MTBF / MTTR / Reliability Calculator

Inspectable records

Ledger state

The current ledger marks this instrument verified clean within the source-bound verification corpus.

Area

Computing & Information

Tranche

tranche-5

Evidence ID

t5-059

Ledger rows

1

No ledger note is attached.

Review findings

The core availability, series, and parallel formulas are standard under the stated independence and steady-state assumptions. The specification needs an explicit downtime-per-year formula, a precise MTBF definition, validation rules for nonpositive inputs and empty topologies, and an honest distinction between availability and reliability before it can receive PASS.

Formula and scope findings

• A=MTBF/(MTBF+MTTR) is the standard steady-state availability formula when MTBF means mean operating time between failures and MTTR means mean repair time, with both expressed in the same time units.
• The series formula is correct for components that are all required to operate and whose availabilities can be treated as independent.
• The parallel formula is correct for independent redundant components where the system operates when at least one component operates.
• The specification claims a downtime-per-year equivalent output but provides no formula for it; an explicit formula such as downtime_per_year=(1-A_sys)*hours_per_year is required.
• The name includes Reliability, but the listed formulas compute availability, not time-dependent reliability. A standard constant-failure-rate reliability formula would be R(t)=exp(-t/MTBF), under additional assumptions.
• MTBF is ambiguous unless defined as mean uptime between failures; if MTBF includes repair time in a failure-to-failure interval, the stated availability formula is not valid as written.

Unit assumptions

• MTBF and MTTR must use identical time units before addition or division.
• The test vectors use 8760 hours per year; leap years and calendar-specific downtime are not modeled.
• The downtime-per-year equivalent is assumed to be (1-A_sys)*8760 hours.
• Availability is interpreted as long-run steady-state availability for repairable systems.
• Series and parallel component availabilities are assumed statistically independent, with no common-cause failures, shared repair dependencies, or switching failures.

Boundary hardening

BREAK_CLAIMED · SELF_FLAGGED

The MTBF = 0 and MTTR = 0 input produces the undefined expression 0/0 and must be rejected. The calculator also has an explicit annual-downtime convention sensitivity when the input time unit is days.

Errata and corrections

No applied erratum is attached to this unique-name record.

Inspectable example

4 review vectors are recorded. One source-bound sample is shown below.

{
  "inputs": {
    "MTBF_hours": 1000,
    "MTTR_hours": 10,
    "topology": "single",
    "component_count": 1
  },
  "expected_outputs": {
    "component_availability": 0.9900990099,
    "system_availability": 0.9900990099,
    "downtime_per_year_hours": 86.7326732673
  },
  "working": "A=MTBF/(MTBF+MTTR)=1000/(1000+10)=1000/1010=100/101=0.9900990099. Downtime/year=(1-A)*8760=(1-100/101)*8760=(1/101)*8760=8760/101=86.7326732673 hours."
}

Related workflows

No source-defined multi-instrument workflow membership is recorded.

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json · VERIFIED_CLEAN
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json