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SLA Availability Calculator

Convert MTBF and MTTR into availability, the nines it earns and the downtime allowed per year, month and week.

About SLA Availability Calculator

Availability is the share of time the system is up, which is repair-time-weighted rather than failure-count-weighted. Truly independent redundant paths multiply the unavailabilities, which is why two 99.9% paths approach 99.9999% — provided they share no fibre, power feed or software version. SLA credits are written in nines but paid in minutes, and the per-month figure is the one an operations team can actually manage against.

Formulas

SLA Availability

availability = MTBF ÷ (MTBF + MTTR); for N independent parallel paths, unavailability is raised to the power N.

MTBF
Mean time between failures
MTTR
Mean time to repair
N
Number of independent redundant paths

Downtime budget

Downtime per year = (1 − availability) × 8760 hours, so 99.9% allows 8.77 hours.

Worked example

With the default values below, SLA Availability Calculator returns Availability of 99.95436%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean time between failuresaccepts 1–100,000,000 hours
8,760 hours
Mean time to repairaccepts 0.01–10,000 hours
4 hours
Independent redundant pathsaccepts 1–8 paths
1 paths

Results

Availability
99.95436%
Nines Achieved
Three nines (99.9%) — 8.77 h/year
Downtime per Year
4.00 hours
Downtime per Month
20.0 minutes
Downtime per Week
4.6 minutes

Step by step

  1. 1Values used
  2. 2SLA Availability
  3. 3Downtime budget
  4. 4Availability= 99.95436
  5. 5Nines Achieved= Three nines (99.9%) — 8.77 h/year
  6. 6Downtime per Year= 4.00 hours
  7. 7Downtime per Month= 20.0 minutes
  8. 8Downtime per Week= 4.6 minutes

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is SLA Availability calculated?

availability = MTBF ÷ (MTBF + MTTR); for N independent parallel paths, unavailability is raised to the power N. Availability is the share of time the system is up, which is repair-time-weighted rather than failure-count-weighted. Truly independent redundant paths multiply the unavailabilities, which is why two 99.9% paths approach 99.9999% — provided they share no fibre, power feed or software version.

Why does SLA Availability matter?

SLA credits are written in nines but paid in minutes, and the per-month figure is the one an operations team can actually manage against.

What values do I need to enter?

This calculator takes 3 inputs: Mean time between failures, Mean time to repair, Independent redundant paths. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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