Black Friday is a month away. You'll probably pick a discount, watch the orders roll in, and enjoy one of the busiest weeks of your year.

But busy isn't the same as better off. A sale can bring in more money than usual and still leave you with less profit than an ordinary week. The reason is simple: your discount doesn't only go to the extra customers the sale brings in. It also goes to everyone who would have bought from you anyway, at full price.

So the question worth answering is this: did the sale earn you more than a normal week would have? Ideally you answer it before you choose your discount, and then again once the sale is over.

The sale that looked like a win

Here's a made-up store with round numbers.

• In a normal week it takes $5,000, and after paying for its stock it keeps about $2,750.

• For Black Friday it runs 25% off everything.

• Sale week takes $6,500. That's 30% up on normal, and it looks like a great week.

But at 25% off, the store had to send out far more stock to reach that $6,500. Once it pays for those goods, it keeps about $2,600.

That's $150 less than an ordinary week. It packed more orders and did more work, and it has more returns to deal with later, yet it made less money.

To match a normal week's profit, this store needed its sale week to take about $6,875, roughly 38% more than usual. Anything below that and the sale cost it money, however busy it felt.

The deeper the discount, the higher the bar

The bar rises fast. With the same margin, 15% off needs only about 17% more takings to break even. At 35% off, the store would need about 79% more.

That's why the most useful number to know before Black Friday isn't your discount. It's how much extra you'd need to sell for that discount to pay for itself. If that target looks unrealistic, a smaller discount might earn you more.

Don't double-count discounts you already give

Many stores already discount every week through welcome codes, loyalty offers and influencer codes. That's easy to misjudge.

Two numbers matter here, and they're not the same:

• How many orders use a discount. For example, 1 in 5.

• How much those discounts actually take off your sales. If those orders average 15% off, that's only about 3% of your total sales.

So in this example a 25% sale costs you about 22 points more than normal, not 25. Counting the full 25 would make the sale look worse than it really is. Assuming your everyday discounts are bigger than they are would make it look better.

PromoMate works this out from your own orders and only counts the discount on top of what you normally give.

After the sale: is the difference real?

No two weeks are the same. Say the example store's ordinary weeks keep anywhere between about $2,400 and $3,200 in profit. Now say its sale week kept $2,800.

That's a little above an average week, but it's well within the range of a normal one. The honest answer is that it's too close to call.

That isn't a failure. It's useful to know, because it tells you a smaller discount, or none, might have done just as well. You should be wary of any report that calls that result a win.

What a quick check leaves out

A quick profit comparison doesn't count:

• ad spend for the sale

• returns, which tend to be higher on sale orders

• customers who would have bought in December anyway

• new customers who come back and buy again

So a sale that only just beats a normal week probably didn't.

Let PromoMate do the thinking

You don't need to work this out by hand.

PromoMate reads your orders export and shows you, free, how much of your trade already carries a discount. Then it can track your sale for you. Your first complete tracked sale is free on promomate.ai (sign in with your email to start it). After that, each sale is US$49. A tracked sale:

• sets your break-even target before the sale

• shows your progress during it

• tells you afterwards whether it was worth running

The after-sale check looks at the four weeks after the sale for quieter-than-normal trading, and allows for extra refunds and new customers. It says plainly when a result is too close to call, or too early to tell. It never connects to your store, and customer details are removed in your browser before anything is uploaded.

On Shopify? The free reading works for Shopify stores here. In PromoMate for Shopify, from the Shopify App Store, your first complete tracked sale is free too, then each tracked sale is US$49.