How to Evaluate the ROI of Hiring an SEO Firm Before You Commit

How to evaluate the ROI of hiring an SEO firm comes down to one sum, and plenty of agencies would rather you didn't do it.

Cost is what you pay them.

Return is the revenue from jobs that came through search.

Subtract the first from the second, divide by the cost, and you have your answer.

Below is the formula, worked through with real numbers.

The formula

ROI = (revenue from search − what you paid) ÷ what you paid.

Multiply by 100 for a percentage.

So £72,000 of work won through search against £18,000 of fees gives you (72,000 − 18,000) ÷ 18,000 = 3, or 300% ROI. This is the standard ROI formula and it's the one worth holding an agency to.

If you want to model the numbers for your own trade and area before you commit to anything, our SEO ROI calculator pulls real search volumes and lets you set your own conversion and close rates.

Step one: work out what it's really costing you

The monthly fee is the obvious part and it's rarely the whole part. Add up everything that only exists because you hired them:

• The monthly retainer, times however many months you're measuring

• Any setup, onboarding or website build fee charged separately

• Ad spend, if they're running Google Ads alongside and invoicing it together

• Tools you're paying for yourself rather than through them

• Content, photography or copy you've commissioned to feed the work

• Your own hours on calls, approvals, sending over job photos and answering questions

  
  

Fees also move. Some agencies quote one number and bill another once ad spend and extras land, which is worth understanding before you sign, and something we've written about in our piece on flexible SEO plans.

Step two: work out what came from search

This is the hard half, and it's where most ROI claims fall apart.

You need the revenue from jobs won through organic search. Not traffic, not rankings, not impressions. Jobs, with invoice values attached.

Attribution without guesswork

Four things make this measurable rather than a feeling:

Ask every caller how they found you and write it down. It's low tech and it's the single most reliable source you have, because the customer (hopefully) knows the answer.

Put a separate tracked phone number on the website. Calls to it can only have come from the site, which removes the largest blind spot in the whole exercise.

Tag the website enquiry form so submissions arrive identifiable, and keep them separate from emails and social messages.

Then reconcile against the jobs you actually invoiced.

Step three: run the sum

Take a kitchen fitter paying £2,500 a month over twelve months. That's £30,000 in fees.

Nothing lands in months one to three, which is normal. From month four the enquiries start, and by the end of the year ten kitchens have come through search at an average of £12,000 each. That's £120,000 of work.

(120,000 − 30,000) ÷ 30,000 = 3

So 300% ROI, or four pounds back for every pound in. On those numbers the decision makes itself, and it's roughly the shape you'd want to see for kitchen fitters and other high job value trades, where a handful of wins covers the year.

Run the same maths on a business with a £400 average job and it looks very different. That's not a failure of SEO, it's arithmetic, and it's why job value decides whether this is worth doing at all before anything else does.

Revenue is the headline, gross profit is the truth

Revenue is the right number for the calculation, because it's the one you can measure without argument and the one both sides can agree on. But you don't bank revenue.

Take that same £120,000 and put it through different margins:

Gross margin	Gross profit on £120,000	ROI after the £30,000 fee
25%	£30,000	0%, break even
30%	£36,000	20%
40%	£48,000	60%
50%	£60,000	100%
60%	£72,000	140%

Same revenue, same fee, and the answer runs from break even to 140% depending entirely on what you keep from each job.

When the number is allowed to be negative

Early on, it should be.

Month one buys a website, page structure and setup, none of which produce an enquiry the day they go live. Google has to crawl and index the pages, links have to be built and picked up, and authority has to register before anything moves. Google's own guidance on hiring an SEO is clear that changes take time to have an effect.

So a negative ROI at month three tells you almost nothing. A negative ROI at month nine tells you a great deal.

Measure at three months to check the leading signals, impressions rising and pages indexing. Measure ROI properly at twelve.

What quietly breaks the calculation

Five things distort the number in practice, and they don't all point the same way:

• Revenue mistaken for profit. Inflates it, as above.

• Judging too early. Understates it, badly, in the first six months.

• Repeat work not counted. Understates it. A customer won through search who comes back twice more should count all three jobs.

• Attribution gaps. Someone finds you on Google, notes the name, and rings the number off your van a fortnight later. That's an SEO win recorded as nothing.

• Mismatched periods. Twelve months of fees measured against nine months of results, or the reverse. Line the dates up.

The first one flatters the agency. The last four flatter nobody and quietly hide work that's going better than it looks.