These two terms come up the moment you try to work out your profit, and they are simpler than the textbooks make them sound.

Opening stock is the value of the goods you had at the start of a period.

Closing stock is the value of the goods you had at the end of it.

The period can be a week, a month or a year. And one becomes the other: this month's closing stock is next month's opening stock. The goods on your shelf at midnight on 31 October are the same goods there on the morning of 1 November.

Why a shop owner should care

Because without them you can't know your profit.

Suppose you sold ₦1,800,000 this month and bought ₦1,500,000 of goods. It's tempting to say you made ₦300,000 before expenses. That's only true if your shelves hold exactly the same value of goods as they did a month ago.

If you ended the month with more stock than you started, some of that ₦1.5 million is still sitting on the shelf. You made more than ₦300,000. If you ended with less, you sold goods you had paid for earlier, and you made less.

Opening and closing stock are what correct for that.

The formula

Cost of goods sold = opening stock + purchases − closing stock

Cost of goods sold is what the things you sold cost you. Take it away from your sales and you have your gross profit.

The same formula, turned around, gives you what your closing stock should be if you know what you sold:

Expected closing stock = opening stock + purchases − cost of goods sold

A worked example

The figures are invented.

A provision store counts its stock on 30 September and values it at ₦2,400,000. That's the opening stock for October.

During October it buys ₦1,500,000 of goods.

On 31 October it counts again. The stock is worth ₦2,450,000. That's the closing stock.

Cost of goods sold = 2,400,000 + 1,500,000 − 2,450,000 = ₦1,450,000

October's sales were ₦1,800,000, so gross profit = 1,800,000 − 1,450,000 = ₦350,000.

Notice the shop ended the month with ₦50,000 more stock than it began with. If the owner had simply taken sales minus purchases, they would have got ₦300,000 and understated their profit by ₦50,000.

The rest of that calculation, down to net profit, is in how to calculate profit and loss for a shop.

How to get the closing stock figure

There are two ways, and the difference between them is useful.

Count it. At the end of the period, count everything, multiply each quantity by its cost, and add up. This is the real figure. The method is in how to take stock in a shop without closing for the day.

Read it from your records. If you keep a stock record book, the balances on the pages, times cost, give you what the stock should be.

The counted figure is what's actually there. The record figure is what ought to be there. The gap between them is your shrinkage: goods lost to damage, expiry, error or theft. A shop that only ever looks at one of the two never sees that number.

Value at cost, not selling price

This is the most common mistake. Stock is valued at what you paid for it, including the cost of getting it to your shop, not at what you hope to sell it for.

If you count 40 tins that cost ₦2,800 each and sell for ₦3,300, the closing stock is 40 × 2,800 = ₦112,000. Use ₦3,300 and you'd be counting profit you haven't earned yet.

If your cost has changed during the month, you have some tins bought at the old price and some at the new. The usual approach for a shop is to assume the oldest were sold first, so what's left is valued at the most recent costs. That's explained in first in, first out: how FIFO works in a provision store.

Goods that are damaged, expired or unsellable shouldn't be in closing stock at full cost. Value them at what you can really get for them, which may be nothing.

Mistakes to avoid

  • Using purchases as cost of sales. They are only equal if stock didn't change.
  • Valuing at selling price. It inflates your stock and your profit.
  • Counting on a different day. If you count on the 3rd, the figure includes three days of the new month's trading. Count on the last day, or adjust for sales and deliveries since.
  • Leaving out the store room. Stock under the counter, in the back and in your car boot all counts.
  • Including goods that aren't yours. Items a supplier left on sale-or-return aren't your stock until you've sold them.
  • Forgetting goods you've paid for that haven't arrived. Leave them out of the count, and make sure the purchase isn't in the month's figures either.

The routine

On the last day of each month, count and value your stock at cost. Write the figure down with the date. That single number closes one month and opens the next.

After three months you'll have something most small shops never get: a profit figure you can trust, and a clear view of whether your stock is growing, shrinking or simply sitting. If it's sitting, see slow-moving stock.