CALCULATOR · FREE
Reorder point and stockout cost calculator
This reorder point calculator tells you the stock level at which to place your next purchase order, so new units arrive before the last ones sell. For marketplace sellers, lead time is longer than the supplier's production time: you also wait for transit and for FBA, WFS or your 3PL to receive and check in the stock. Safety stock covers the days that sell faster than average, and the stockout line shows what each day out of stock costs you.
What the reorder point calculator works out
Lead time in days, safety stock, lead-time demand, your reorder point, days of supply on hand and inbound, days until you must reorder, the daily cost of a stockout in lost contribution margin and an Amazon flag when days of supply at FBA fall under 28.
The inputs
| Input | Default | Label | Note |
|---|---|---|---|
| Average units sold per day | 20 | Example | Use the last 30 to 90 days, leaving out days you were out of stock |
| Standard deviation of daily units | 6 | Example | Or switch to the simple method and enter your highest daily sales |
| Supplier production days | 30 | Example | From the supplier's confirmed lead time |
| Transit days | 25 | Example | Port to port plus drayage, or courier time |
| Receiving and check-in days at FBA, WFS or your 3PL | 7 | Example | Use your own inbound history; times vary by site and season |
| Service level | 95% (Z = 1.65) | Assumption | Options: 90% (1.28), 95% (1.65), 97.5% (1.96), 99% (2.33), from the standard normal table |
| Units on hand plus inbound | 1,600 | Example | Count only sellable units |
| Contribution margin per unit | $9 | Example | Price minus landed cost minus fees and ad spend per unit |
| Sales lost when out of stock | 66% | Sourced | FMI and GMA global out-of-stock study (Oct 14, 2002): 31% buy at another store, 26% switch brand, 9% buy nothing |
A note on Z values: a 95% service level means a 95% chance of not running out during lead time, which is the one-sided value, 1.65. Some calculators online use 1.96 for 95%; that is the two-sided value and overstates safety stock.
The formula and method
- Lead time (L) = production days + transit days + receiving days
- Safety stock = Z × standard deviation of daily units × √L
- Simple method instead: safety stock = maximum daily units × maximum lead time − average daily units × average lead time
- Reorder point = average daily units × L + safety stock
- Days of supply = (on hand + inbound) ÷ average daily units
- Days until you must reorder = (on hand + inbound − reorder point) ÷ average daily units
- Stockout cost per day = average daily units × lost share × contribution margin per unit
- Amazon low-inventory flag = days of supply at FBA under 28
The Amazon flag is a warning, not a fee calculation. Amazon's low-inventory-level fee applies only when both your 30-day and 90-day historical days of supply are below 28 days, and Amazon applies exemptions, including one for new FBA products in their first 180 days of sales history, according to Seller Labs (vendor, updated Aug 3, 2026).
Worked example
Illustrative · sample figures, using the defaults above.
| Step | Calculation | Result |
|---|---|---|
| Lead time | 30 + 25 + 7 | 62 days |
| Safety stock | 1.65 × 6 × √62 (7.87) | about 78 units |
| Lead-time demand | 20 × 62 | 1,240 units |
| Reorder point | 1,240 + 78 | about 1,318 units |
| Days of supply | 1,600 ÷ 20 | 80 days |
| Days until you must reorder | (1,600 − 1,318) ÷ 20 | about 14 days |
| Stockout cost per day | 20 × 66% × $9 | about $118.80 in contribution margin |
So the purchase order for this SKU should go out within about two weeks. Every day it is late and the shelf empties costs about $119 in margin, before any effect on rank.
Limitations
- It assumes demand is steady around the average. Seasonal products, launches and promotions need their own forecast.
- Lead-time variation is not modeled separately in the statistical method. If your supplier or freight is unreliable, add buffer days or use the simple method.
- The stockout cost counts lost margin only. On Amazon and Walmart, a stockout can also cost search rank and the featured offer, which this does not price.
- Receiving times at fulfillment centers change by site and season. Use your own recent inbound history.
- Since Mar 31, 2026, Amazon resellers must label every unit with an FNSKU for new inbound shipments, and units that arrive without required barcodes are not eligible for reimbursement (PPC Land, Mar 24, 2026). Build labeling time into receiving days.
Under every result: this estimate uses the figures you enter and published rates. It is not an audit, an offer or a promise of savings, sales or profit.
What is the reorder point formula?
Reorder point = average daily units × lead time in days + safety stock. For marketplace sellers, lead time includes supplier production, transit and the days FBA, WFS or your 3PL take to receive the stock.
How much safety stock should I keep?
Enough to cover demand above average during lead time at the service level you choose. At 95%, safety stock is 1.65 × the standard deviation of daily sales × the square root of lead time in days. Higher service levels cost more stock.
Should I keep more than 28 days of supply at Amazon?
For most steady sellers, yes. Amazon's low-inventory-level fee applies when both 30-day and 90-day historical days of supply fall below 28 days, unless an exemption applies. Running too lean costs fees as well as sales.
See all free calculators.
Sources5 sources
- 01fmi.orgOct 14, 2002; checked Oct 20, 2026Primary
- 02sellingpartners.aboutamazon.comOct 15, 2025; checked Oct 20, 2026Primary
- 03sellerlabs.comlow-inventory-level fee scope and new-product exemption, updated Aug 3, 2026; vendor; checked Oct 20, 2026Vendor
- 04ppc.landMar 24, 2026; checked Oct 20, 2026Primary
- 05sellercentral.amazon.comAmazon staff, "Low-inventory-level fee further explained": fee only when both 30-day and 90-day historical days of supply are below 28 days; about 2023; checked Oct 20, 2026Primary
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