ACoS formula: how to calculate it step by step with a real example

The ACoS formula is simple: ad spend ÷ sales generated by those ads × 100. If you spent $2,000 on advertising and those ads generated $10,000 in sales, your ACoS is 20%. That’s it. There’s no secret formula or hidden calculation: it’s a division, expressed as a percentage, that tells you what share of your advertised revenue went to paying for clicks.

What confuses most sellers isn’t the formula itself, but where to get the two numbers that feed it. “Ad spend” and “ad sales” live in different reports, under different names, and they change depending on the attribution window you pick. It gets worse if you sell across several marketplaces: Amazon gives you its ACoS with its own logic, MercadoLibre reports Product Ads with another, and you end up building the math by hand in a spreadsheet just to compare them.

In this article we break down the ACoS formula step by step, with a real example from a seller who sells a stainless-steel tumbler sleeve on Amazon Mexico and MercadoLibre. You’ll see exactly which number goes into each part of the equation, where to find it, and why the same product can give you two different ACoS values depending on the channel.

the acos formula, no fluff

ACoS (Advertising Cost of Sales) measures how much you spent on advertising for every dollar of sales that advertising generated. The formula is:

ACoS = (Ad spend ÷ Sales attributed to ads) × 100

The two ingredients are:

• Ad spend: the total you paid for clicks (or impressions, depending on the format) in a period. Amazon calls it Spend; MercadoLibre calls it investment in Product Ads.
• Sales attributed to ads: the revenue from sales the marketplace credits to those ads within an attribution window (for example, 7 or 14 days after the click).

The result is a percentage. An ACoS of 20% means that for every $100 of advertised sales, you spent $20 on ads. The lower the number, the more “efficient” the spend was in terms of gross revenue. Watch that last phrase: gross revenue, not profit. ACoS doesn’t know what your product cost, or how much you paid in commission or fulfillment. That’s a separate story we’ll get to at the end.

Glossary: ACoS is ad spend divided by the sales attributed to those ads, expressed as a percentage; it measures spend efficiency against revenue, not profitability.

where each number comes from

This is where most people get stuck, because the two numbers aren’t together on any single screen. On Amazon, spend and attributed sales come from the Amazon Ads campaign report (or the campaigns tab in Seller Central). On MercadoLibre, they come from the Product Ads panel. Each platform calculates its own ACoS and shows it pre-digested, but with rules that don’t always match.

The attribution window matters a lot. If Amazon attributes sales up to 14 days after the click and MercadoLibre uses a shorter window, the same advertising effort will produce different “ad sales” on each channel, and therefore different ACoS values, even if you spent the same. Comparing the two ACoS values without understanding their windows is comparing apples to oranges.

And there’s a detail almost nobody adjusts: the ACoS you see in the marketplace uses gross sales, including ones that later get canceled or returned. If you measure your real ACoS against net sales — subtracting canceled and returned orders — the percentage goes up. That’s why it helps to have a clear ACoS definition and know which report each data point comes from before drawing conclusions.

a real example, step by step

Take a seller who sells a stainless-steel tumbler sleeve. She sells it on Amazon Mexico and on MercadoLibre. During May she ran campaigns on both channels. Let’s calculate the ACoS for each, step by step.

Step 1: gather ad spend. On Amazon she spent $3,400 on Sponsored Products during the month. On MercadoLibre she spent $1,900 on Product Ads. These are the Spend / investment figures from each panel.

Step 2: gather attributed sales. Amazon attributed $17,000 in sales to those campaigns (7-day window). MercadoLibre attributed $8,300 (shorter window). These are the revenues each platform credits to the ads.

Step 3: apply the formula, channel by channel.

• Amazon: (3,400 ÷ 17,000) × 100 = 20%
• MercadoLibre: (1,900 ÷ 8,300) × 100 = 22.9%

Step 4: read the result. Same product, same seller, same month, two different ACoS values. On Amazon she converted her spend better (20%), on MercadoLibre it cost her a bit more (22.9%). It’s not that MercadoLibre is “worse”: the attribution window is shorter, the competition for the keyword is different, and pricing against other sellers isn’t identical. The formula is the same; the inputs change by channel.

Step 5: the combined ACoS (optional). If you want a single number for the whole business, don’t average the two percentages: that gives a misleading result. Add up the spend, add up the sales, and divide:

• Total spend: 3,400 + 1,900 = 5,300
• Total attributed sales: 17,000 + 8,300 = 25,300
• Combined ACoS: (5,300 ÷ 25,300) × 100 = 20.9%

That 20.9% is your real aggregate ACoS. Averaging 20% and 22.9% would have given you 21.45%, a number that represents nothing because it ignores that Amazon moved more volume.

the mistake of assembling it by hand every month

The example looks clean in an article. In real life, those five numbers — two spends and two attributed sales plus the combined figure — come from two different platforms, in two different tabs, with two different attribution windows, and you copy them into a spreadsheet to do the division. Multiply that by twenty SKUs and you have a whole afternoon of work that’s stale the moment you finish: by the time the table is done, the campaigns have kept spending and the data is from yesterday.

That’s the hidden cost of the ACoS formula. The equation is trivial; gathering its inputs is not. When the spend and attributed sales from all your channels live in a single real-time dashboard, the formula calculates itself, by SKU and by channel, and the combined ACoS recomputes on its own every time a sale comes in. You stop rebuilding the math and start reading the answer. And because the same dashboard knows your real-time inventory, you can see right away whether an ACoS that went up is due to the campaign or to running out of stock on your best-selling color, which dropped conversion.

from acos to profit: the step the formula doesn’t take

The ACoS formula ends at an efficiency percentage, but it doesn’t tell you whether you made money. An ACoS of 20% is excellent on a product with a 45% margin and ruinous on one with 18%. To know whether advertising left you a profit, you have to subtract ACoS from your margin after all costs: product, commission, fulfillment, and tax.

That full margin is what really rules. If your tumbler sleeve leaves a 30% real net margin before advertising, an ACoS of 20% leaves you 10 points of profit. But if the real margin were 22%, that same 20% ACoS would leave barely 2 points, and a single return would push you into the red. The ACoS formula is the first step; the second is checking it against the real margin by channel.

Glossary: real net margin is what’s left after ALL costs — product, commission, fulfillment, shipping, and tax — not just price minus cost; it’s against this number that you should read your ACoS.

in short: the formula and what comes after

Keep this: ACoS = ad spend ÷ attributed sales × 100. Gather spend and attributed sales for each channel, apply it separately, and if you want a global number, add spend and sales before dividing instead of averaging percentages. With the tumbler-sleeve example you saw that the same product yields 20% on Amazon and 22.9% on MercadoLibre, and that the real combined figure was 20.9%.

The challenge was never the arithmetic. It was having the right inputs, from the right report, in the right window, and in the same place across all your channels, up to date. When that happens, the formula stops being an end-of-month chore and becomes a real-time read. And when you also read it alongside your real margin and your real available stock, you go from “how much did I spend?” to the only question that matters: “did this spend leave me a profit today?”.

Glossary: real available stock is the sellable inventory net of reserves and in-transit units; if it drops, your conversion falls and your ACoS rises without the campaign having changed anything.