Uptime / SLA calculator

Turn a 99.9% SLA into real allowed downtime per day, month and year.

Uptime SLA (%)

43m 11s

allowed downtime per month at 99.9% uptime

Per day1m 26sPer week10m 4sPer month (30d)43m 11sPer quarter (90d)2h 9m 35sPer year (365d)8h 45m 35s

Allowed downtime is the SLA gap (100% minus your SLA) applied to each period. ServerCake targets a 99.9% uptime SLA. Calculated in your browser.

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What does an uptime SLA mean in real downtime?

An uptime SLA is the availability a provider promises over a period, written as a percentage such as 99.9 percent. The gap below 100 percent is the downtime you are allowed, and small-looking percentages translate into very different real numbers. This calculator turns any figure into the maximum permitted outage per day, week, month and year: 99.9 percent leaves about 43 minutes a month, 99.99 percent leaves about four minutes, and 99 percent allows over seven hours. Enter your target and read the windows across each timescale. Use it to judge whether a supplier's number is strong enough for your workload, to size an error budget for planned maintenance, or to translate a contract clause into the concrete interruption your users would actually experience over a month or a year of continuous operation.

Key facts

Allowed downtime equals (1 minus uptime/100) times the length of the period, where a year is 8,760 hours and a 30-day month is 43,200 minutes.
A 99.9 percent uptime SLA (three nines) allows about 8 hours 46 minutes of downtime per year, or roughly 43 minutes per 30-day month.
A 99.99 percent uptime SLA (four nines) allows about 52 minutes 34 seconds of downtime per year, or roughly 4 minutes 19 seconds per month.
Each additional nine of uptime cuts the allowed downtime by roughly ten times, so 99.9 percent to 99.99 percent is a tenfold reduction.
Most SLAs pay out in service credits toward future invoices rather than cash, and typically exclude scheduled maintenance, DDoS attacks and consequential losses.

What an SLA uptime percentage actually means

An availability SLA is a promise about the fraction of time a service is up over a period. 99.9% uptime means the service may be down for at most 0.1% of the period; the allowed downtime is simply (100% - uptime%) times the length of the period.

The formula is: allowed downtime = (1 - uptime / 100) x period. A year is 8,760 hours (365 x 24) and a 30-day month is 43,200 minutes, so 99.9% over a year is 0.001 x 8,760 = 8.76 hours, and over a 30-day month is 0.001 x 43,200 = 43.2 minutes.

Providers usually quote a monthly figure because SLA credits are calculated per billing month, but buyers often think in yearly terms. This tool shows both so the number you compare against is the one that matters to you.

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The nines: allowed downtime per year and per month

99% (two nines): about 3.65 days a year, or roughly 7 hours 18 minutes a month. This is a weak SLA for anything customer-facing.

99.9% (three nines): about 8 hours 46 minutes a year, or roughly 43 minutes a month. This is the common baseline for shared hosting and many managed services.

99.95%: about 4 hours 23 minutes a year, or roughly 21 minutes 36 seconds a month. A frequent midpoint for business-tier services.

99.99% (four nines): about 52 minutes 34 seconds a year, or roughly 4 minutes 19 seconds a month. Typical for serious cloud infrastructure and needs redundancy to hit.

99.999% (five nines): about 5 minutes 15 seconds a year, or roughly 26 seconds a month. Rare, expensive, and usually requires multi-region failover and automated recovery.

Each extra nine cuts the allowed downtime by roughly ten times, so the jump from 99.9% to 99.99% is a tenfold reduction in tolerable outage, not a small tweak.

Error budgets: the flip side of an SLA

The allowed downtime is your error budget. If your target is 99.9% over a month, your error budget is about 43 minutes of downtime; as long as you stay inside it you are meeting the objective, and every outage spends part of it.

SRE teams use this budget to balance reliability against shipping speed. If the budget is healthy, you can take more deployment risk; if a bad month has already burned most of it, you freeze risky changes and focus on stability until the budget refills at the start of the next period.

Note the difference between an internal SLO (the target your team aims for) and the contractual SLA (the number you are held to). Teams usually set the SLO tighter than the SLA so they have warning before they breach the promise to customers.

SLA credits: what you actually get for downtime

Most SLAs pay out in service credits, not cash and not compensation for your losses. If a provider misses the target, you typically get a percentage of that period's bill back as credit toward future invoices - for example 10% of the monthly fee if uptime falls below 99.9%, more if it falls further.

Credits almost never cover consequential loss such as lost sales, missed transactions or reputational damage; SLA terms usually exclude those explicitly. The credit is a penalty on the provider, not insurance for your business.

Read the fine print: many SLAs exclude scheduled maintenance windows, third-party or DDoS-driven outages, and problems caused by your own configuration from the downtime count. You usually also have to request the credit within a set window rather than receiving it automatically.

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Glossary

SLA (Service Level Agreement): An availability SLA is a contractual promise about the fraction of time a service is up over a period. 99.9 percent uptime means the service may be down for at most 0.1 percent of that period.
Allowed downtime formula: Allowed downtime equals (1 minus uptime/100) times the length of the period. A year is 8,760 hours and a 30-day month is 43,200 minutes, so 99.9 percent over a year allows about 8.76 hours.
The nines: The nines describe availability by the count of leading nines: 99 percent is two nines, 99.9 percent is three nines and 99.99 percent is four nines. Each extra nine cuts allowed downtime roughly tenfold.
Error budget: The error budget is the allowed downtime seen as a spendable amount. Against a 99.9 percent monthly target it is about 43 minutes of outage, and every incident spends part of it until the period resets.
SLO vs SLA: An SLO is the internal reliability target a team aims for, while the SLA is the contractual number it is held to. Teams usually set the SLO tighter than the SLA to get warning before a breach.
Service credits: Service credits are the usual SLA remedy: a percentage of the period's bill refunded toward future invoices if the target is missed. They exclude consequential losses and often exclude scheduled maintenance.

Questions, answered.

How much downtime does 99.9% uptime allow?+

99.9% (three nines) allows about 8 hours 46 minutes of downtime a year, which works out to roughly 43 minutes 12 seconds per 30-day month, about 10 minutes 5 seconds a week, or about 1 minute 26 seconds a day.

What does 99.9% uptime mean in hours and minutes?+

It means the service can be unavailable for at most 0.1% of the time. Over a 8,760-hour year that is about 8.76 hours (8 hours 46 minutes); over a 30-day month it is about 43 minutes.

What is the difference between 99.9% and 99.99%?+

One extra nine cuts the allowed downtime roughly tenfold. 99.9% permits about 43 minutes of downtime a month, while 99.99% (four nines) permits only about 4 minutes 19 seconds a month, or about 52 minutes 34 seconds a year.

How much downtime is five nines (99.999%)?+

99.999% allows about 5 minutes 15 seconds of downtime per year, or roughly 26 seconds a month. Hitting it usually requires redundancy, multi-region failover and automated recovery, which is why it is rare and expensive.

What is the SLA downtime formula?+

Allowed downtime = (1 - uptime / 100) x period. A year is 8,760 hours and a 30-day month is 43,200 minutes, so 99.95% over a year is 0.0005 x 8,760 = 4.38 hours (about 4 hours 23 minutes).

What is an error budget?+

An error budget is the allowed downtime seen as a spendable amount. If your target is 99.9% a month, your budget is about 43 minutes of outage; every incident spends part of it, and teams slow down risky changes once most of it is used up.

What do I get if a provider misses its SLA?+

Usually service credits, not cash or compensation for your losses. A typical SLA refunds a percentage of that period's bill as credit toward future invoices, and explicitly excludes consequential losses like lost sales or reputational damage.

Does the SLA count planned maintenance?+

Often not. Many SLAs exclude scheduled maintenance windows, DDoS attacks and problems caused by your own configuration from the downtime count, so read exactly what 'availability' means in the specific agreement before comparing numbers.

What uptime does ServerCake offer?+

ServerCake targets a 99.9% uptime SLA, which is about 43 minutes of allowed downtime a month. Always read the SLA terms for what counts as downtime and how credits are calculated.

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