Ecommerce Profit Metrics: How to Calculate MER, AOV, CAC, LTV and Contribution Margin

Most ecommerce dashboards are full of numbers that look good and explain little. A campaign shows a strong ROAS, the store hits a revenue record, and the bank balance still goes down.

The fix is a small set of metrics that connect marketing to profit: marketing efficiency ratio (MER), average order value (AOV), customer acquisition cost (CAC), customer lifetime value (LTV) and contribution margin. Knowing the definitions is not enough, though. The value comes from calculating them correctly, from the same data, and using them to set targets.

This guide walks through the data you need, the exact formula for each metric and its variants, how to work out your break-even MER and ROAS, three ways to calculate LTV, payback period, a full worked example for one month of a store, a spreadsheet template and the mistakes that most often distort the numbers.

Why ROAS Is Not Enough

ROAS divides the revenue an ad platform attributes to its ads by the ad spend. It is useful for comparing campaigns inside one platform, but each platform counts conversions its own way, several platforms can claim the same order, and ROAS ignores product, shipping and payment costs. A 3x ROAS can be profitable for one store and loss making for another. The metrics below use your own store data, which makes them harder to inflate and closer to real profit.

Step 1: Collect the Right Data

Pull these numbers for the same period, usually one calendar month, from Shopify, your shipping and payment providers and your ad accounts:

• Net revenue: gross sales minus discounts and returns. Exclude sales tax. Decide once whether shipping fees charged to customers are included, and stay consistent.
• Orders: total orders, split into first orders from new customers and repeat orders from returning customers.
• New customers: the number of customers who placed their first ever order in the period.
• Product cost (COGS): the landed cost of the products sold, including inbound freight and duties.
• Fulfillment costs: outbound shipping, packaging and pick and pack fees.
• Payment fees: card and payment gateway fees.
• Marketing spend: all paid media plus any marketing costs you want to hold accountable, such as agency fees, influencer payments and affiliate commissions. Write down what you include.

Contribution Margin: The Base for Everything Else

Most of the formulas below depend on contribution margin, so calculate it first, in layers:

• CM1 = net revenue minus COGS.
• CM2 = CM1 minus fulfillment costs and payment fees. This is the margin before marketing.
• CM3 = CM2 minus marketing spend. This is what is left to cover fixed costs and profit.
• Each layer as a ratio = that layer divided by net revenue. CM2 ratio is the number you will use most.

Our full contribution margin guide covers these layers in more detail. Here, the key idea is simple: CM2 ratio tells you how many cents of every dollar of revenue are available to pay for marketing and still leave something over.

MER: Formula, Break-Even and Target

MER measures how much revenue your total marketing spend produces:

• MER = net revenue divided by total marketing spend.
• New customer MER = revenue from first orders divided by acquisition spend. It shows how efficiently you buy new customers, without returning customers flattering the number.

The most useful MER number is your break-even. Marketing is profitable as long as the margin it brings in exceeds what it costs, which means revenue multiplied by CM2 ratio must be larger than marketing spend. Rearranged:

• Break-even MER = 1 divided by CM2 ratio.
• Target MER for a desired CM3 ratio = 1 divided by (CM2 ratio minus target CM3 ratio).

For example, if your CM2 ratio is 57 percent, break-even MER is 1 divided by 0.57, about 1.75. Below 1.75, every dollar of marketing loses money. If you want CM3 to be 20 percent of revenue, target MER is 1 divided by 0.37, about 2.7. These two numbers turn MER from a vanity metric into a decision rule. Some teams call MER blended ROAS; see blended ROAS.

A real example of why this matters: for an Australian merchandise brand we worked with, Google Ads returned 10x ROAS in Q1 2024 and email generated AUD 158,500 in five months from an existing list of more than 100,000 subscribers. Owned channels like that email list add revenue with very little extra marketing spend, which lifts MER for the whole business. Tracking MER alongside channel ROAS shows that effect; channel ROAS alone would credit only the ads.

Break-Even ROAS for Each Channel

The same logic gives you a break-even ROAS to use inside each ad platform:

• Break-even ROAS = 1 divided by CM2 ratio.
• Target ROAS = 1 divided by (CM2 ratio minus target CM3 ratio).

Treat platform ROAS targets as a guide rather than truth, because platforms attribute revenue in their own way. A common practice is to manage MER as the business level check and use ROAS only to compare campaigns within a platform.

AOV: Formula and How to Value an Increase

AOV is net revenue divided by orders. Track it separately for first orders and repeat orders, because they usually differ.

AOV matters because some costs are paid per order, not per dollar. Shipping, packaging and pick and pack fees stay roughly the same whether a basket is 50 or 55 dollars. That means an AOV increase is worth more than the same revenue from extra orders:

• Extra contribution per order from a higher AOV = AOV increase multiplied by (1 minus COGS ratio minus payment fee ratio).
• Extra contribution per month = that amount multiplied by monthly orders.

With a 30 percent COGS ratio and 3 percent payment fees, raising AOV by 5 dollars adds about 5 multiplied by 0.67, or 3.35 dollars of contribution to every order. On 2,100 orders a month, that is roughly 7,000 dollars, with no extra marketing spend. Use the same formula to judge bundles, volume pricing or a free shipping threshold, and subtract any extra shipping cost you absorb.

CAC: Blended vs New Customer CAC

CAC tells you what it costs to win a customer, but there are two versions and they answer different questions:

• Blended CAC = total marketing spend divided by new customers. Simple, but it charges all spend, including retention, to new customers.
• New customer CAC = acquisition spend divided by new customers. Acquisition spend is the part of your budget aimed at people who have not bought before, such as prospecting campaigns.
• Channel CAC = channel spend divided by new customers attributed to that channel. Useful for comparison, but subject to attribution limits.

Never divide by total customers or total orders. Including returning buyers makes acquisition look far cheaper than it is. The related metric CPA is cost per action, such as cost per purchase inside an ad platform, which mixes new and returning buyers.

LTV: Three Ways to Calculate It

LTV estimates what a customer is worth over time. For profit decisions, always calculate it on contribution, not revenue. Choose the method that fits your data:

Method 1: Simple formula

• LTV = AOV multiplied by CM2 ratio multiplied by orders per customer per year multiplied by expected years as a customer.
• Example: 50 dollars multiplied by 0.57 multiplied by 2.1 orders a year multiplied by 1.5 years is about 90 dollars of contribution.
• Quick to calculate, but lifespan is a guess, so treat the result as a rough ceiling.

Method 2: Cohort based LTV

This is the most reliable method for most stores. Group customers by the month of their first order, then track the cumulative CM2 they generate per customer over time:

• Take all customers whose first order was in January.
• For each month after, add up the CM2 from all their orders and divide by the number of customers in the cohort.
• Plot the cumulative figure at month 0, 3, 6 and 12. That curve is your LTV over time.
• Example: a cohort earns 27 dollars of CM2 per customer on the first order, 38 dollars by month 3, 45 by month 6 and 58 by month 12. Twelve month LTV is 58 dollars.

Method 3: Subscription formula

• For subscription products: LTV = monthly contribution per subscriber divided by monthly churn rate.
• Example: 20 dollars of contribution a month and 8 percent monthly churn gives 20 divided by 0.08, or 250 dollars.

LTV to CAC Ratio and Payback Period

LTV and CAC become useful when you compare them:

• LTV to CAC ratio = contribution based LTV divided by new customer CAC. A widely used rule of thumb is around 3 to 1, but treat it as a guide, not a law.
• First order payback = first order CM2 minus new customer CAC. If it is positive, you are profitable from the first purchase.
• Payback period = the month in which a cohort's cumulative CM2 per customer first exceeds CAC. Read it straight off the cohort curve.

Payback period often matters more than the ratio, because it decides how much cash you need to fund growth. A 3 to 1 ratio that takes 18 months to pay back can still run a store out of cash.

A Worked Example: One Month, Step by Step

Here is a complete month for an illustrative store. Replace the numbers with your own.

The inputs

• Gross sales 120,000 dollars, discounts 8,000, returns 7,000, so net revenue is 105,000.
• 2,100 orders: 800 first orders from new customers and 1,300 repeat orders.
• First order revenue: 800 orders at 48 dollars, so 38,400. Repeat revenue: 66,600.
• COGS 31,500. Shipping and fulfillment 5 dollars per order, so 10,500. Payment fees 3 percent of net revenue, so 3,150.
• Marketing spend 30,000, of which 24,000 is acquisition spend.

The calculations

• CM1 = 105,000 minus 31,500 = 73,500 (70 percent).
• CM2 = 73,500 minus 10,500 minus 3,150 = 59,850 (57 percent).
• CM3 = 59,850 minus 30,000 = 29,850 (about 28 percent), or about 14.20 dollars per order.
• MER = 105,000 divided by 30,000 = 3.5. Break-even MER = 1 divided by 0.57 = 1.75. Target MER for 20 percent CM3 = 1 divided by 0.37 = 2.7.
• AOV = 105,000 divided by 2,100 = 50 dollars. First order AOV is 48.
• New customer CAC = 24,000 divided by 800 = 30 dollars. Blended CAC = 30,000 divided by 800 = 37.50.
• First order CM2 = 48 multiplied by 0.57 = about 27.40 dollars, which is below the 30 dollar CAC, so a new customer pays back on their second order.
• With a 12 month cohort LTV of 58 dollars, LTV to CAC = 58 divided by 30 = about 1.9.

What the numbers say

MER of 3.5 is well above break-even and target, so the business is profitable overall. But new customer economics are thinner: CAC is not covered by the first order, and LTV to CAC is below the common 3 to 1 guide. The priorities would be raising first order AOV, for example with bundles, improving conversion rate to lower CAC, and building retention flows so more customers reach their second and third orders. Scaling acquisition spend before fixing these would grow revenue faster than profit.

How to Set Your Own Targets

• Start from margin: calculate CM2 ratio from the last three months.
• Decide the CM3 ratio the business needs to cover fixed costs and profit. Fixed costs divided by expected revenue gives a minimum.
• Calculate break-even and target MER from those two ratios.
• Set a maximum new customer CAC: for example, first order CM2 plus the CM2 you are confident a customer generates within your payback window.
• Review monthly and update when prices, costs or the product mix change.

A Monthly Dashboard Template

Build one sheet with a row per month and these columns. The formulas assume each input sits in its own column:

• Inputs: net revenue, orders, first orders, first order revenue, new customers, COGS, fulfillment, payment fees, marketing spend, acquisition spend.
• CM2 = revenue minus COGS minus fulfillment minus payment fees. CM2 ratio = CM2 divided by revenue.
• CM3 = CM2 minus marketing spend.
• MER = revenue divided by marketing spend. Break-even MER = 1 divided by CM2 ratio.
• AOV = revenue divided by orders. First order AOV = first order revenue divided by first orders.
• New customer CAC = acquisition spend divided by new customers.
• First order payback = first order AOV multiplied by CM2 ratio, minus new customer CAC.
• A separate cohort tab for LTV, with first order month as rows and months since first order as columns.

Common Mistakes That Distort the Numbers

• Using gross sales instead of net revenue, which hides discounts and returns.
• Counting returning customers in CAC, which makes acquisition look cheap.
• Calculating LTV on revenue instead of contribution, which overstates what customers are worth.
• Adding up ROAS across platforms, which double counts the same orders.
• Mixing periods, for example spend from one month and revenue from another.
• Changing what counts as marketing spend from month to month, which breaks trend comparisons.

Where to Start

This week, calculate CM2 ratio, MER and break-even MER for last month; they only need numbers you already have. Next, split new and returning customers to get new customer CAC, then build your first cohort tab for LTV. Within a month you will have a profit dashboard that tells you more than any platform report.

If you want a Shopify store designed to lift order value and conversion, our Shopify development service can help. Contact us to review your metrics together.