calculators give slightly different answers?

They define a month differently. Some use exactly 30 days, others use a twelfth of a 365-day year. The difference is small, but an SLA should say which definition it uses.

Is 99.99 percent uptime realistic for a small client site?

It allows about 52 minutes of downtime a year, which usually needs redundant hosting and fast failover. A typical small site on a single host aims for 99.9 percent, and the cost of each extra nine rises quickly.