SEO and SEM are the two ways of appearing on Google and the other search engines. SEO (search engine optimisation) works on the organic results: you don't pay for the click, you pay for the work that earns it. SEM (search engine marketing), in the sense most people use it day to day, is paid advertising on search engines, meaning Google Ads and Microsoft Advertising: every click has a price and the visibility ends when the budget does.
Put like that, it sounds like a choice between unpaid and paid. Two things are missing from most pages on this question: the word SEM doesn't mean the same thing to everyone, and the real comparison is made on the cost of a click in your sector, not on definitions. Below are the numbers to do it: UK cost-per-click data from Google Ads, our own figures from three Google Ads accounts and a thousand queries measured in Search Console, and Google's European click data collected by SparkToro.
SEM means two different things, and it's worth knowing which
Before the comparison there's a vocabulary problem. For some sources SEM is just paid search advertising, which some also call SEA (search engine advertising). For others it's an umbrella term that includes SEO as well. On Google UK the first page for "seo vs sem" is split between the two meanings.
| Source | Market | What it means by SEM |
|---|---|---|
| Shopify | United States | Paid ads only, to increase visibility in the SERPs |
| Semrush | United States | A broad term combining SEO and PPC ads |
| Search Engine Land | United States | An umbrella covering paid and organic traffic |
| SEO.com | United States | Promotion on search engines, both organic and pay per click |
| The AI Overview and ten organic results for "seo vs sem" | United Kingdom | Split: the AI Overview calls SEO "a subset of SEM" but adds that in everyday practice SEM usually means paid ads; HawkSEM and UNV Digital use the umbrella meaning, Americaneagle and First Page (Australia) the paid one |
The practical consequence concerns quotes. If an agency offers you an "SEM service", ask in writing whether it includes organic work or only campaign management: with the broad definition, two proposals can have the same name and contain different things. In this article SEM means paid advertising on search engines, the meaning most practitioners use day to day. The cost model is pay per click.
The differences between SEO and SEM in one table
The table puts the two activities against the same criteria, with a number wherever we have one.
| Criterion | SEO | SEM (Google Ads) |
|---|---|---|
| Where you appear | Organic results, and as a source in AI answers | Spaces labelled as sponsored, above and below the results |
| What you pay for | The work: content, technical fixes, links. The click costs nothing | Every click at auction price, plus the 2% fee Google adds to ads served in the UK to cover the Digital Services Tax |
| Cost of a click | The monthly fee divided by the clicks that come in | £3.03 for "led wall", £9.49 for "accountant london", £35.94 for "divorce solicitor london" (Google UK, October 2026) |
| Time to first result | Months: only 1.74% of new pages reach the top 10 within a year | Days, plus a learning period for automated bidding of up to around 50 conversion events or 3 conversion cycles |
| When you stop paying | Rankings decline slowly | The ads disappear at once |
| What first position is worth | CTR of 19.62% across 1,000 Italian queries | Ads shown on fewer than 20% of searches; CTR estimated at 5-10% when they're at the top |
Two numbers in the table need a note. The 1.74% comes from the 2025 Ahrefs study of a million URLs: of the pages that do reach the top 10, 40.82% get there within the first month, so SEO isn't slow for everyone, it's slow for most. The learning period is what Google gives in its documentation on the learning period, and those days are paid at full price. The organic timescales, with our own data, are in how long SEO takes to show results.
How many clicks go to organic results and how many to ads
The difference in scale between the two channels surprises people comparing them for the first time. SparkToro, using the Datos clickstream panel (a Semrush company), measured Google searches between September 2022 and May 2024: in the European Union, for every 1,000 searches, 374 clicks go to sites not owned by Google, and 59.7% of searches end without a click. In the United States it's 360 clicks and 58.5%. The study doesn't report the UK separately; a 2026 SparkToro update based on Similarweb data covers only the US so far (68.01% zero-click searches) and says UK data will follow.
Paid ads take about 1% of clicks. Not because nobody clicks them: according to the 2023 Google figure quoted in the same study, ads appear on fewer than 20% of searches, and SparkToro estimates their average CTR at between 5% and 10% when they're at the top. The panel covers desktop and Android and almost no iPhones, so these figures should be read as orders of magnitude. We've written a separate article on zero-click searches.
This explains the division of labour better than any definition. SEM buys space on the minority of searches where someone has decided to run an auction, almost always the commercial ones. SEO competes on all of them, including the informational questions where an ad usually doesn't appear (choosing search by search, with the data, is covered in SEO or Google Ads).
In the UK, spending tells the same story from the other side. According to IAB UK's Digital Adspend study (with Oliver Wyman, March 2026), UK digital advertising was worth £40.5bn in 2025 and search accounted for 44% of it, £17.9bn, up 6% on the year. Google had 91.39% of searches made in the UK in September 2026 according to StatCounter, Bing 6.08%. For a UK business, SEM means almost entirely Google Ads, with Microsoft Advertising as second choice, and the same goes for SEO: the differences are in the Google vs Bing comparison. More market figures are in our digital marketing statistics.
The sum that decides it: what a click is worth in your sector
The "SEO or SEM" question is better settled with a division than with a list of pros and cons. Take the monthly SEO fee and divide it by the cost per click of your searches on Google Ads: the result is the number of organic clicks a month that makes SEO equivalent to advertising, for the same traffic.
With a fee of £1,000 a month and the UK CPCs from Google's Keyword Planner in October 2026, the break-even point changes more than tenfold from one search to another.
| Search | CPC | Organic clicks a month worth £1,000 | Organic impressions needed (CTR 4.25%) |
|---|---|---|---|
| led wall | £3.03 | 330 | about 7,760 |
| electrician london | £5.22 | 192 | about 4,520 |
| plumber london | £7.32 | 137 | about 3,220 |
| accountant london | £9.49 | 105 | about 2,470 |
| divorce solicitor london | £35.94 | 28 | about 660 |
The result runs against many people's instinct. Where clicks are cheap, advertising is the simple choice, and SEO has to bring in thousands of visits to justify itself. Where a click costs £10, around a hundred organic visits a month are enough for the fee to be repaid in equivalent clicks; at £36, fewer than thirty. The more expensive your sector is on Google Ads, the sooner SEO pays off.
Don't use American benchmarks for this comparison. The WordStream 2026 report, across 13,474 US campaigns between April 2025 and March 2026, gives a median CPC of $5.42 and a cost per lead of $66.69: a US sample, and an average that says nothing about your sector when UK searches in our table run from £3.03 to £35.94 a click.
Two caveats on the sum. An organic click and a paid one don't necessarily convert in the same way, so the next step is cost per lead: across the three Italian Google Ads accounts we manage, the average cost per conversion between September 2025 and August 2026 was €117.80, so a campaign with a €1,500 budget and €500 of management brings in one lead for roughly every €160, all in (the detail is in how much Google Ads costs). And SEO doesn't deliver those clicks from the first month: whether it holds up over time depends on the margin and conversion calculation we worked through in is SEO worth it.
When SEM makes sense, and when SEO does
With the sum in hand, the choice depends on three variables: how quickly you need leads, how much a click costs in your sector and whether people are already searching for what you sell.
| Situation | Where to start | Why |
|---|---|---|
| You need enquiries this quarter | SEM | Ads go live in days; in a year fewer than two new pages in a hundred reach the top 10 |
| High CPC in your niche (above £5) | SEO, with a supporting campaign | Break-even in equivalent clicks comes with a few hundred visits a month or fewer |
| Low CPC and a large catalogue | SEM on the best sellers, SEO on the categories | Clicks are cheap to buy, but organic volume is needed across hundreds of pages |
| Informational questions ("how to", "how long", "difference between") | SEO | Ads appear on fewer than 20% of searches, rarely on these |
| A new product nobody searches for | Neither, for now | Both capture existing demand; the demand has to be created through other channels |
| You want to know which searches bring leads | SEM for two or three months, then SEO | The search terms report shows which queries convert and at what cost |
The "when" also depends on the demand calendar, which varies a lot from sector to sector: we worked it out in when Google Ads is worth it. If you want to run this sum on your own searches, it's the first step in both our SEO work and our management of Google Ads campaigns.
What AI answers have changed, on both sides
Google's AI Overviews have been live in the UK since 15 August 2024 and they affect the two channels in different ways.
On the organic side they take clicks away. The most quoted figure is from the US: the Pew Research Center followed 900 American adults across 68,879 searches in March 2025, and with an AI summary on the page a traditional result was clicked in 8% of visits, against 15% without one. On the Macropix cluster in Italy we measured that 32.7% of tracked keywords show an AI Overview and that, when it appears, 43% of searches end without a click: just over 10% of the cluster's volume is traffic that reaches no site at all. The method for measuring it on your own site is in the impact of AI Overviews on SEO. In exchange SEO has a new place to appear, among the sources cited inside the answer, and that's the work we call GEO.
On the paid side the ads are where they were, but the page around them has changed. Google Ads Help says ads can show above and below AI Overviews in all 200+ markets where they exist, the UK included. Inside the answer, on the other hand, they only appear in English in twelve countries (among them the US, Canada and Australia), and the UK isn't one of them. There's no option to opt out. The details are in our article on Google Ads and AI Overviews.
As we read it, the gap between the two channels has widened above all on informational searches, where the AI answer absorbs organic clicks and ads barely appear.
Together, in the right order: the Macropix case
Google says it in Google Ads Help: running online ads has no impact on where your website appears in organic search results. And on its page about how Search works it adds that nobody can buy a better ranking in the organic results. So the two channels don't help each other technically. They help each other through data and through the space they take up on the same page.
Macropix, a Milan-based maker of custom LED screens, is the project we've worked on longest. It started in 2020 with SEO, when the site didn't appear for any category keyword, and only later expanded to Google Ads, Bing and visibility in AI answers. Over seven years the project has spent €420,000 across SEO and advertising and brought in around 7,000 enquiries: roughly €60 an enquiry, all in. Today the site holds more than a hundred keywords in the top three positions on Google Italy, including "ledwall" (LED wall), 4,400 searches a month, where it ranks first.
The comparison with the €117.80 per conversion of the three Google Ads-only accounts isn't clean: different sectors, different years, conversions counted differently. One thing holds, though: over a horizon of years the organic side lowers the average cost of a lead, because it keeps bringing in traffic without paying for the click. The full story is on the Macropix project page, and the method used for the site is in SEO for manufacturing.
The Macropix order, SEO first and ads later, isn't a rule. With a budget that has to pay back within a few months the opposite makes sense: campaigns straight away, and the search terms that convert as the starting list for keyword research. How to make the two channels work together is explained in SEO and Google Ads together.
Frequently asked questions about SEO and SEM
SEO works on the organic results and doesn't pay for the click. SEM, in everyday use, is paid advertising on search engines, such as Google Ads, where every click has an auction price. SEO takes months and the results last; SEM starts in days and stops when the budget runs out.
In everyday practice, yes. SEA (search engine advertising) means only advertising on search engines, and that's the meaning most people give to SEM too. In some sources, for example Semrush and Search Engine Land, SEM is instead the combination of SEO and advertising.
It depends on the cost of a click in your sector and how quickly you need leads. Divide the SEO fee by the CPC of your searches: with a £1,000 fee and a £10 CPC, around 100 organic visits a month match the value of the traffic; with a £3 CPC you need more than 330.
No. Google Ads Help says that running online ads has no impact on where your website appears in organic search results. The two channels help each other in other ways: data on the search terms that convert, and a double presence on the same results page.
SEM brings clicks from day one, with a learning period for automated bidding that Google puts at up to around 50 conversion events or 3 conversion cycles. SEO is measured in months: in the 2025 Ahrefs study only 1.74% of new pages reach the top 10 within a year.
It varies enormously by search. In Google Keyword Planner data for the UK in October 2026, converted from dollars at the Bank of England rate of 8 October 2026, a click cost £3.03 for led wall, £7.32 for plumber london, £9.49 for accountant london and £35.94 for divorce solicitor london.
Something the industry rarely says
The Google Ads search terms report is the best keyword research tool a business has, and it's almost always locked inside an account the SEO people never see. It shows the exact words typed by the people who then asked for a quote, how many times and at what cost. No SEO tool estimates this: only the person paying for the clicks measures it. When SEO and SEM are handled by two different suppliers, the first thing to ask is whether they share that report every month.
