Aftermarket, Field Service & Service Parts worked example

Service Parts Days of Supply with service parts inventory on hand of 930 parts: a worked example

Here is what the math looks like when conditions slip. We hold every other input steady and drop service parts inventory on hand to 930 parts, then walk the calculation through step by step. Calculate protected days of supply for service parts from inventory on hand, daily usage, and safety-stock factor.

The inputs for this scenario

  • Service parts inventory on hand: 930 parts (the input this scenario stresses; the baseline uses 1,850)
  • Average service parts usage: 42 parts/day (held at the documented default)
  • Demand and lead-time safety factor: 1.25 x (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Unprotected days of supply = on-hand service parts inventory รท average daily usage.
  • Protected service parts days of supply works out to 17.71 protected days at these inputs, and this is the headline figure for the scenario.
  • Unprotected days of supply works out to 22.14 days at these inputs.
  • Service parts inventory works out to 930 parts at these inputs.
  • Average service parts usage works out to 42 parts/day at these inputs.

How this compares with the baseline

  • Against the tool's baseline example, where service parts inventory on hand sits at 1,850 parts and the headline result is 35.24 protected days, this scenario comes in 49.73% below the baseline at 17.71 protected days.
  • The practical read: the gap between this scenario and the baseline is entirely attributable to service parts inventory on hand, so recovering it is worth quantifying in dollars before considering equipment or staffing changes. It uses a single average daily usage, so for intermittent or lumpy service-part demand - which is common for low-failure-rate components - it can badly misstate real cover; pair it with a demand-pattern review.

Results at a glance

  • Protected service parts days of supply: 17.71 protected days (headline result)
  • Unprotected days of supply: 22.14 days
  • Service parts inventory: 930 parts
  • Average service parts usage: 42 parts/day

Run it with your numbers

  • To rerun this with your own numbers, open the live Service Parts Days of Supply calculator, set service parts inventory on hand to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.