Return on investment (ROI) is the profit an activity produces, expressed as a percentage of what it cost. In marketing, the formula is: profit attributable to the marketing, minus the cost of the marketing, divided by the cost of the marketing, multiplied by 100. A positive figure means the activity earned back more than it cost; a negative one means it lost money.
How ROI works
The formula is easy. The work is in deciding what goes into each part.
A worked example in pounds
A VAT-registered kitchen fitter in Leeds spends £3,000 in a quarter on Google Ads and a freelancer to manage them. The campaign brings in jobs worth £24,000 including VAT.
- Remove VAT first: £24,000 ÷ 1.2 = £20,000 of net revenue. VAT is collected for HMRC, not earned.
- Remove the cost of delivering those jobs: units, worktops, fitters’ wages and van costs. Say these come to £14,000, leaving £6,000 of gross profit.
- ROI = (£6,000 − £3,000) ÷ £3,000 × 100 = 100%. Every pound spent on marketing came back along with another pound of profit.
Had the business used turnover instead, it would have reported (£24,000 − £3,000) ÷ £3,000 × 100 = 700%, a figure seven times too flattering. That gap is why I always ask whether a reported ROI is built on revenue or on profit.
ROI compared with ROAS
Return on ad spend divides revenue by ad cost and usually ignores product costs, fees and management time. It is useful for day-to-day bidding inside an ad platform. ROI answers the business question: did this make money? A campaign can show a healthy ROAS and a negative ROI when margins are thin.
Why it matters
ROI lets you compare unlike things on the same basis: an SEO retainer, a trade show stand, a Meta Ads campaign, a new website. That turns budget conversations from opinion into arithmetic. For a UK small business, where marketing spend comes straight out of profit, a clear ROI is often the difference between a budget that grows and one that is cut after the first quiet month.
It also exposes activity that looks busy but does not pay. A social media programme with impressive reach and no traceable sales has an ROI nobody can calculate, which is a finding in itself.
Common mistakes
- Using turnover instead of profit. Shown above, and still the error I meet most often in marketing reports.
- Leaving VAT in. For a VAT-registered business at the standard rate, revenue including VAT overstates net income by 20%, because a sixth of every gross sale belongs to HMRC.
- Counting only media spend. Agency or freelancer fees, software, design, photography and your own time are all part of the investment.
- Judging too early. SEO and content often cost money for months before returning any. A three-month ROI on a twelve-month programme will look poor by design.
- Ignoring repeat business. If a customer won by an ad keeps buying for years, first-order ROI understates the return. Use customer lifetime value where you have the data.
- Claiming sales that would have happened anyway. Brand search ads and retargeting often take credit for customers already on their way. Incrementality testing shows how much the spend genuinely caused.
How to act on it
Start by finding your gross margin for each main product or service; your accountant or bookkeeping software will have it. Without margin, ROI cannot be calculated honestly.
Next, make sure each channel’s results can be traced: conversion tracking on the website, call tracking where phone enquiries matter, and a simple way to record which enquiries became paying jobs. For lead-generation businesses, a monthly spreadsheet matching leads to closed sales is often enough.
Then calculate ROI per channel each quarter using net revenue, real margins and full costs, and agree a break-even point for each. Decide in advance what you will do if a channel stays below it for two quarters running. Building that measurement and budget discipline is a large part of the performance marketing work I do for UK businesses.