Every shop owner knows the feeling of telling a regular customer "it has finished, come back on Thursday" and watching them walk to the shop next door. Running out of a fast seller costs more than the one sale. It teaches the customer where else to go.

The reorder level is the number that prevents this. It is the stock level at which you place the next order, set high enough that the new goods arrive before the old ones run out.

The formula

Reorder level = (average daily sales × days the supplier takes) + safety stock

Three inputs, all of which you can get from your own records.

Average daily sales is how many you sell on a normal day. Take the last four weeks from your sales book, add up the units sold, and divide by the number of days you were open.

Days the supplier takes, often called lead time, runs from the moment you decide to order until the goods are on your shelf. Count all of it: the day you call, the day they load, the day it travels, the morning you unpack. If you go to the market yourself, it's the number of days until your next trip.

Safety stock is extra, kept for the week when sales jump or the supplier is late. A reasonable starting point for a small shop is two or three days of sales.

Worked through

Take tinned milk in a provision store. The numbers here are invented for the example.

Over the last four weeks, open six days a week, the shop sold 144 tins. That's 24 selling days, so 144 ÷ 24 = 6 tins a day.

The distributor delivers 4 days after an order.

The owner wants 2 days of safety stock, which is 12 tins.

Reorder level = (6 × 4) + 12 = 36 tins.

So when the shelf and store room together drop to 36 tins, the order goes in. During the four days of waiting the shop sells about 24, and the new stock arrives with roughly 12 still on hand. If the truck is two days late, those 12 tins cover it.

What it costs to hold

The formula has a price. At a cost of ₦2,800 a tin, 36 tins is ₦100,800 sitting on the shelf at the moment you reorder.

That's the trade you are making. A bigger safety stock means fewer disappointed customers and more money tied up in goods. A smaller one frees cash and raises the chance of running out. For your top sellers, lean towards holding more. For slow lines, hold less, and read about what to do with slow-moving stock before it builds up.

How much to order

The reorder level tells you when. It doesn't tell you how much.

A simple rule for a small shop: order enough to last until the next order you plan to make, plus the safety stock, minus what you'll still have when it arrives. If you want to order milk every two weeks, that's 12 selling days at 6 a day, so 72 tins.

Then adjust for what the supplier actually sells. If cartons hold 24, order 3 cartons. If there is a price break at 5 cartons and you can sell them before they expire, that may be worth taking. If it only looks cheap because you'd be holding two months of stock, it isn't.

When the number changes

A reorder level is only right until something moves. Check it again when:

  • the season turns. December, Easter, Ramadan and back-to-school weeks change what sells. Raise the level a few weeks ahead for the goods affected.
  • the supplier gets slower. Fuel scarcity, bad roads in the rains and port delays all stretch lead time. If 4 days has become 7, the level for milk goes from 36 to 54.
  • sales shift. A new estate opens nearby, or a competitor does. Rework the daily figure from the last four weeks, not from last year.

Putting it to use

You don't need this for every item. Start with the 20 products you would be most embarrassed to run out of. Work out the level for each one and write it at the top of that item's page in your stock record book.

Then the rule for whoever keeps the book is simple. When the balance column reaches the number at the top of the page, tell the owner the same day. That one habit removes most stock-outs, and it works with a notebook and a pen.