Spare Parts Stock Level Calculator

Work out spare parts stock level instantly with clear inputs, formula shown and shareable results.

About Spare Parts Stock Level Calculator

The reorder point is expected demand during the supplier lead time plus safety stock sized from the variability of that demand and the service level required. For spares the service level should reflect the cost of a stockout, which for a critical machine part is the whole downtime cost.

Formulas

Reorder point

ROP = daily usage x lead time + z x standard deviation of lead-time demand

Service factors

z = 1.282 for 90%, 1.645 for 95%, 2.326 for 99%

Worked example

With the default values below, Spare Parts Stock Level Calculator returns Reorder Point of 12.33. Change any input above and every figure here is recalculated in your browser.

Inputs used

Annual Usageminimum 0.01 units: 60 units
Supplier Lead Timeminimum 0.01 days: 45 days
Standard Deviation Of Lead-Time Demandminimum 0 units: 3 units
Target Service Levelone of 90%, 95%, 99%: 95%

Results

Reorder Point: 12.33
Expected Lead-Time Demand: 7.40
Safety Stock: 4.94
Maximum Stock Level: 27.33

Step by step

1Values used
2Reorder point
3Service factors
4Reorder Point= 12.33
5Expected Lead-Time Demand= 7.40
6Safety Stock= 4.94
7Maximum Stock Level= 27.33

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Should every spare be stocked?

No. Criticality analysis should split spares into insurance items held regardless of usage, routine consumables managed by reorder point, and items best sourced on demand.

What if usage is very low and lumpy?

Normal-distribution safety stock breaks down for intermittent demand. Use a Poisson or Croston-based model, or set stock from the consequence of a stockout rather than a service level.