Reorder point calculator — ROP formula with safety stock
Calculate your reorder point from daily demand, lead time and safety stock, with or without lead-time variability. Includes ERP min/max levels.
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Short answer
Reorder point = average daily demand × average lead time + safety stock. At 420 units a day, a 12-day lead time and 513 units of safety stock, the reorder point is 5,553 units. Add lead-time variability of 2.5 days and safety stock rises to 1,802 units, pushing the reorder point to 6,842.
The reorder point is the on-hand level that triggers a replenishment order. Set it too low and you run out while the supplier is still loading the truck. Set it too high and you carry weeks of stock you did not need. It is the level, not the quantity — how much to order is a separate decision.
Enter your demand, lead time and target service level. The calculator returns the reorder point, the safety stock inside it, and the ERP min and max levels you would key into an item record. The second mode adds supplier lead-time variability, which usually matters more than demand variability.
Your numbers
Lead-time variability
Probability of getting through a replenishment cycle without a stockout.
units
How much you buy each time. Sets the ERP maximum and average inventory, not the reorder point itself.
Result
Reorder point, fixed lead time
5,553 units
13.2 days of cover when the order is raised
Demand during lead time5,040 units
Safety stock inside the reorder point513 units
ERP minimum (reorder point)5,553 units
ERP maximum (ROP + order quantity)11,553 units
Average inventory (Q ÷ 2 + safety stock)3,513 units
Replenishment orders per year25.6
Demand during lead time = 420 × 12 = 5,040 units
σ over the lead time = 90 × √12 = 311.8 units
Safety stock = 1.6449 × 311.8 = 513 units
ROP = 5,040 + 513 = 5,553 units
Fixed lead time (513 units of buffer)5,553 units
Variable lead time (1,802 units of buffer)6,842 units
Raise a 6,000-unit order when inventory position reaches 5,553 units. This mode assumes the supplier always arrives on day 12. Allowing lead time to vary by 2.50 days would lift the reorder point to 6,842 units — 1,289 more, or 3.1 extra days of cover.
Same demand and lead time, different service levels. The right-hand column is what an unreliable supplier costs you in stock.
Service level
z
ROP fixed LT
ROP variable LT
90%
1.2816
5,440 units
6,444 units
95%
1.6449
5,553 units
6,842 units
97.5%
1.9600
5,651 units
7,187 units
98%
2.0537
5,680 units
7,289 units
99%
2.3263
5,765 units
7,588 units
99.9%
3.0902
6,003 units
8,425 units
Everything is computed in your browser. Nothing you type is sent anywhere or stored.
The formula
ROP = (Average daily demand × Average lead time) + Safety stock
Average daily demand
Units consumed per day, averaged over a recent, representative window. Use calendar days if you ship on weekends, working days if you do not — and keep lead time on the same basis.
Average lead time
Days from raising the order to the stock being available to pick. Includes supplier transit, receiving and put-away.
Safety stock
Buffer for variability. With fixed lead time: z × σdemand × √lead time. With variable lead time: z × √(lead time × σdemand² + demand² × σleadtime²).
z
One-sided normal value for the target cycle service level: 1.28 at 90%, 1.65 at 95%, 2.33 at 99%.
The two safety-stock variants are the whole story of this calculator. Ignoring lead-time variability is the most common reason a correctly-calculated reorder point still stocks out — a supplier five days late costs 2,100 units on this item, four times the demand-only buffer.
Worked example
Average daily demand
420 units
Standard deviation of daily demand
90 units
Average lead time
12 days
Standard deviation of lead time
2.5 days
Target service level
95% (z = 1.6449)
Order quantity
6,000 units
Result
ROP = 5,553 units fixed lead time · 6,842 units with lead-time variability
Demand during lead time is 420 × 12 = 5,040 units. With lead time treated as fixed, safety stock is 1.6449 × 90 × √12 = 513 units, so the reorder point is 5,553 units — trigger a 6,000-unit order when on-hand plus on-order falls to that level. Allowing lead time to vary by 2.5 days, the combined deviation is √(12 × 90² + 420² × 2.5²) = 1,095 units, safety stock becomes 1,802 units and the reorder point rises to 6,842. Lead-time variability contributes 72% of the buffer.
Demand variability versus lead-time variability
Most reorder point formulas cover demand going up and quietly assume the supplier always arrives on day 12. Real suppliers do not. The safety stock calculator handles the demand-only case; the combined formula here treats both as random and adds their variances: z × √(LT × σdemand² + demand² × σleadtime²).
Source of variance
Contribution
As a share
Demand during 12 days (12 × 90²)
97,200
8.1%
Lead time ±2.5 days (420² × 2.5²)
1,102,500
91.9%
Combined variance
1,199,700
100%
Combined σ (√1,199,700)
1,095 units
—
Why a late supplier hurts more than a busy week: demand deviation scales with √lead time, lead-time deviation scales with the full daily demand rate.
That ratio is typical for a fast-moving item with an unreliable supplier, and it points at the cheapest fix: reducing lead-time variability from 2.5 days to 1.0 day cuts the combined deviation from 1,095 to 523 units and the reorder point from 6,842 to 5,900. No extra cash, no extra warehouse space — just a supplier conversation and an accurate lead time in the item master.
Measured receipt date minus order date, not the supplier's quote
12 days
Most ERP replenishment screens ask for a minimum and a maximum rather than a formula. This is the mapping.
Periodic review changes the interval
If you review stock weekly rather than continuously, demand can pass the reorder point unnoticed for up to seven days. Add the review period to lead time inside the formula — the protection interval becomes 12 + 7 = 19 days — or you will be short by roughly one review period of demand on average. This calculator assumes continuous review, which is what an ERP with a real-time replenishment engine gives you.
Common ways the reorder point goes wrong
Mixed time bases. A calendar-day demand rate multiplied by a lead time counted in working days understates the trigger by about 29%.
Stale lead times. The item master says 7 days; the last 20 receipts average 12. The reorder point inherits the fiction.
Safety stock counted twice — once inside the reorder point and again as a separate ERP safety stock field, quietly doubling the buffer.
Demand taken from shipments during a period when you were stocked out, which flatters both the average and the deviation.
Never revisited. Demand drifts, suppliers change. Recheck the top items quarterly and after any supplier switch.
The inputs are all in your ERP and none of them are on a screen: 90 days of shipped units by item, order-to-receipt dates for the last 20 receipts by supplier, current inventory position. Ask ERPray for them in a sentence and read the query it shows before you set replenishment policy on the answer.
Frequently asked questions
What is the reorder point formula?
Reorder point = average daily demand × average lead time in days + safety stock. At 420 units a day and a 12-day lead time, demand during lead time is 5,040 units; adding 513 units of safety stock gives a reorder point of 5,553 units. Trigger the order when inventory position reaches that level.
How do you calculate a reorder point with variable lead time?
Replace the safety stock term with z × √(lead time × σdemand² + demand² × σleadtime²). With 12 days, σdemand of 90, demand of 420 and σleadtime of 2.5 days, the combined deviation is 1,095 units, so safety stock at 95% service is 1,802 units and the reorder point is 6,842.
Does the reorder point include safety stock?
Yes. The reorder point is demand during lead time plus safety stock. If your ERP has a separate safety stock field that it adds to the reorder point, enter only the lead-time demand in the reorder point field, or you will hold the buffer twice.
What is the difference between reorder point and reorder quantity?
The reorder point is a level that triggers an order; the reorder quantity is how much you then buy. The level comes from lead time and variability, the quantity from ordering and holding costs. Changing one has no effect on the other, and both belong on the item record.
Should I use calendar days or working days?
Either, as long as demand and lead time use the same basis. Calendar days are simpler when suppliers ship any day. If you only consume stock on 5 working days out of 7, calendar-day demand will be 29% lower than working-day demand, and mixing the two produces a reorder point that is wrong by that margin.
How often should reorder points be reviewed?
Quarterly for A-items, annually for the tail, and immediately after a supplier or lead-time change. Demand drifts slowly but lead times change in steps, so a switch of supplier or freight mode should trigger a recalculation the same week rather than waiting for the next cycle.
This calculator needs you to find the inputs first. ERPray pulls them from your own ERP account and computes the answer live — with the exact query shown so you can check it.