Spare Parts Min/Max and Reorder Point Calculator
Safety stock, reorder point, maximum stock and tied-up capital from usage, lead time and the variability of both.
Calculator
From usage, lead time and the variability of both to safety stock, the reorder point and the capital it ties up.
Result
The calculation runs entirely in your browser; nothing you type is sent to our servers. Only if you press “Turn this into a quote request” is the result written to your browser’s session storage, so it can be carried into the quote form.
Worked example
Opened with its default case — Average daily usage (pcs): 2 · Standard deviation of daily usage: 0.8 · Average lead time (days): 21 — the calculator returns the figures below. They are written out here so the output is readable without running JavaScript: in print, with scripts disabled, or by a search engine.
| Input | Value |
|---|---|
| Average daily usage (pcs) | 2 |
| Standard deviation of daily usage | 0.8 |
| Average lead time (days) | 21 |
| Standard deviation of lead time (days) | 5 |
| Target service level | 95% — general purpose |
| Order quantity / minimum order (pcs) | 0 |
| Unit price (TRY) | 0 |
| Cost of one day of downtime (TRY) | 0 |
| Result | Value |
|---|---|
| Safety stock | 18 pcs |
| Reorder point (ROP) | 60 pcs |
| Maximum stock | Enter an order quantity |
| Reorder point coverage | 30 days of demand |
| Capital tied up | Enter a unit price |
Change any field above and the calculator recomputes; this table is the default case only.
How much safety stock does a spare part need?
Safety stock covers variability, not average demand — average demand is covered by the cycle stock. The standard formula combines both sources of variability: σ = √(LT × σ_d² + d² × σ_LT²), then safety stock = z × σ and reorder point = (d × LT) + safety stock, where d is average daily usage, LT is the average lead time and z comes from the service level (90% → 1.28, 95% → 1.65, 97.5% → 1.96, 99% → 2.33). The term most people leave out is the second one: an unreliable supplier drives safety stock harder than erratic consumption does, because σ_LT is multiplied by daily usage. Fixing the lead time is usually cheaper than financing the stock that compensates for it.
Why the average is not enough
If usage were exactly 2 a day and the supplier delivered exactly on day 21, you would need 42 pieces on the shelf when you place the order and no safety stock at all. Neither is ever exactly true. Safety stock is the price of that uncertainty, and it is not linear: going from 95% to 99% service does not cost 4% more stock, it costs about 40% more, because z rises from 1.65 to 2.33 while everything else stays the same.
| Source | Symbol | How to estimate it | What reduces it |
|---|---|---|---|
| Demand variability | σ_d | Standard deviation of daily or weekly issues from the store; roughly 40% of the mean if you have no history | Condition monitoring, planned replacement instead of run-to-fail |
| Lead-time variability | σ_LT | Spread of actual delivery dates against the promised one, in days | A supplier who holds the item; a framework order; consignment stock |
| Lead time itself | LT | Order date to shelf date, including inbound inspection | Local stock, faster shipping mode, earlier release of the order |
| Service level | z | A policy choice, set by what the stockout costs | Nothing — this one is deliberate |
The calculator applies both variability terms together, which is why its answer is higher than the “z × σ_d × √LT” version found in most textbooks and spreadsheets. That simpler formula assumes the supplier never varies, and in practice the supplier is the larger term.
The four numbers you need
You can start with worse data than you think. Daily usage comes from annual issues divided by working days. Lead time comes from your own purchase orders, not the supplier’s catalogue promise. If you have no deviation figures at all, 40% of the mean for demand and a third of the lead time for supply are workable opening assumptions — then replace them with measured values after two or three cycles.
A useful sanity check: the reorder point should cover roughly the lead time plus a margin. If the calculator returns a reorder point that covers 60 days when your lead time is 21, the deviation figures you entered are too high — or the item genuinely is unpredictable, which is itself worth knowing before you commit capital to it.
Service level is a money decision
The right service level is not a matter of taste; it follows from the cost of not having the part. Enter the cost of one day of downtime and the calculator compares the extra stock needed to reach 99% against what a single day of stoppage costs. For a €40 bearing on a line that loses €20,000 a day, the answer is obvious and the discussion is over. For a consumable that can be bought locally in an afternoon, 90% is generous.
| Consequence of a stockout | Suggested level | Typical items |
|---|---|---|
| Production stops; no workaround | 99% | Single-sourced drive components, control boards, custom seals |
| Production continues degraded | 97.5% | Filters, belts, sensors with a spare channel |
| Maintenance is delayed, output unaffected | 95% | General bearings, fasteners, lubricants |
| Easily bought locally within a day | 90% | Standard consumables, hand tools, common fixings |
The table under the calculator prices all four levels side by side for your own item, so the choice is made against a number rather than a feeling.
Min/max, ROP and order quantity
The three figures are related but answer different questions. The reorder point answers when to buy: raise the order when free stock crosses it. The order quantity answers how many, and is usually set by the supplier’s minimum, a price break or a packaging unit rather than by an economic-order-quantity formula. Maximum stock is simply the reorder point plus the order quantity — the level the shelf reaches when a delivery lands just as the previous stock ran down to the reorder point.
Minimum stock, in the min/max convention, is the reorder point. Systems that ask for “min” and “max” are asking for those two numbers, and the most common error is entering safety stock as the minimum — which triggers the order a full lead time too late.
Which parts to hold at all
Not every part deserves a shelf. Classify on two axes before you calculate anything: criticality (what happens when it fails) and availability (how quickly it can be replaced).
| Consequence of failure | Available quickly | Long or uncertain lead time |
|---|---|---|
| Stops production | Hold a small quantity; the reorder point is what matters | Hold generously; this is where 99% belongs |
| Does not stop production | Do not stock — buy on demand | Hold a minimum, or agree consignment stock with the supplier |
The bottom-right box is the one that quietly consumes working capital: slow-moving items with a long lead time that nobody is willing to delete because “we might need it”. Review that box annually against actual issues.
What to send us for a quote
Send the part list with manufacturer references, the annual usage per line and, where you have it, the reorder point from this calculator. Quantities plus usage let us quote against a framework rather than line by line, hold agreed items and deliver against call-offs — which is what actually shortens the lead time you just paid safety stock to cover.
Frequently Asked Questions
What is the formula for safety stock?
Safety stock = z × √(LT × σ_d² + d² × σ_LT²), where d is average daily usage, LT the average lead time in days, σ_d the standard deviation of daily usage, σ_LT the standard deviation of the lead time and z the service factor (1.65 for 95%, 2.33 for 99%). The reorder point is (d × LT) plus that safety stock.
What is the difference between the reorder point and the minimum stock?
In min/max systems they are the same number: the level at which an order is raised. Safety stock is a different and lower figure — the buffer that remains untouched if everything runs to plan. Entering safety stock as the minimum triggers replenishment one full lead time too late.
How do I estimate the deviation if I have no data?
Start with about 40% of the average for daily usage and roughly a third of the lead time for supply, then replace both with measured values once you have two or three cycles of history. Being approximately right beats waiting for perfect data while the shelf is empty.
Why is my safety stock so high?
Almost always lead-time variability rather than demand variability, because σ_LT is multiplied by average daily usage in the formula. If the supplier delivers anywhere between 15 and 40 days, no reasonable amount of stock feels safe. Fixing the supply is cheaper than financing the buffer.
Should the same service level apply to every part?
No. Set it by consequence: 99% for anything that stops production with no workaround, 90% for items you can buy locally within a day. A flat policy across the catalogue over-stocks the cheap items and under-stocks the dangerous ones.
Can you hold stock for us?
For items with regular usage, yes — a framework order with agreed prices and call-off deliveries, so the stock sits with us rather than tying up your capital. Send the list with annual quantities and we will tell you which lines it works for.
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Read moreSend your list — your quote is ready within 1 business day.
A list, a photo or an Excel file — whatever is easiest. On standard items most quotes go out the same business day. Custom production, imports or 50+ line lists can take longer; when they do, we tell you the timeline in writing on the day we receive the request.