Allowed Downtime Calculator
See how much downtime the availability target you set allows.
In short
Formula: Allowed downtime = Period × (1 − Target availability)
What this calculator does
The complement of the target applied to the length of the period. The target is always entered by the user; the calculator neither suggests nor defaults one.
Use it to turn Availability target you have set, Length of the period 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
- Availability target you have set
- — Your own objective or contractual figure. No target is suggested or assumed..
- Length of the period
- — Days in the window the target applies to..
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
Allowed downtime = Period × (1 − Target availability).
Inputs used: Availability target you have set, Length of the period.
Edge handling: Availability target you have set is required.; Availability target you have set cannot be negative.; Availability target you have set must be no more than 100.; Length of the period is required.; Length of the period cannot be negative..
Worked example
A 99.9% target over 30 days
- Start with Availability target you have set: 99.9, Length of the period: 30.
- Apply the method: Allowed downtime = Period × (1 − Target availability).
- The target is the one you entered; the calculator does not recommend one.
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 allowed downtime calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Reference: Google — Implementing SLOs
Last reviewed:
Common questions
Where does this formula come from?
Implementing SLOs — a service level objective is the share of good events among valid events (or of available time in the period); the error budget is the remainder that the objective permits. See the source link on this page.
What kind of calculation is this?
The unavailable time an availability target leaves in the period. 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. Allowed downtime is an arithmetic consequence of the target, not advice about how much downtime is acceptable.
How do I use the Allowed Downtime Calculator?
Enter the required values for Availability target you have set, Length of the period. 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 Allowed Downtime Calculator use?
Allowed downtime = Period × (1 − Target availability) 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 Allowed Downtime 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 unavailable time an availability target leaves in the period.
The question it answers: How much error budget does our objective allow, how much is left, and did we meet the commitment in the agreement?
The formula
Allowed downtime = Period × (1 − Target availability)
- T — Target availability
- (%). The user's own objective or contractual figure.
- P — Period
- (days). Length of the window the target applies to.
Units: Days in, minutes and hours out.
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
The complement of the target applied to the length of the period. The target is always entered by the user; the calculator neither suggests nor defaults one.
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: The target applies to the whole period with no exclusions unless the user has agreed them separately.
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.
- No 'nines' figure is offered as a recommended or typical target.
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.
- Allowed downtime is an arithmetic consequence of the target, not advice about how much downtime is acceptable.
Source and version
- Standard or reference
- Google SRE Workbook — Implementing SLOs — Google SRE (Implementing SLOs — a service level objective is the share of good events among valid events (or of available time in the period); the error budget is the remainder that the objective permits.)
- Published source
- Google — SRE Workbook
- 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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