Uptime and SLA Calculator

See what an uptime percentage allows in real downtime, how much redundancy adds, and what an outage is worth in SLA credits.

Inputs

Calculation

Uptime to downtime Downtime to uptime Combined availability SLA credit

Availability (uptime)

Result

Enter an availability percentage to see the allowed downtime.

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How the calculation works

Uptime to downtime. Allowed downtime = (1 − availability) × the length of the period. The calculator uses a year of 365.25 days, which averages in leap years, a month of 365.25 ÷ 12 = 30.44 days and a quarter of 365.25 ÷ 4 = 91.31 days. Some tables use a 365-day year or a 30-day month, so their figures differ slightly from these.

Downtime to uptime. Availability = 1 − downtime ÷ period, for a day, week, average month, quarter or year.

Combined availability. List the components a service depends on, in order:

In series (every one must be up, such as a circuit, then a router, then a firewall): multiply the availabilities. A = a₁ × a₂ × …
In parallel (any one can carry the load, such as two internet circuits with failover): A = 1 − (1 − a₁) × (1 − a₂) × …

Components joined “in parallel” form one stage; stages are then combined in series. The parallel formula assumes the components fail independently. Shared conduits, power, building entrances, upstream carriers or a single failover device all break that assumption, and the true availability will be lower than the result.

SLA credit. Availability for the month = 1 − outage minutes ÷ (days in the month × 1,440). The credit is the percentage set by the lowest tier the month falls below, times the monthly fee. A month that lands exactly on a threshold is not below it. Enter the tiers from your own contract: the defaults are placeholders, and many SLAs credit by outage duration or by incident rather than by monthly percentage.

Worked example

What 99.9% allows. 1 − 0.999 = 0.001 of the time can be down:

per day: 0.001 × 86,400 s = 1 min 26.4 s
per month: 0.001 × 30.4375 × 86,400 s = 2,629.8 s = 43 min 49.8 s
per year: 0.001 × 365.25 × 86,400 s = 31,557.6 s = 8 h 45 min 57.6 s

Going the other way. 45 minutes of downtime in an average month is 1 − 2,700 ÷ 2,629,800 = 99.897%, just short of three nines.

Two circuits and a firewall. An office has a 99.9% fiber circuit and a 99.5% broadband circuit in parallel, then a 99.99% firewall in series:

circuits in parallel: 1 − (0.001 × 0.005) = 99.9995%
then the firewall in series: 0.999995 × 0.9999 = 99.9895%, about 4 min 36.1 s of downtime a month and 55 min 13.5 s a year

With the fiber circuit alone, the same firewall gives 0.999 × 0.9999 = 99.89%, about 48 min 12.5 s a month. Once the circuits are redundant, the single firewall is the limit: a second firewall in parallel would raise the result further.

An SLA credit. A $2,000-a-month service with tiers of 10% below 99.9%, 25% below 99.5% and 50% below 99% has a 4-hour (240-minute) outage in a 30-day month. Availability is 1 − 240 ÷ 43,200 = 99.444%, below 99.5%, so the credit is 25% × $2,000 = $500.00. The first tier starts after 43 min 12 s of downtime that month.

Frequently Asked Questions

How much downtime does 99.9% uptime allow?

About 43 minutes 50 seconds a month, using an average month of 30.44 days, and about 8 hours 46 minutes a year. Per day it is 1 minute 26.4 seconds. In a 30-day billing month it is exactly 43 minutes 12 seconds.

What does five nines (99.999%) mean in practice?

About 5 minutes 16 seconds of downtime a year, or 26.3 seconds a month. Each extra nine cuts the allowed downtime by a factor of ten: 99.99% allows about 52 minutes 36 seconds a year and 99.9% about 8 hours 46 minutes.

Why does adding a component in series lower availability?

In series, every component has to be up for the service to work, so their availabilities multiply. A 99.9% circuit behind a 99.99% firewall gives 99.9% × 99.99% ≈ 99.89%, slightly worse than either part. Redundancy in one place does not help a single point of failure elsewhere in the path.

Do two 99.9% internet circuits give 99.9999%?

Only if they never fail at the same time for the same reason. The parallel formula, 1 − (0.001 × 0.001) = 99.9999%, assumes independent failures. Two circuits from different carriers that share a conduit, a building entrance or an upstream provider can be cut together, so the real figure is lower. Ask for route diversity and separate building entrances before counting on the math.

Do SLA credits cover the cost of an outage?

Usually not. Credits are a share of the monthly fee for the affected service, not of what the outage cost you. They are often capped, and many contracts require you to claim them within a set time. Scheduled maintenance and some causes are commonly excluded. Read the definitions in your contract, and compare the credit with what the outage costs your business.

Which month length does the calculator use?

For downtime per month it uses the average month, 365.25 ÷ 12 = 30.44 days, so twelve months add up to a year of 365.25 days. For SLA credits it uses the billing month you choose, 28 to 31 days, because providers measure availability over the actual month.