How to calculate reorder quantity for Amazon FBA
By Nitesh Sharma · 15/10/2026 · 7 min read
Reorder quantity = sales per week × (lead time + weeks of cover you want) × (1 + safety %), minus the stock you already have and what is on the way. The hard part is not the maths. It is getting honest numbers for each piece, and that is what this guide covers.
The formula in one line
Write it down once and use it for every product: Reorder quantity = Sales per week × (Lead time in weeks + Cover weeks) × (1 + Safety %) − (Stock on hand + Stock inbound).
Read it in two halves. The first half is how much stock you need to be safe from the day you order until the next order arrives. The second half is what you already have. The gap is what you order.
There is also a second number that matters as much: the reorder point. That is Sales per week × Lead time × (1 + Safety %). When your stock on hand plus inbound falls to this level, you order. Not a week later.
Step 1: get a sales speed you can trust
Sales speed is units sold per week, and it is the number everything else hangs on. Most sellers get it wrong in one of three ways.
- Using too short a window. Last 7 days is noisy. Use 6 to 8 weeks for a steady product.
- Counting weeks when you were out of stock. If you had zero stock for 4 days, those days sold nothing because there was nothing to sell. Leave them out, or your speed is too low and you under-order again.
- Counting one-off spikes. A single deal day that sold 10 times the normal rate should not set next quarter's plan. Plan sale events separately (see the guide on sale events).
- Counting cancelled orders. Use shipped units, not orders placed.
Step 2: lead time is longer than the supplier's date
Lead time is the number of days from the day you decide to order to the day the stock is sellable on Amazon. Sellers quote the factory or supplier date only. The real chain is longer:
- Supplier makes or picks the goods.
- Pick-up and road or sea transit to your warehouse.
- Quality check, labelling and packing.
- Booking the inbound shipment and getting a delivery slot.
- Travel to the fulfilment centre, and the days Amazon takes to check it in.
Step 3: pick your cover weeks
Cover weeks is how long you want one order to last once it arrives. A fast, easy-to-buy product can be ordered every 3 to 4 weeks. A product with a long supplier cycle or a high minimum order quantity needs 8 to 12 weeks of cover, because you order less often.
Cover also decides your cash tied up. Every extra week of cover on a ₹420 product that sells 30 a week is ₹12,600 sitting on a shelf. Do not set cover the same for every product. Set it by lead time and by how much each unit costs.
Step 4: add safety stock, in percent
Safety stock covers two things: demand coming in higher than your average, and the supplier or Amazon check-in running late. Keep it simple and use a percentage of the lead-time demand.
A steady product with a reliable supplier can use 10%. A product that swings week to week, or a supplier who is often late, should use 20 to 30%. Look at the last 8 weeks: if your best week sold 40% more than your average, 10% will not be enough.
| Step | Number | How we got it |
|---|---|---|
| Units sold in last 8 weeks | 240 | From the sales report, cancellations removed |
| Sales per week | 30 | 240 ÷ 8 |
| Lead time | 5 weeks | Supplier 2 + transit 1 + inbound booking 1 + check-in 1 |
| Cover weeks | 4 | One order should last a month |
| Safety % | 20% | Supplier is sometimes a few days late |
| Stock needed | 324 units | 30 × (5 + 4) × 1.2 |
| On hand + inbound | 170 units | 110 on hand + 60 on the way |
| Reorder quantity | 154 units | 324 − 170 |
| Cash to put in | ₹64,680 | 154 × ₹420 |
| Reorder point | 180 units | 30 × 5 × 1.2. You hold 170, so order today |
Put your own numbers in the free reorder calculator to see the same maths with a date on it.
Mistakes we see again and again
These are the four that cost money, in the order we usually find them.
- Forgetting inbound. Stock that is already on the way is counted nowhere, so the plan orders the same units twice.
- Using order date instead of sellable date. Stock that is still at Amazon receiving is not on sale.
- One cover number for every product.
- Never re-running the numbers. Sales speed changes every week; a plan made last month is already wrong.
Do this weekly
- Refresh sales per week for every active product (6 to 8 weeks, no out-of-stock days).
- List every shipment on the way and when it is expected to be sellable.
- Compare stock on hand plus inbound with the reorder point; list what has crossed it.
- Check the supplier's real dates against the lead time you assumed.
- Round the order to a full carton or the supplier's minimum, and write down why.
- Mark products with a sale event in the next 6 weeks for a separate plan.
Quick questions
How many weeks of stock should I keep on Amazon FBA?
There is no single number. Keep enough to last your lead time plus a safety buffer, then add the cover you want between orders. For many fast sellers that lands between 6 and 10 weeks in total.
Should I use average sales or my best week?
Use the average of 6 to 8 weeks, then cover the swing with safety stock. Using your best week for everything ties up cash in slow weeks.
Does the formula change for a new product with little history?
Yes. With under 4 weeks of sales, order small, treat the number as a guess, and re-run it weekly until you have a steady speed.
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