Order too late and customers find an empty shelf. Order too early and cash sits in the stockroom. The reorder point tells you the stock level at which to place the next order so that it arrives just before you run out — with a small buffer for the unexpected.
This guide explains the reorder point formula, how to measure demand and supplier lead time, three ways to calculate safety stock, and four fully worked examples. All calculations have been checked; you can reproduce them in a spreadsheet.
In this guide
The reorder point formula
Reorder point (ROP) = average daily demand × lead time in days + safety stock
- Average daily demand (d): how many units you sell per day on average.
- Lead time (L): the number of days between placing an order and the stock being available to sell.
- Safety stock (SS): extra units held to absorb higher-than-average demand or late deliveries.
The first part, d × L, is the stock you expect to sell while waiting for the delivery. Safety stock covers the uncertainty around that expectation.
Measuring demand correctly
Use your own sales history rather than guesses. A POS that records every sale by product makes this straightforward.
- Use a representative period: the last 8–12 weeks for steady products; the same season last year for seasonal products.
- Use units, not revenue. Price changes distort revenue-based estimates.
- Exclude one-off events such as a single bulk order from a business customer, or treat them separately.
- Count days the shop is open. If you trade six days a week, divide weekly sales by six, and use lead time in trading days.
- Watch for stockouts in the history: days with zero stock show zero sales, which understates real demand.
Sales reports by product and period make this quick — see reports and analytics.
Measuring supplier lead time
Lead time is not only the shipping time. It runs from the moment you decide to order until the stock is on the shelf:
| Component | Example |
|---|---|
| Time to place the order | 1 day (you order on your weekly ordering day) |
| Supplier processing | 2 days |
| Transport | 3 days |
| Receiving, checking and shelving | 1 day |
| Total lead time | 7 days |
Record the actual date you ordered and the date stock was available for each delivery. Recording supplier purchases with dates gives you this history automatically — see purchase management and supplier records.
Safety stock: three methods
1. Fixed buffer (simple)
Choose a number of days of cover — for example, three days of average demand. Easy to apply and explain, but it ignores how variable demand actually is.
2. Statistical method (demand varies)
SS = Z × σd × √L, where σd is the standard deviation of daily demand and Z is the service-level factor.
The service level here is the probability of not running out during a replenishment cycle. Common Z values from the standard normal distribution:
| Cycle service level | Z value |
|---|---|
| 90% | 1.28 |
| 95% | 1.645 |
| 97.5% | 1.96 |
| 99% | 2.33 |
Use the same time unit throughout. If σ is measured per day, lead time must be in days; mixing a weekly or monthly standard deviation with a daily lead time is a common error that inflates safety stock dramatically.
3. Statistical method (demand and lead time vary)
SS = Z × √( L × σd² + d² × σL² ), where σL is the standard deviation of lead time in days.
Use this version when suppliers are unreliable: the d² × σL² term often dominates, which is why late deliveries hurt more than demand noise.
Alternative: the max–average method
Without enough data for statistics, some retailers use SS = (maximum daily demand × maximum lead time) − (average daily demand × average lead time). It is simple but conservative and tends to overstate stock.
Worked examples
Example 1 — Fixed buffer
A café sells an average of 12 bags of coffee beans per day. The roaster delivers in 5 days. The owner keeps 20 bags as safety stock.
ROP = 12 × 5 + 20 = 60 + 20 = 80 bags. When stock falls to 80, place the order.
Example 2 — Statistical safety stock
A grocery store sells an average of 15 units/day of a cooking oil, with a standard deviation of 4 units/day. Lead time is 9 days. Target service level: 95% (Z = 1.645).
- Safety stock = 1.645 × 4 × √9 = 1.645 × 4 × 3 = 19.74 → round up to 20 units.
- Lead-time demand = 15 × 9 = 135 units.
- ROP = 135 + 20 = 155 units.
Example 3 — Demand and lead time both vary
A hardware store sells 20 units/day of a fastener pack (σd = 5). The supplier’s lead time averages 7 days but varies (σL = 2 days). Target: 95% (Z = 1.645).
- Combined deviation = √(7 × 5² + 20² × 2²) = √(175 + 1,600) = √1,775 ≈ 42.13.
- Safety stock = 1.645 × 42.13 ≈ 69.3 → 70 units.
- ROP = 20 × 7 + 70 = 140 + 70 = 210 units.
Notice that the lead-time variability (1,600) contributes far more than demand variability (175). Improving supplier reliability would reduce this buffer more than better forecasting.
Example 4 — Max–average method
Maximum daily demand 30, maximum lead time 10 days; average demand 20, average lead time 7 days. Safety stock = 30 × 10 − 20 × 7 = 300 − 140 = 160 units — much higher than Example 3, illustrating why this method is conservative.
Calculating reorder points in a spreadsheet
You do not need special software to start. Export daily unit sales for each product, then build one row per SKU:
| Column | Content | Example formula (Excel / Google Sheets) |
|---|---|---|
| A | Daily sales history (one column per day) | — |
| Avg demand (d) | Average units per trading day | =AVERAGE(B2:BI2) |
| σd | Standard deviation of daily demand | =STDEV.S(B2:BI2) |
| Lead time (L) | Days from order to shelf | entered manually |
| Z | Service level factor | =NORM.S.INV(0.95) → 1.645 |
| Safety stock | Z × σd × √L, rounded up | =ROUNDUP(Z*σd*SQRT(L),0) |
| Reorder point | d × L + safety stock | =ROUNDUP(d*L,0)+SS |
NORM.S.INV returns the Z value for any service level, so you can test what moving from 95% to 97.5% does to stock — in Example 2 it would raise safety stock from about 20 to about 24 units (1.96 × 4 × 3 = 23.5).
Special cases
- Seasonal products: calculate demand from the same season last year, not from the last few weeks.
- New products: start with a conservative estimate based on similar items and review after four to six weeks of sales.
- Promotions: add the expected promotional uplift to demand during the promotion period only.
- Perishable goods: safety stock is limited by shelf life; a lower service level is often the right trade-off.
- Slow movers (a few units a month): statistical formulas are unreliable; use a simple minimum such as one or two units on hand.
- Weekly ordering days: if you can only order once a week, add the review period to the lead time (periodic review), or you will run out between order days.
How much to order once you hit the reorder point
The reorder point tells you when; you still need to decide how much. Common retail approaches:
- Fixed quantity: always order the same amount (for example, a full case or a supplier minimum).
- Order-up-to level: order enough to bring stock back to a target maximum, such as two weeks of demand plus safety stock.
- Days of cover: order a number of days of expected demand, adjusted for upcoming promotions or seasons.
Balance three costs: the cost of placing orders, the cost of holding stock, and the cost of running out. Case sizes and minimum order values usually decide the final number.
Reorder point vs. min/max
Many POS and inventory tools use a minimum and maximum per product. The two ideas fit together: the minimum is effectively your reorder point, and the maximum is your order-up-to level. When stock falls to the minimum, you order enough to reach the maximum. Using the method in this guide to set the minimum — instead of a round number picked once and never revisited — is what turns a min/max setting into a real replenishment policy.
Revisit both values when demand, lead time or case sizes change, and after every peak season.
Preventing stockouts in practice
- Set reorder points for your class A products first — see ABC inventory analysis.
- Review the low-stock list every day at the same time.
- Recalculate demand and lead time every quarter, and before peak seasons.
- Keep stock records accurate with regular cycle counts — a reorder point is useless if the stock figure is wrong.
- Record deliveries on the day they arrive so the system stock is current.
- Agree on emergency options with key suppliers.
For the bigger picture, read the retail inventory management guide. Low-stock visibility is part of inventory management in the POS.
Track daily sales and low stock for every product: the free edition includes inventory tracking and sales history.
Frequently asked questions
What is the reorder point formula?
Reorder point = average daily demand × lead time in days + safety stock. When stock falls to this level, place a new order.
How do I calculate safety stock?
A common statistical formula is Z × standard deviation of daily demand × square root of lead time. If lead time also varies, use Z × √(L × σd² + d² × σL²).
What Z value should I use?
Z depends on your target cycle service level: about 1.28 for 90%, 1.645 for 95%, 1.96 for 97.5% and 2.33 for 99%.
Is the reorder point the same as the order quantity?
No. The reorder point is the stock level that triggers an order; the order quantity is how much you order, often based on case sizes, minimums or an order-up-to level.
How often should I recalculate reorder points?
Review them at least quarterly and before seasonal peaks, or whenever demand or supplier lead times change significantly.
Related guides
- Retail inventory management guide
- ABC inventory analysis
- Inventory cycle counting
- How to choose a POS system