Source: https://guthlabs.ai/docs/instruments/mtbf-mttr-reliability-calculator/
instrument / verified-clean
# MTBF / MTTR / Reliability Calculator
The current ledger marks this instrument verified clean within the source-bound verification corpus.
Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**
## Inspectable records
[01**Overview**Purpose and lifecycle]()[02**Mechanism**Method and assumptions]()[03**Implementation**Binding and validation]()[04**Qualification**Evidence and errata]()
## 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`