Allowed Downtime per SLA Calculator
A service level agreement promises a percentage of uptime, but engineers think in minutes and hours of outage they can afford. This calculator turns an uptime target into an allowed downtime budget. The rule is simple: take the period the SLA covers, multiply it by one minus the uptime fraction, and the result is the maximum time the service may be unavailable before the target is missed. A promise of 99.9 percent uptime over a 30 day month leaves 0.1 percent for outages, which is 0.001 times 30 days, or 43.2 minutes. Each extra nine in the target cuts the budget by roughly ten, which is why the jump from three nines to four nines is so costly to engineer. The page reports the budget in days, hours and minutes so you can compare it against a single incident or a month of small blips. Use it to set realistic targets, size on-call capacity, or check whether a vendor's promise matches your needs. The figure scales directly with the period, so pick the window your SLA measures over. Every value is computed deterministically from your two inputs, never estimated, with the method and a worked example shown below for verification.
Allowed downtime is the period times one minus the uptime fraction: downtime = (1 - SLA) x period. A 99.9% target over a 30 day month allows 43.20 minutes of outage.
Allowed downtime formula
downtime = (1 - SLA fraction) x period
SLA fraction = uptime percent / 100
period in minutes = days x 24 x 60
downtime minutes = (1 - SLA fraction) x period minutes
The fraction of the period not covered by the uptime promise is the downtime budget. Scaling it to minutes and hours makes it easy to compare against real incidents.
Worked example
An SLA promises 99.9 percent uptime measured over a 30 day month.
- SLA fraction = 99.9 / 100 = 0.999
- Downtime fraction = 1 - 0.999 = 0.001
- Period in minutes = 30 x 24 x 60 = 43,200 minutes
- Downtime = 0.001 x 43,200 = 43.2 minutes
The budget is 43.20 minutes. These are the calculator's default inputs, so the result above matches the widget exactly.
Downtime by number of nines (per year)
Each extra nine cuts the budget by roughly a factor of ten.
| Uptime | Allowed per year |
|---|---|
| 99% | 3.65 days |
| 99.9% | 8.77 hours |
| 99.99% | 52.6 minutes |
| 99.999% | 5.26 minutes |
Availability and reliability measurement concepts: US National Institute of Standards and Technology (NIST).
Allowed downtime calculator: frequently asked questions
How do you calculate allowed downtime?
Multiply the period by one minus the uptime fraction. If the SLA promises 99.9 percent uptime over a 30 day month, the allowed downtime is 0.001 times 30 days, which is 0.03 days, or 43.2 minutes.
What does the number of nines mean?
Each extra nine in an uptime target cuts allowed downtime by roughly a factor of ten. Over a year, 99 percent allows about 3.65 days down, 99.9 percent about 8.77 hours, 99.99 percent about 52.6 minutes, and 99.999 percent, called five nines, about 5.26 minutes.
Should I use the month or the year as the period?
Use the period your SLA measures over. Many cloud agreements report uptime monthly, often using a 30 day month, while internal targets are sometimes annual. The allowed downtime scales directly with the period you choose.
Does this include planned maintenance?
That depends on the SLA. Some agreements exclude scheduled maintenance windows from the downtime calculation, so the real budget for unplanned outages is the figure here minus any agreed maintenance. Check the wording of your specific SLA.
Is the result computed automatically?
Yes. The page multiplies the period by one minus the uptime fraction deterministically and converts to minutes and hours. No value is estimated or hard-coded, so changing either input updates the answer instantly.
Official sources
- Availability and reliability measurement concepts: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.