In short
A reorder point is the moment you need to place your next order so it arrives before you run out. It depends on how fast you sell, how long a reorder takes to become sellable on Amazon (production, shipping and check-in), and how many days of safety stock you want. Days of stock is units available plus inbound, divided by daily sales including the sales ads add. Subtract lead time and safety stock to get the days left before you must reorder. In the example, 2,400 units selling 60 a day last 40 days, so with 30 days of lead time and 7 of safety stock, the reorder is due within 3 days. Raising ad spend shortens that window.
What is a reorder point?
Your reorder point is the moment you need to place your next order so it arrives before you run out. It depends on three things: how fast you sell, how long a reorder takes to reach Amazon, and how much spare stock you want as a cushion.
Ads change the first of those. A strong campaign can double daily sales, which halves the time you have left. Running out on Amazon costs more than lost sales for a few weeks: your listing can lose rank, and ads you were paying for stop delivering. That's why ad plans and stock plans belong together.
How to calculate your reorder point
stock = units available + units inbound daily sales = organic units per day + ad units per day days of stock = stock ÷ daily sales reorder within = days of stock − lead time − safety stock days most ad units = stock ÷ (lead time + safety days) − organic units per day
- Lead time: days from placing a reorder to units being sellable on Amazon, including production, shipping and Amazon's check-in.
- Safety stock: days of sales you want left when the reorder lands, in case it's late or sales jump.
- Most ad units: if you reorder today, the most ad-driven units a day your stock can carry while still keeping the safety stock. Above that, slow ads or expect to run short.
In units, the classic reorder point is daily sales × (lead time + safety days). Once your stock falls to that number, it's time to order.
Worked example
A made-up brand has 1,800 units available and 600 inbound. It sells 35 units a day without ads and ads add 25 more. A reorder takes 30 days to land, and it wants 7 days of safety stock. These are the calculator's example values.
- Stock = 1,800 + 600 = 2,400 units, selling 60 a day.
- Days of stock = 2,400 ÷ 60 = 40 days.
- Reorder within = 40 − 30 − 7 = 3 days. The reorder point is 60 × 37 = 2,220 units.
- Most ad units = 2,400 ÷ 37 − 35 = about 30 a day, so today's 25 is fine if the reorder goes in on time.
If the brand had only 1,500 units and nothing inbound, it would already be 12 days late. To keep its safety stock, it would need to slow ads to about 5.5 units a day while the reorder is on its way.
How much stock is enough?
There's no single right number, but these are common rules of thumb:
- Safety stock of 1 to 2 weeks: enough to absorb a late shipment or a good week. More if your supplier is often late.
- Reorder window under a week: time to act. Place the order now, or plan to slow ads.
- Not too much: Amazon charges storage fees, and more for stock that sits a long time. Check current FBA fees and limits in Seller Central.
Common mistakes
- Planning stock on last month's sales. If you're about to raise ad spend, plan on the higher sales rate.
- Forgetting Amazon's check-in time. Units can take days or weeks to become sellable after they arrive at Amazon. Count it in lead time.
- Treating all sales as organic. If ads drive a big share of sales, pausing them is the fastest way to stretch stock, but only if you know how much they add.
- Running out, then restarting ads at full spend. After a stockout, rank and conversion can take time to recover. Ramp back up.
How this connects to true incrementality
This calculator needs one number most brands don't have: how many daily sales the ads actually add. Ad reports can't tell you, because they count sales that would have happened anyway. A holdout test measures it. Plan one with the holdout test calculator, and turn the result into incremental sales with the incremental ROAS calculator.
If you're planning a push, the launch traffic calculator shows the sales a budget should bring. Spillmark measures what your Meta ads add on Amazon, so stock plans start from caused sales. See how lift is calculated.
Frequently asked questions
How do I calculate a reorder point?
Multiply daily sales by lead time plus safety stock days. When your stock falls to that many units, place the next order.
How much safety stock should I keep on Amazon?
One to two weeks of sales is a common rule of thumb. Keep more if your supplier or shipping is often late, less if storage fees are a concern.
Should I pause ads when stock is low?
Slowing ads is the fastest way to stretch stock. The calculator shows the most ad-driven sales a day your stock can carry until the reorder lands.
Where do I find my inventory numbers in Seller Central?
Inventory → FBA Inventory shows available and inbound units. Business Reports show units sold per day.
Why does the calculator show days instead of dates?
So the answer stays right whenever you open it. Count forward from today: reorder within 3 days means by 3 days from now.