Which Cost Counts When Your Stock Prices Keep Changing

Imagine you make scented candles and sell them online. In spring you buy a batch of wax for a low price. By autumn the same wax costs a third more. Now you sell a candle. Which wax did it come from? The cheap spring wax, or the pricey autumn wax? Both sit in the same bin, and nobody can tell them apart.

That small puzzle is the whole topic of stock costing. The answer changes how much profit your books show, so it is worth understanding even if you only sell a few products.

Why the answer is a choice, not a fact

When you sell something, you record two things: the money that came in and the cost of the item that went out. The second number is called the cost of goods sold. If every unit cost the same, this would be easy. But prices move, so identical units sit on your shelf with different price tags in your purchase records.

Because you cannot always say which physical unit left, you pick a rule that decides which cost to assign. The rule is an assumption about the order of costs, not a claim about what happened in the warehouse. A florist can sell the newest stems first and still record costs the other way around, as long as the method is one that is allowed and used consistently.

Three common rules

Oldest cost first

This rule assumes the oldest purchases leave first. When prices climb, your older units were cheaper, so the cost of each sale looks lower and your profit looks higher. What remains on the shelf is valued at the latest, higher prices, which tends to match what it would cost you to replace it.

Newest cost first

This rule assumes the most recent purchases leave first. When prices climb, each sale carries a higher cost, so profit looks smaller on paper. The leftover stock is valued at old, low prices, which can drift far from reality over the years. Be careful here: some countries and accounting standards do not accept this rule at all, so check what applies to you before you pick it.

Average cost

This rule blends everything. Add up what you paid for all the units on hand, divide by the number of units, and use that average for each sale. It smooths out the jumps and is the easiest to explain to a client, a partner or an accountant.

A worked example with made-up numbers

Say you bought three batches of candle jars, 50 each:

• Batch one: 2 per jar
• Batch two: 3 per jar
• Batch three: 4 per jar

You now sell 60 jars. Here is what each rule says the cost was:

1. Oldest first: 50 jars at 2 plus 10 jars at 3 gives a cost of 130.
2. Newest first: 50 jars at 4 plus 10 jars at 3 gives a cost of 230.
3. Average: total spend of 450 across 150 jars is 3 per jar, so 60 jars cost 180.

Same sales, same shelf, three different profits. The gap between the highest and lowest cost is 100, and that 100 simply moves between "cost of sales" and "value of stock left over". The money you spent does not change. Over the full life of the business, the totals meet in the end. The rule only changes when the cost shows up.

What this does to your numbers

Pay attention to three effects:

• Reported profit. In a rising market, the newest-first rule shows the lowest profit and the oldest-first rule the highest. In a falling market it flips.
• Stock value. The value of unsold goods on your balance sheet depends on the same choice. A bank or buyer looking at your books will read this number.
• Taxes. Profit usually drives what you owe, so the rule can shift when you pay. Rules about what is permitted differ widely, so this is a question for a local accountant, not a blog post.

How to choose without overthinking it

Start with the practical questions:

1. What is allowed where you operate? This can settle the question before anything else.
2. Can you track individual batches? If you can, an oldest-first or newest-first rule is possible. If everything is mixed together, averaging is more honest.
3. Does your stock expire or go out of fashion? Food, cosmetics and seasonal items naturally leave oldest first, so that rule matches reality.
4. Do prices swing a lot? Averaging keeps your monthly results calm, which helps you see the real trend.

Then stick with your choice. Switching methods every year makes your results impossible to compare, and a change usually has to be explained and justified.

Keep the paper trail clean

Any method depends on one habit: record every purchase with its date, quantity and unit price. Keep supplier receipts and a simple list. Without this, none of the rules can be applied, and your accountant will have to guess.

If you only sell services, you can skip all of this. But if you sell physical goods next to your services, say a designer who also sells printed posters, then the cost of those items belongs in the same picture as your income. Reports that show what you invoiced against what came in, such as the ones in TickPaid, make it easier to see the revenue side clearly, so that only the cost side needs the extra thought.

A quick checklist

• Write down the unit price of each purchase on the day you buy.
• Pick one rule that is allowed where you work.
• Use it for every sale, every period.
• Count your shelf stock at least once a year and compare it with your records.
• Ask an accountant before you change methods.

The takeaway

Costing your stock is less about maths and more about consistency. The formulas are short. The real work is keeping honest purchase records, choosing a method that fits your goods and your rules, and not changing your mind every few months. Do that, and your profit figures will mean something.