Aftermarket, Field Service & Service Parts worked example

Service Parts Days of Supply with service parts inventory on hand of 4,600 parts: a worked example

This scenario runs the service parts days of supply calculation on the strong side: service parts inventory on hand of 4,600 parts, with every other input held at its documented default. a parts inventory analyst needs to understand how long current service stock will support demand

The inputs for this scenario

  • Service parts inventory on hand: 4,600 parts (raised for this scenario; the documented default is 1,850)
  • Average service parts usage: 42 parts/day (unchanged)
  • Demand and lead-time safety factor: 1.25 x (unchanged)

Working through the calculation

  • Applying the documented formula (Unprotected days of supply = on-hand service parts inventory รท average daily usage) to the inputs above produces each figure below.
  • At this operating point the engine returns 87.62 protected days for protected service parts days of supply, the number this scenario is built around.
  • At this operating point the engine returns 110 days for unprotected days of supply.
  • At this operating point the engine returns 4,600 parts for service parts inventory.
  • At this operating point the engine returns 42 parts/day for average service parts usage.

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 149% above the baseline at 87.62 protected days.
  • Use it when reviewing spares stocking levels at a depot or van, setting reorder triggers for critical parts, or stress-testing whether current inventory supports an uptime SLA. Treat this as a target state: the delta against the baseline quantifies what the improvement is worth before you commit to chasing it.

Results at a glance

  • Protected service parts days of supply: 87.62 protected days (headline result)
  • Unprotected days of supply: 110 days
  • Service parts inventory: 4,600 parts
  • Average service parts usage: 42 parts/day

Run it with your numbers

  • Every input above is editable in the live Service Parts Days of Supply calculator, which recalculates instantly and can be shared with the inputs intact.

Last reviewed 2026-05-12.