Dashboards

How to calculate MER on Shopify, and the UK VAT mistake that inflates it.

Published 30 October 2026 · 5 min read

MER is the simplest honest number a Shopify brand has: total sales divided by total ad spend. This guide shows how to calculate MER Shopify brands can actually trust, which figures to pull from Shopify and the ad platforms, and the one UK mistake that makes it look a fifth better than it is. The sum takes a minute. Picking the right inputs is where most stores go wrong.

The formula

MER (marketing efficiency ratio) = total sales ÷ total ad spend

Unlike ROAS, it does not care which ad gets the credit for a sale. It puts every order from every channel against everything you paid the ad platforms, so it cannot be double counted. If you want the background on what MER is and what a good one looks like, our guide What is MER covers it. This post is about getting the two inputs right.

How to calculate MER Shopify brands can trust, step by step

1. Pick a complete period. A full week or a full month works best. A single day swings too much, and a half finished week mixes busy and quiet days. Use the same start and end dates for every figure.
2. Take net sales from Shopify. In Analytics, open the sales reports for your dates and find net sales. Shopify works it out as gross sales, less discounts, less returns and other reversals. It leaves out shipping and taxes, which is exactly what you want.
3. Add up ad spend from every platform. Meta, Google Ads, TikTok, Pinterest and anything else you paid for ads in those dates. Take each figure from the platform's own spend column or billing page, before any VAT.
4. Divide. Net sales ÷ total ad spend. Write down which figures you used, so next month's number is worked out the same way.

The UK VAT mistake that inflates it

The biggest number at the top of most Shopify sales reports is total sales. It is what customers paid: net sales plus shipping plus taxes. For a VAT registered UK store, those taxes are VAT at the standard 20% on most products. That money belongs to HMRC. Shipping mostly goes straight back out to your courier. Neither is money your ads earned you.

Here is a made up store's month:

• Net sales: £22,000
• Shipping charged: £800
• VAT collected: £4,560
• Total sales: £27,360
• Ad spend: £5,500 Meta, £2,000 Google, £500 TikTok, so £8,000 in all

Using total sales, MER is £27,360 ÷ £8,000 = 3.42. Using net sales, it is £22,000 ÷ £8,000 = 2.75. Same month, same ads, and the first figure is about 24% higher.

Now say this store keeps 34% of each pound of net sales after products, packaging, delivery and payment fees. Its break even MER is 1 ÷ 0.34, about 2.9. At 3.42 the ads look comfortably profitable. At 2.75 they are losing money. Raise the budget on the strength of the first number and you raise the loss.

Getting the spend side right

VAT can creep into the bottom of the sum too.

• VAT registered. If your ad accounts carry your VAT number, invoices from the big platforms usually show no UK VAT added, so the spend is already net. If an invoice does show VAT that you reclaim, use the figure before VAT.
• Not VAT registered. You charge no VAT, so your sales have none to strip out (taxable turnover under £90,000 means you do not have to register). Any VAT you pay on ads is a real cost you cannot reclaim, so leave it in the spend.
• Fees. Some teams add agency fees and tool subscriptions to spend. Either way is fine as long as you always do it the same way and say which one you use.

Three other things that skew it

• A mixed VAT basket. Children's clothes and some food are zero rated or reduced rate. Dividing total sales by 1.2 is wrong for those stores. Net sales uses the tax Shopify actually recorded, so it avoids the problem.
• Returns landing later. Shopify usually records a return on the day it happens, not against the original order. A busy sale month can look great, then the following month carries its returns. Look at MER over a few months, not one.
• Time zones. Your store and your ad accounts may run on different time zones. Over a month the difference is small. Over a single day it can move the number noticeably.

What we do on client dashboards

Every custom dashboard we build takes net sales straight from Shopify and spend from each ad platform, both excluding VAT, and works out MER from those two figures every day. Targets like break even MER use the same basis, so nothing on the screen flatters you. You can click around a working example with made up data in our live dashboard demo. If you want the same for your store, every build starts with a consultation and a fixed quote. For more on VAT in your numbers, read ROAS and VAT.

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