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Lifetime Buy Spares Estimate

This tool forecasts the number of spare parts needed to support a system through the end of anticipated system useful life based on characteristics about the population of fielded units, current depot inventory levels and unit repairability. The tool is intended to provide a long range forecasting estimate of the number of spare parts that should be procured near-term to support a given number of field operating units (nf); therefore, it takes as input a time frame of years and ignores any short term shortages that may arise during the time window when units are repaired and restocked to depot inventory. It also does not take into account any degradation that may occur with depot inventory units over long periods of time.

Calculation Inputs:

1. Fielded Units:

a) Number of units in the field (nf)
c) Remaining time units will be in service (years)

2. Depot Inventory:

a) Number of units currently in depot inventory (ni)
b) Fraction of failed units that will be repaired and put back into inventory (0 - 1.0):
c) Calculate inventory depletion based on:

Expected failures
Sparing requirements, at a confidence of:

Decimal places:

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Reliability Analytics Corporation


  1. MIL-HDBK-338, Electronic Reliability Design Handbook.
  2. Bazovsky, Igor, Reliability Theory and Practice.