Reorder Point Calculation: A 20-Minute Weekly Method

Why reordering by feel keeps failing you

Most shop owners reorder the way they always have: a quick look at the shelf, a gut feel that something's running low, maybe a mental note from last week's sales. It works — until it doesn't. You run short on a Friday before a long weekend, or you over-order before a festival and sit on stock that quietly eats into your margin.

The problem isn't that you have bad instincts. It's that feel doesn't account for two things: how fast a product is actually moving right now, and how long your supplier will take to replenish it. Those two numbers, combined, tell you exactly when to order and how much. Everything else is noise.

This post walks through a simple reorder point calculation you can run in about 20 minutes each week — no software required, no spreadsheet wizardry. Then we'll look at where the formula breaks, and what to do about it.

The formula: three numbers, one decision

The reorder point formula comes down to this:

Reorder point = (sell-through per day × lead time in days) + safety buffer − units on hand

When your on-hand stock drops to or below this number, it's time to place an order. Here's what each part means.

Sell-through per day is simply how many units you sell on an average day. Pull your last two weeks of sales for a SKU and divide by 14. That's your baseline.

Lead time is the number of days from when you place the order to when the stock is actually on your shelf and ready to sell — not when the truck arrives, but when it's counted, checked, and put out.

Safety buffer is the cushion you hold for demand spikes and supplier delays. A simple starting point: multiply your daily sell-through by half your lead time. So if you sell 10 units a day and your lead time is 6 days, your buffer is 30 units.

A worked example, step by step

Suppose you run a grocery store and you're looking at your 1-litre full-cream milk packets.

• Sales over the last 14 days: 280 units sold
• Sell-through per day: 280 ÷ 14 = 20 units/day
• Supplier lead time: 3 days (order Monday morning, stock on shelf Wednesday evening)
• Safety buffer: 20 × (3 ÷ 2) = 30 units

Reorder point = (20 × 3) + 30 = 90 units

So the moment your on-hand count drops to 90 packets, you place an order. If you're currently holding 200 units, you've got roughly 5–6 days before you need to act.

Now, how much do you order? That depends on your order cycle — how often you want to place orders. Say you order weekly:

Order quantity = sell-through per day × 7 days + safety buffer − on hand at reorder point

Worked example continued: 20 × 7 + 30 − 90 = 80 units per order

Adjust up if your supplier has a minimum order quantity, or if you're heading into a high-demand period.

Where feel fails: two situations the formula handles better

Festivals and seasonal spikes. Your average daily sell-through from the last fortnight won't reflect what happens the week before Diwali or Eid. For those windows, manually scale your sell-through estimate — say, 1.5× or 2× normal — before plugging it into the formula. The seasonal inventory planning calendar has a full approach to this, but even a rough multiplier beats pure instinct.

Suppliers who slip. Say your dairy supplier usually delivers in 3 days, but twice in the last two months they've taken 5. Use 5 as your lead time input, not 3. Your safety buffer exists precisely for this, but if a supplier is consistently unreliable, the formula should reflect the actual worst-case lead time, not the promised one.

This is where feel fails hardest. When a supplier slips by two days and your buffer was sized for a reliable 3-day window, you're out of stock before the next delivery lands. The hidden cost of out-of-stock perishables covers how quickly those gaps compound — it's not just one lost sale.

The 20-minute weekly routine

You don't need to run this for every SKU every week. Pick your top 20–30 by revenue or by spoilage risk. That's where stockouts and overstock hurt most.

Here's the routine:

• Minutes 1–5: Pull last 14 days of sales for your priority SKUs. Your billing software or POS should give you this in one report.
• Minutes 6–12: For each SKU, calculate sell-through per day and compare to your recorded lead time. Flag any where lead time has changed recently.
• Minutes 13–18: Run the reorder point formula. Mark which SKUs are at or below their reorder point right now.
• Minutes 19–20: Write your order list. One supplier at a time, grouped by delivery day.

That's it. The first time takes longer. By week three it's a habit.

What a notebook can and can't do here

A notebook or a simple spreadsheet handles this well for 20–30 SKUs. You update the sell-through number manually, keep a column for lead time per supplier, and calculate the reorder point by hand or with a basic formula. It's not glamorous, but it works.

Where it breaks down: when you're managing more SKUs than you can reasonably track weekly, when your sell-through changes fast (perishables, seasonal items), or when you have multiple suppliers with different lead times that shift without notice. At that point, the manual update cycle starts lagging behind reality. You're always working from last week's numbers on a product that moved this week.

The gap between what your inventory system tracks and what you actually need for expiry and reorder decisions is worth reading if you're running into this ceiling. And if you want to understand how AI-assisted reordering works in Indian retail contexts, the smart reorder and procurement guide covers the landscape honestly.

A note on cost basis

The formula above is quantity-first. But quantity without cost context can mislead you. Suppose you're calculating a reorder for a slow-moving premium SKU — say, an imported cheese or a specialty supplement. The sell-through per day might be low, but the cost per unit is high. Over-ordering ties up cash and risks expiry write-offs.

For high-cost, slow-moving SKUs, shrink your safety buffer. For low-cost, fast-moving staples, a bigger buffer is cheap insurance. The arithmetic is the same; the judgment call is yours.

Sizing reorders automatically

ShelfSense by ShelfLifePro does exactly this calculation daily — sell-through rate, cost basis, lead time it learns from your store's own order history — and surfaces a ranked reorder recommendation with the arithmetic shown. It recommends; you approve. Nothing is ordered automatically.

ShelfSense's free audit reads one CSV export, no account needed — so you can see how it sizes your reorders before committing to anything. The Watch tier is free forever at 30 SKUs — a daily scan and a health report.

If you want to try it: start free at /shelfsense.

The point

Reordering by feel isn't laziness — it's what you do when you don't have a better system. The sell-through formula gives you one. Run it for your top SKUs this week. Adjust the lead time inputs to reflect what actually happens with your suppliers, not what they promise. Build in a buffer for the weeks when things slip.

Twenty minutes. A cleaner order list. Fewer stockouts, less overstock sitting on the shelf past its best date.