Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the Spare parts reorder & stock coverage calculator
Check your reorder point, stock coverage and replenishment gap for one spare part. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Average daily usage (units/day). Average units used per day, from your issue history.
- Supplier lead time (days). Days from placing an order until stock is on the shelf.
- Safety stock (units). Buffer units you choose to hold against late delivery or higher use.
- Stock on hand (units). Physical units on the shelf.
- Confirmed incoming orders (units). Units on confirmed orders that have not yet arrived.
- Reserved or backordered stock (units). Units reserved for jobs or already owed to a requester.
- Coverage after lead time (days). Days of cover you want to hold after the lead time.
- Unit cost (your currency). Cost of one unit, in your currency.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
Inventory position and physical stock are different. Confirmed orders can reduce the replenishment requirement, but they cannot supply a repair today. Lead time should include purchasing, manufacturing, transport and receiving delays. For a rarely used critical spare, an average demand formula alone may underestimate the risk of having no replacement available.
Reorder point = ceil(daily usage × lead time + safety stock). Inventory position = on hand + incoming − reserved/backordered. Target = ceil(daily usage × (lead time + coverage period) + safety stock). Suggested replenishment = max(0, target − inventory position).
This simple policy assumes reasonably steady demand and manually chosen safety stock. Intermittent critical spares need equipment-risk and failure-history review. Incoming orders count toward inventory position but not physical days of cover; check their promised dates separately. Reserved and backordered quantities should not overlap. Reorder trigger uses inventory position at or below the reorder point.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 25 units reorder point. Reorder threshold reached. Review incoming delivery dates, criticality and supplier order constraints before ordering.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These illustrative rows retain the worked example assumptions except for the varied input. They are not certified ratings or manufacturer data. Use the form for intermediate values.
Common mistakes
Counting promised stock as physically available; double-counting reserved units and backorders; applying a steady-demand formula blindly to intermittent or obsolete parts. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The results give the stock level at which to reorder, the current inventory position and a suggested order quantity. They rely on steady demand and your chosen safety stock, so review critical spares with failure history.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. This simple policy assumes reasonably steady demand and manually chosen safety stock. Intermittent critical spares need equipment-risk and failure-history review. Incoming orders count toward inventory position but not physical days of cover; check their promised dates separately. Reserved and backordered quantities should not overlap. Reorder trigger uses inventory position at or below the reorder point. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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