Availability Calculator
Work out availability from your own MTBF and MTTR.
In short
Formula: A = MTBF / (MTBF + MTTR)
What this calculator does
Inherent availability as defined in the NIST/SEMATECH handbook: mean operating time as a share of the mean full cycle. Expected downtime over 30 days and 365 days is the unavailability share applied to those periods.
Use it to turn Mean time between failures (MTBF), Mean time to repair (MTTR) into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this professional & industry calculation.
Inputs and what they mean
- Mean time between failures (MTBF)
- — Average operating time between one failure and the next..
- Mean time to repair (MTTR)
- — Average time to restore service once a failure has happened..
How to use it
- Enter your own figures — the calculator never fills in a rate, price or benchmark for you.
- Press Calculate to see the result.
- Read the formula, variables, assumptions and source below the result before you rely on it.
Formula
A = MTBF / (MTBF + MTTR).
Inputs used: Mean time between failures (MTBF), Mean time to repair (MTTR).
Edge handling: Mean time between failures (MTBF) is required.; Mean time between failures (MTBF) cannot be negative.; Mean time between failures (MTBF) must be no more than 1000000000000.; Mean time to repair (MTTR) is required.; Mean time to repair (MTTR) cannot be negative..
Worked example
720 hours between failures, 2.5 hours to repair
- Start with Mean time between failures (MTBF): 720, Mean time to repair (MTTR): 2.5.
- Apply the method: A = MTBF / (MTBF + MTTR).
- Availability from your own reliability data. No target availability is implied.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Inputs outside the supported range are rejected rather than forced into a result.
- The result depends on the values you enter for this availability calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Reference: National Institute of Standards and Technology — Reliability measures: MTBF, MTTR and availability
Last reviewed:
Common questions
Where does this formula come from?
Section 8.1.2.4 — mean time between failures is operating time divided by the number of failures; mean time to repair is total repair time divided by the number of repairs; inherent availability is MTBF / (MTBF + MTTR). See the source link on this page.
What kind of calculation is this?
The share of a failure-and-repair cycle during which the service is operating. Check the formula, example, and limitations on this page before using the result for a real professional & industry decision.
What are its limits?
Arithmetic over the figures entered — it cannot tell whether the underlying monitoring, incident records or change records are complete. A mean over a short period, or over few events, is a weak description of behaviour. Inherent availability excludes delays outside repair itself, so it is usually higher than availability as the user experiences it.
How do I use the Availability Calculator?
Enter the required values for Mean time between failures (MTBF), Mean time to repair (MTTR). The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Availability Calculator use?
A = MTBF / (MTBF + MTTR) The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone. Check the formula, example, and limitations on this page before using the result for a real professional & industry decision.
Can the Availability Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
Formula, source and verification
The share of a failure-and-repair cycle during which the service is operating.
The question it answers: What availability, uptime, mean time between failures and mean time to repair does our own data show?
The formula
A = MTBF / (MTBF + MTTR)
- MTBF — Mean time between failures
- (h). Operating time divided by failures, from the user's own data.
- MTTR — Mean time to repair
- (h). Total repair time divided by repairs, from the user's own data.
Units: Hours in, percentage out. Both times must be in the same unit.
What kind of calculation this is
Deterministic formula. The same inputs always give the same answer. The maths is fixed and does not depend on judgement.
Method
Inherent availability as defined in the NIST/SEMATECH handbook: mean operating time as a share of the mean full cycle. Expected downtime over 30 days and 365 days is the unavailability share applied to those periods.
Assumptions built into the result
- Mathematical: The figures entered all cover the same measurement period.
- Mathematical: Incidents, changes and requests are counted consistently with the definition the team has written down.
- Mathematical: Both times are means over the same population of failures.
- Mathematical: Only repair time is counted as downtime; logistic and administrative delay are not included unless the user included them in MTTR.
Figures this calculator will never guess for you
- No target, benchmark, industry average or performance band is supplied for any figure.
- A definition is never presented as a goal: the availability formula is not an availability target, and the change failure rate formula is not an acceptable failure rate.
Limitations
- Arithmetic over the figures entered — it cannot tell whether the underlying monitoring, incident records or change records are complete.
- A mean over a short period, or over few events, is a weak description of behaviour.
- Inherent availability excludes delays outside repair itself, so it is usually higher than availability as the user experiences it.
Source and version
- Standard or reference
- NIST/SEMATECH e-Handbook of Statistical Methods — NIST (Section 8.1.2.4 — mean time between failures is operating time divided by the number of failures; mean time to repair is total repair time divided by the number of repairs; inherent availability is MTBF / (MTBF + MTTR).)
- Published source
- National Institute of Standards and Technology — Section 1.4
- Formula version
- Version 1
- Verification
- Reviewed against the cited source on
- How much weight the source carries
- Standards or government
- Applies to
- Currency
- The result is a ratio or index, so it does not depend on currency.
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