The Reorder Point Formula Every FBA Seller Should Have Memorized by Now

The FBA Reorder Point Formula That Turns “We Should Probably Order Soon” Into a Real Plan

There are several ways to decide when to reorder an Amazon SKU.

You can check inventory every morning.

You can stare at the little red “low stock” warning.

You can wait until your best seller is down to 47 units and suddenly discover your supplier needs six weeks.

Or… and hear us out… you could use math.

Not scary math. Not “find X while questioning every life choice that led you back to algebra” math.

One simple formula:

That’s the basic reorder point formula, and if you sell through Amazon FBA, it deserves a permanent spot somewhere between your purchase-order process and your muscle memory.

Amazon describes the same basic calculation as average daily sales multiplied by lead time, plus buffer stock. The point is simple: instead of guessing when inventory is “getting low,” you determine in advance exactly when replenishment should be triggered.

And heading into peak selling periods, that distinction becomes much more important.

Because your reorder point isn’t really an inventory number.

It’s a decision point.

Let’s calculate it properly.

What Is the Reorder Point Formula?

The reorder point is the inventory level at which you should trigger your next replenishment order.

In its simplest form:

The formula answers one question:

“How much inventory should I have left when it becomes time to reorder?”

It does not tell you how much to buy. We’ll get to that.

First, let’s break down the three numbers you need.

1. Daily Demand

Daily demand is the number of units you expect a SKU to sell each day.

The easiest approach is:

If a SKU sold 900 units over the last 30 days:

Easy.

But there’s a catch.

And its name is Q4.

More on that shortly.

2. Lead Time

Lead time is how long it takes to go from “We need more inventory” to “That inventory is actually available to sell.”

For an FBA seller, that can include more than supplier production time.

Depending on your supply chain, consider:

• Supplier production
• Supplier processing
• Freight
• Customs
• Prep or labeling
• Delivery to Amazon
• Receiving and check-in time
• Other predictable delays between ordering and sellable inventory

If your factory takes 20 days, transportation takes 12, prep takes 3, and you normally allow another 5 days before everything is actually available:

Your practical lead time isn’t 20 days.

It’s closer to 40 days.

That difference matters.

A lot.

3. Safety Stock

Safety stock is the buffer between your forecast and reality.

Because reality occasionally enjoys improvising.

Demand spikes.

Suppliers run late.

Containers get delayed.

Amazon receiving takes longer than expected.

A promotion performs much better than anticipated.

Safety stock gives you additional inventory to absorb some of that uncertainty without immediately heading toward a stockout.

A Simple FBA Reorder Point Example

Suppose one SKU averages:

• 20 units sold per day
• 30-day total replenishment lead time
• 200 units of safety stock

Your calculation is:

Your reorder point is 800 units.

In plain English:

When the SKU reaches approximately 800 units of inventory position, it’s time to initiate replenishment.

Not when it hits 400.

Not when someone happens to notice it on Friday afternoon.

Not when you hear the faint sound of an Amazon seller screaming somewhere in the distance.

At 800 units.

That’s the beauty of a reorder point.

It turns replenishment from a reaction into a rule.

Why a Basic 30-Day Average Can Betray You Heading Into Q4

Here is where the simple reorder point formula gets more interesting.

Using average daily demand works well if the future behaves roughly like the period you’re averaging.

Peak season tends to have other plans.

Imagine this sales pattern:

A flat 90-day average gives you:

But your most recent 30 days are already running at:

If you’re heading into a period when demand is expected to rise further, building your reorder point around 22 units per day could leave your plan looking wonderfully accurate right up until the moment it isn’t.

This Is Where Weighted Demand Becomes More Useful

Instead of treating sales from three months ago exactly the same as sales last week, you can give more recent or more relevant periods greater weight.

For example:

• Last 30 days: 50% weight
• Previous 30 days: 30% weight
• Prior 30 days: 20% weight

Using our example:

That gives you approximately:

Still conservative compared with the most recent sales rate—but more responsive than simply averaging everything equally.

And a sophisticated forecasting model can go further by accounting for things like seasonality, changing sales velocity, historical patterns, and other demand signals rather than relying on one trailing average.

Amazon itself says its FBA restock recommendations use sales history, demand forecasting, and seasonality to help determine when and how much inventory sellers should send. Amazon also recommends forecasting future demand when planning inventory for holidays and major sales periods such as Black Friday.

The Data-Driven Forecasting Advantage

The broader inventory research points in the same direction.

One academic comparison tested forecasting approaches across roughly 200,000 forecasts and found that optimally weighted moving-average methods performed better overall than the traditional forecasting methods evaluated.

In another real-world example, McKinsey described a distributor that built a data-driven demand forecasting model across more than 100,000 SKUs. The company reduced inventory by 20% while significantly reducing out-of-stocks after improving its forecasts, safety stock calculations, and lead-time planning.

That doesn’t mean every Amazon seller who changes a spreadsheet formula magically gets 20% less inventory and zero stockouts.

Would be nice.

It does illustrate the important principle:

And when demand is accelerating, simple trailing averages are more likely to miss what’s happening next.

Your Reorder Point Should Change When Demand Changes

This is the part that occasionally gets lost.

A reorder point shouldn’t necessarily be carved into stone.

If a SKU’s:

• Demand increases
• Demand declines
• Lead time changes
• Supplier reliability changes
• Seasonality changes
• Safety-stock requirements change

…then the appropriate reorder point changes too.

Let’s revisit our earlier SKU.

Same product.

Same supplier.

Same safety stock.

But you need to trigger your reorder 240 units earlier because inventory is disappearing faster.

If you’re still operating from the old 800-unit reorder point, your formula isn’t technically wrong.

Your assumptions are.

Reorder Point vs. Reorder Quantity: Please Don’t Make Them the Same Thing

This distinction is important.

Your Reorder Point Tells You When to Order

The reorder point answers:

“At what inventory level should I initiate replenishment?”

Your Reorder Quantity Tells You How Much to Order

That’s a separate decision.

Your reorder quantity may depend on:

• Forecasted demand
• Target days of supply
• Existing inventory
• Inbound inventory
• Supplier minimum order quantities
• Case-pack quantities
• Purchase-order frequency
• Storage considerations
• Available cash
• Expected margin or return
• Seasonality
• How much demand you expect after the inventory arrives

So:

Confusing the two is a great way to turn a useful inventory formula into a very expensive guessing game.

Safety Stock Isn’t “Whatever Number Feels Safe”

Safety stock is sometimes treated like the emergency granola bar in your desk drawer.

Just throw some extra in there.

But safety stock should reflect actual risk.

More safety stock isn’t automatically better.

Too little creates stockout risk.

Too much ties up cash.

Welcome to inventory management, where both sides of the seesaw have invoices attached.

The SKU-by-SKU Reorder Rule FBA Sellers Actually Need

For each SKU, work through the same five questions.

1. What demand should I realistically expect?

Don’t automatically assume the last 30, 60, or 90 days will repeat.

Ask whether demand is:

• Stable
• Increasing
• Declining
• Seasonal
• Promotion-driven
• Temporarily distorted

Use the demand estimate that best represents what is likely to happen during the upcoming lead-time period.

2. How long will replenishment really take?

Use total operational lead time—not just the number on the supplier quote.

If it takes 45 days from purchase order to sellable inventory, use 45 days.

Your factory’s production schedule does not care that your spreadsheet says 30.

3. How much uncertainty do I need to protect against?

Set safety stock based on the SKU’s actual demand and supply risk.

4. What is my reorder point?

Now calculate:

5. Am I approaching it?

If yes, the SKU has entered the reorder decision zone.

Notice we didn’t say:

“Place a giant purchase order immediately.”

Because there’s one more question.

And it might be the most important one.

The Formula Can Tell You When to Reorder. It Can’t Tell You Whether You Should.

Here is where inventory planning becomes a business decision instead of an arithmetic exercise.

Imagine two products.

Both SKUs may need inventory.

But if your purchasing budget is limited, should they receive the same share of your next inventory dollar?

Probably not.

This is the bigger idea behind profit-aware replenishment.

RestockPro already supports forecasting, restock suggestions, purchase orders, shipments, suppliers, and broader inventory planning. RestockPro Plus adds SKU Economics, bringing profitability information into that replenishment decision so sellers can think beyond simply what is running low.

The goal becomes:

• What should I reorder?
• When should I reorder it?
• And is this SKU worth investing more cash in?

That third question matters.

Because a fast-selling SKU can still be a lousy place to put your money.

Your Next Purchase Order Shouldn’t Be a Popularity Contest

One of the easiest Amazon inventory mistakes is assuming:

Not necessarily.

Sales velocity tells you that a product is moving.

It doesn’t automatically tell you whether replenishing it is the best use of your available capital.

Before committing more inventory dollars, look beyond units sold and consider:

• Margin
• Product costs
• Amazon fees
• Advertising costs
• Returns
• Inventory investment
• Inventory turns
• Expected profitability
• Upcoming demand
• Cash requirements across the rest of your catalog

That’s especially important in Q4, when several SKUs may all be waving their hands asking for more inventory at exactly the same time.

Your cash, meanwhile, remains inconveniently finite.

RestockPro Plus is designed around this exact progression: connect inventory planning with SKU-level economics so replenishment can become a profit-aware decision, not simply a response to sales velocity.

5 Common Reorder Point Mistakes That Create Avoidable FBA Inventory Problems

Calculate Your Reorder Point Before Peak Demand Calculates It for You

For every important SKU, start with three numbers:

1. Expected daily demand
2. Total replenishment lead time
3. Safety stock

Then calculate:

Do it SKU by SKU.

Update it when demand changes.

And when a SKU reaches that threshold, don’t stop at “How much should I buy?”

Ask the better question:

“Is this where my next inventory dollar should go?”

Still Doing Reorder Math in Your Head? Let’s Clear Up a Few Things.

Know When to Reorder. Then Know Whether It’s Worth It.

Running out of your most profitable inventory hurts.

So does spending thousands of dollars replenishing a SKU that’s great at generating revenue and considerably less enthusiastic about generating profit.

RestockPro Plus helps connect forecasting and replenishment planning with SKU-level economics, giving you a clearer view of what to reorder, when to reorder it, and where your inventory dollars may work hardest.

Because the goal isn’t to keep every SKU fully stocked at any cost.

It’s to put the right inventory (and the right amount of cash) in the right place.