How is inherent availability calculated from MTBF and MTTR?

Inherent availability A = MTBF ÷ (MTBF + MTTR), where MTTR is mean time to repair in the same time units as MTBF.

How does the spares allowance calculator work?

It models constant-rate demand with a Poisson distribution. Consumables use mission and lead-time demand; repairables use an induction-rate times turnaround-time pipeline approximation. It is transparent decision support, not an approved provisioning analysis.

Does the CSV spares calculator upload inventory data?

No. CSV files are read and processed locally by JavaScript in the browser and are not uploaded or retained by SILS.

Supportability Toolkit

Q: What is failure rate lambda?

Under a constant failure-rate assumption, lambda (λ) is the reciprocal of MTBF: λ = 1 ÷ MTBF, expressed as failures per hour.

Transparent, browser-only MTBF and risk-based spares allowance screening for ILS, inventory, and reliability teams.

Live

MTBF & Failure Rate Calculator

Compute mean time between failures and constant failure rate from total operating exposure and observed failures. Optional MTTR and planning period for availability and expected failures.

Formulas: MTBF = T ÷ F · λ = 1 ÷ MTBF · A = MTBF ÷ (MTBF + MTTR) · Expected failures ≈ N × Period ÷ MTBF

Total operating time (T) Hours (or unit-hours for a fleet)

Number of failures (F) Observed failures in that exposure

Identical units (N) — optional Used with planning period for expected failures

MTTR (hours) — optional Mean time to repair → inherent availability

Planning period (hours) — optional 8760 ≈ 1 year continuous; 720 ≈ 30 days

Results

MTBF

—

hours

Failure rate (λ)

—

failures per hour

Inherent availability

—

requires MTTR

Expected failures

—

in planning period

Educational / screening tool assuming constant failure rate. Results are not a substitute for programme LSA, warranty analysis, or certified RAM studies. Use consistent time units throughout. For formal reliability work, contact SILS.

Live

Risk-Based Spares Allowance Calculator

Separate economic ABC class from consequence-based criticality, then calculate an explainable allowance. Everything runs locally in this browser.

Local processing: CSV data is never uploaded or retained.

Decision-support scope: This single-location, single-echelon screening model assumes independent, constant-rate Poisson demand and deterministic lead/turnaround times. It is not an approved provisioning analysis, FMECA, LORA, RAM model, safety assessment, or engineering authorization.

Method context and references

The workflow uses S2000M provisioning concepts and the supportability-planning context of MIL-HDBK-502A. Constant-rate event counts follow the Poisson model described in the NIST/SEMATECH handbook. Repairable allocation is intentionally a transparent approximation; classic METRIC identifies the richer multi-echelon method that remains outside V1. These references provide context, not standards approval or compliance.

Item and demand

Item ID

Item type

Installed quantity

Annual hours per unit

MTBF (hours)Or use exposure and failures below

Quantity per failure

Use observed history instead of MTBF

Exposure hours

Observed failures

Planned annual demand

Unit cost

Policy and support

ABC classPortfolio mode derives this from annual usage value

Criticality

Criticality consequence gates

Safety consequence?

Mission loss?

Expected downtime (hours)

Substitute available?

Lead time (days)

Current stock

Mission / protection period (days)

No-stockout target override (%)Policy default is calculated from ABC and criticality.

Drop a CSV here or select a local file (maximum 5,000 rows)

Download sample / template

Item	Type	ABC	Risk	Annual demand	Current	Recommended	Achieved	Gap cost	Quality	Details

Portfolio availability is a criticality-weighted average of per-item no-stockout probabilities. Recommendations satisfy policy minimums first, then greedily add the highest marginal weighted-availability gain per unit cost. This is not a full multi-echelon METRIC optimization.

FAQ

Supportability Toolkit — Common Questions

How do you calculate MTBF?

MTBF (Mean Time Between Failures) equals total operating time divided by the number of failures: MTBF = T ÷ F. Use consistent units (usually hours or unit-hours for a fleet).

What is failure rate lambda (λ)?

Under a constant failure-rate assumption, λ = 1 ÷ MTBF — failures per hour. Higher λ means more frequent failures.

How is inherent availability calculated?

With MTTR (mean time to repair): A = MTBF ÷ (MTBF + MTTR). This is a screening estimate — programme RAM studies may use additional downtime factors.

Is this calculator suitable for defence / ILS programmes?

It is a free screening tool for engineers and ILS teams. Formal LSA, warranty, or certified RAM deliverables should use programme-specific data and standards — contact SILS for that work.

How are spares allowances calculated?

Consumables use Poisson mission and lead-time demand. Repairables use the explainable pipeline approximation mean = annual inductions × turnaround days ÷ 365, with expected backorders shown separately.

What standards and methods provide context?

The workflow is informed by S2000M provisioning concepts, NIST guidance on Poisson processes, and the supportability context of MIL-HDBK-502A. METRIC is cited as the multi-echelon benchmark; this calculator does not claim compliance, approval, or full METRIC optimization.

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