Reorder point formula and calculator
The reorder point is the stock level at which you place your next order, so it arrives before you sell out. Put in your sales and your lead time below to get yours, with safety stock worked out the way you prefer.
Reorder point = average daily sales × lead time in days + safety stock
to see how it works.
Results
| Sales during the lead time | - |
|---|---|
| Safety stock | - |
| Reorder point | - |
| Stock on hand and on order | - |
| When to reorder | - |
| Cubic feet per unit | - |
| Units your capacity limit holds | - |
| Reorder point and your limit | - |
| Room left under your limit | - |
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The reorder point formula, term by term
- Average daily sales: units per day. If you know units per month, divide by the days in that month, and keep everything in days.
- Lead time: days from placing the order to having stock you can sell. Count every step, not just shipping.
- Sales during the lead time is the first two multiplied: what you expect to sell while you wait.
- Safety stock covers the times you sell faster than average or the order arrives late. Without it, you run out about half the time.
When your stock on hand plus what's already on order drops to the reorder point, place the next order.
Worked examples
1. Safety stock as days of sales. You sell 20 a day and your supplier takes 30 days. You keep 10 days of sales in reserve. Sales during the lead time: 20 × 30 = 600. Safety stock: 20 × 10 = 200. Reorder point: 800 units.
2. Safety stock from a 95% service level. You sell 20 a day on average, with a standard deviation of 6, and the lead time is always 25 days. The spread of sales over 25 days is 6 × √25 = 30 units. For 95%, z is 1.645, so safety stock is 1.645 × 30 = 49.3. Reorder point: 500 + 49.3, rounded up to 550 units.
3. The same, but the lead time varies. Deliveries take 25 days on average, give or take 4 days (standard deviation). Now the spread is √(25 × 6² + 20² × 4²) = √(900 + 6,400) = 85.4 units, and safety stock is 1.645 × 85.4 = 140.5. Reorder point: 641 units. A supplier who's unreliable by four days costs you almost 3 times the safety stock of one who's always on time.
4. When to order. With example 1's reorder point of 800, you have 1,100 on hand and 300 on order: 1,400 in all. At 20 a day you reach 800 in (1,400 - 800) ÷ 20 = 30 days.
Amazon FBA in Q4: longer lead times and capacity limits
On Amazon the same formula works, but in the fourth quarter three things change at once: you sell more, stock takes longer to become sellable, and Amazon may cap how much you can send.
- Lead time includes Amazon's check-in. Amazon's Q4 2026 peak readiness playbook says its fulfillment centers "focus on receiving shipments in September and October, then shift to processing customer orders in November and December," and that "inbound delivery window slots will be limited." It asks you to schedule delivery appointments "at least 7 days before the dates listed," and partnered carrier pickups 14 days before. Amazon doesn't publish a receiving time, so use the days your own past shipments took from delivery to sellable stock, and add more in peak.
- Arrival dates, not ship dates. For Prime-badge eligibility during Black Friday Week and Cyber Monday, Amazon's holiday announcement asks for stock to arrive by October 14 for AWD shipments, October 21 for FBA shipments with minimal shipment splits, and October 28 for FBA shipments with Amazon-optimized splits ("most sellers select this option").
- Capacity limits. Amazon sets "one capacity limit for each storage type per month," in cubic feet, usually announced "during the fourth week of the month." If "your on-hand inventory plus shipments on the way to our fulfillment centers exceeds your capacity limit, you'll be blocked from creating new shipments." The playbook warns "you may see lower estimated capacity limits for October and November." Individual selling accounts have a fixed 15 cubic feet; professional accounts in FBA for less than 39 weeks get no limit.
- Cubic feet: length × width × height in inches ÷ 1,728. Amazon's example: 47 × 12 × 10 inches is 3.3 cubic feet.
- How much Amazon wants you to keep: for new products without a recommendation, "aim to keep at least 28 days of inventory." Before a deal, "make sure you have 4-6 weeks of inventory available" to avoid the low-inventory level fee, and 28 days after it.
Worked Q4 example
5. Restocking for November and December. These numbers are an example. You normally sell 20 a day, and Amazon's demand forecast shows about 45 a day for the holiday weeks, so use 45: the order you place now sells during the peak. Your supplier and freight take 30 days, and you add 14 days for the delivery appointment and check-in in peak, so the lead time is 44 days. With 10 days of safety stock:
- Sales during the lead time: 45 × 44 = 1,980. Safety stock: 45 × 10 = 450. Reorder point: 2,430 units, against 800 the rest of the year.
- Now the capacity check. Each unit is 10 × 8 × 4 inches: 320 ÷ 1,728 = 0.185 cubic feet. With a 400 cubic foot limit for standard-size, you can hold 2,160 units on hand and inbound. The reorder point of 2,430 doesn't fit.
When the reorder point is bigger than your limit, you'll run out before the next shipment arrives unless you change something. Amazon's options, from its capacity page and playbook:
- Amazon Warehousing and Distribution (AWD): "Inventory that is auto-replenished from AWD to FBA isn't subject to 'FBA capacity limits' during peak." With bulk stock in AWD, the lead time that matters for FBA becomes AWD to the fulfillment center, not factory to Amazon. If that were 14 days (an example, not an Amazon figure), the FBA reorder point drops to 1,080 units, which fits.
- Capacity Manager: request more space for a reservation fee you set. Amazon grants "requests with the highest reservation fee per cubic foot first," and you earn "a $0.15 performance credit for every dollar of sales" made with the extra space. The playbook points it at products that "sell out in under four weeks," and Amazon warns that if you don't sell enough to offset the fee, you pay the difference.
- Our note: a smaller safety stock or a shorter lead time also lowers the reorder point. Cutting safety stock raises your stockout risk in exactly the weeks it costs most, so shorten the lead time first.
Safety stock: which method?
- Days of sales is the easiest and fine for most small sellers. Pick more days if your supplier is often late or a sale spike would hurt.
- Service level sizes the buffer to how much your sales really vary. It needs a standard deviation, which the sales history box works out for you.
- Lead time that varies usually matters more than sales that vary, as example 3 shows. If you only know a delivery's range, a rough standard deviation is the range divided by 4.
The service-level formula is safety stock = z × √(L × σd² + d² × σL²), where d is average daily sales, σd its standard deviation, L the average lead time in days and σL its standard deviation. With a fixed lead time it shortens to z × σd × √L. It assumes days are independent and the sales during a lead time are roughly normal.
Service level and z
Higher service levels cost more safety stock, and the cost climbs fast near 100%. Going from 95% to 99% raises z from 1.645 to 2.326.
| Service level | z | Safety stock compared with 95% |
|---|---|---|
| 90% | 1.282 | 78% |
| 95% | 1.645 | 100% |
| 97.5% | 1.960 | 119% |
| 98% | 2.054 | 125% |
| 99% | 2.326 | 141% |
| 99.5% | 2.576 | 157% |
| 99.9% | 3.090 | 188% |
Common mistakes
- Mixing units of time: monthly sales times a lead time in days gives a reorder point 30 times too big.
- Forgetting stock already on order: compare the reorder point with on-hand plus on-order stock, or you'll order twice.
- Averaging over sold-out days: days with no stock show zero sales, which pulls the average down and the reorder point with it.
- Counting only shipping as lead time: production, customs and check-in at the warehouse all count.
- Using normal sales for a holiday order: an order that arrives in November sells at November's pace. Use the sales you expect while that order lasts.
Sources
- z values for each service level: NIST/SEMATECH e-Handbook of Statistical Methods, Cumulative Distribution Function of the Standard Normal Distribution. The calculator matches 409 of the table's 410 entries to all five decimals and the last within rounding.
- The safety stock formulas are checked by simulation in our tests: 200,000 simulated lead times give the spread and the 95% service level the formula predicts.
- Amazon Seller Central, FBA capacity limits (how limits are set, what counts, cubic feet, Capacity Manager), read October 24, 2026.
- Amazon Seller Central, Q4 2026 peak readiness playbook (receiving in September and October, delivery appointments, capacity in October and November, AWD, days of inventory), read October 24, 2026.
- Amazon Seller Central News, Holiday 2026: Same fees, same eligibility, earlier deadlines (July 2026; inbound arrival dates), read October 24, 2026.
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