ROAS, MER and Cost Per Purchase: What These Meta Ads Metrics Actually Mean

You're running Meta ads and the dashboard is full of numbers. Here's what the ones that actually matter are telling you.

You boosted a post. Or maybe you set up your first proper campaign. Either way, you're now staring at a Meta Ads Manager dashboard full of acronyms and numbers and wondering which ones you're actually supposed to care about.

ROAS. MER. Cost per purchase. CPM. CTR. The list goes on.

Most of it you can ignore for now. But three metrics will tell you almost everything you need to know about whether your ads are working. This post covers what each one means, how to find it, and what to do with it.

No jargon. No assumptions about what you already know. Just the plain English version.

First, a quick note on Meta Ads Manager

All of these metrics live inside Meta Ads Manager. That's the platform where you build, run and monitor your Facebook and Instagram ads.

If you're not sure where to find a specific number, you can customise your columns inside Ads Manager to show exactly the metrics you want. There's a "Columns" dropdown at the top of your campaign view. Click it, select "Customise columns" and you can add or remove whatever you need.

Right. Into the metrics.

ROAS: Return on Ad Spend

ROAS is the one you'll hear about most. It's also the one that gets misread most often, so let's cover it properly.

ROAS tells you how much revenue you made for every dollar you spent on ads.

The formula is simple. Revenue from ads divided by amount spent on ads.

So if you spent $500 on ads and those ads generated $2,000 in sales, your ROAS is 4. Which is sometimes written as 4x. For every dollar you put in, four came back in revenue.

That sounds straightforward. And it is, mostly. But here's the thing ROAS doesn't tell you.

It doesn't account for your costs. A ROAS of 4x sounds great until you factor in that your product costs $60 to make and ship, you're paying a management fee, and your actual profit margin is sitting somewhere around 30 percent. At that point, a 4x ROAS might be profitable. Or it might not be. ROAS alone can't tell you.

So ROAS is useful as a quick pulse check. Is this campaign generating revenue relative to what I'm spending? But it shouldn't be the only number you look at.

What's a good ROAS? It depends entirely on your margins. A business with a 60 percent margin can be profitable at a 2x ROAS. A business with a 20 percent margin might need a 5x ROAS to break even. There's no universal "good" number. There's only what works for your specific business, which is why knowing your numbers matters so much.

Where to find it in Meta Ads Manager: ROAS shows up as "Purchase ROAS" in your columns. If you can't see it, add it via the customise columns option.

Cost Per Purchase

Cost per purchase is exactly what it sounds like. How much did it cost you in ad spend to get one sale.

The formula: total ad spend divided by number of purchases.

So if you spent $300 and got 10 purchases, your cost per purchase is $30.

This is the metric that tells you whether your ads are actually profitable at a unit level. Not whether they're generating revenue. Whether they're generating profit.

Here's how to use it. Take your cost per purchase and compare it to your gross profit per sale (that's your selling price minus the cost of making and delivering the product). If your gross profit per sale is $50 and your cost per purchase is $30, you're making $20 per sale from ads before other overheads. If your cost per purchase is $60 and your gross profit is $50, every sale from ads is costing you $10. That's a problem worth knowing about.

Cost per purchase is the most grounded metric for ecommerce because it connects ad spend directly to unit economics. It's the number I look at first when I'm reviewing whether a campaign is working.

Where to find it in Meta Ads Manager: "Cost per result" when your campaign objective is set to purchases, or add "Cost per purchase" via customise columns.

MER: Marketing Efficiency Ratio

MER is the one most founders haven't heard of yet. And it's arguably the most useful of the three, especially once you're running ads at any kind of volume.

MER stands for Marketing Efficiency Ratio. Here's the formula.

Total revenue (across your entire store) divided by total marketing spend (across all channels).

So if your store made $10,000 in revenue this month and you spent $2,000 total on ads (across Meta, Google, or anything else), your MER is 5.

Now here's why this matters. Meta's own reporting has attribution problems. It counts conversions based on people who saw or clicked your ad, but it doesn't know whether that person would have bought anyway via Google, organic search, or direct. This means Meta often overcounts its contribution to your revenue. Your ROAS inside Meta Ads Manager might say 6x, but your actual business-level return is lower because some of those sales would have happened regardless.

MER cuts through that. It doesn't care about attribution. It just looks at total money in versus total money spent on marketing. It's a blunt tool, but it's an honest one.

The way to use MER is alongside ROAS, not instead of it. If your Meta ROAS is high but your MER is low, something doesn't add up and it's worth digging into why. If both are healthy, your ads are genuinely contributing to business growth.

Where to find it: MER isn't a metric inside Meta Ads Manager. You calculate it yourself using your Shopify revenue and your total ad spend for the same period.

How to use all three together

Here's a simple way to think about it.

ROAS tells you whether your Meta campaigns are generating revenue relative to spend. It's the quick check.

Cost per purchase tells you whether that revenue is actually profitable at a unit level. It's the gut check.

MER tells you whether your ads are genuinely contributing to overall business growth, or whether Meta is just taking credit for sales that would have happened anyway. It's the honest check.

Run all three. Look at them together. If your ROAS looks great but your cost per purchase is eating your margin and your MER is flat, you have a problem even if the dashboard looks happy.

That's the thing about metrics. They're only useful if you know what they're telling you.

A quick example to tie it together

Say you run a skincare brand. Your hero product sells for $80. It costs you $30 to make and ship. Your gross profit per unit is $50.

This month you spent $1,000 on Meta ads. Meta is reporting a ROAS of 4x, meaning it's attributing $4,000 in revenue to your ads. Your cost per purchase is $25.

At $25 cost per purchase against a $50 gross profit, you're making $25 per sale from ads before other costs. That's healthy.

Now you check your MER. Your store did $6,000 in total revenue this month. You spent $1,000 on ads. MER is 6. Strong.

Everything lines up. Your ads are working.

Now imagine the same scenario but your cost per purchase is $55. You're spending more to acquire each customer than you're making on the sale. ROAS might still look okay because Meta is reporting revenue, but the unit economics are broken.

That's the scenario MER and cost per purchase catch that ROAS alone misses.

What to do if your numbers don't look right

First, don't panic. And don't immediately turn off your campaigns.

Check your cost per purchase against your gross profit first. That's the most important relationship. If cost per purchase is consistently higher than your gross profit per unit, you either need to reduce what you're spending to acquire customers (through better creative, better targeting, or better conversion on your store) or increase your average order value so each sale is worth more.

If your ROAS looks high but your MER is low, look at what else is driving revenue. If organic or direct traffic is doing the heavy lifting and Meta is overclaiming, you might be able to reduce ad spend without losing much revenue.

If all three numbers look off, it might not be an ads problem. It might be a store problem. (More traffic to a store that isn't converting doesn't fix anything. It just makes it more expensive.)

The honest truth about running ads

Meta Ads Manager is designed to make your ads look like they're working. It optimises for the metrics it can measure and takes credit wherever it can find it.

That doesn't mean ads don't work. They absolutely do. But they work best when you know which numbers to trust, which ones to question, and what they're actually telling you about your business.

ROAS, MER and cost per purchase, used together, give you that picture.

Want someone to look at your numbers with you?

If you're running Meta ads and not sure whether your numbers are healthy, or if you're thinking about starting ads and want to make sure the foundations are right first, that's exactly the kind of thing we can work through together.

Ads management starts from $1,300 plus GST per month, and includes campaign setup, ongoing management and regular performance reviews so you always know what your numbers are telling you.

Get in touch here to find out if it's the right fit.