How do you work out PPC agencies return on investment?

What to take away

Step 1: agree the numbers you will compare

Start with three figures: media spend, agency fee and gross profit per sale. A £500 service at 40% gross margin carries £200 of profit per sale. An agency charging £1,500 a month plus £6,000 of media needs £7,500 of gross profit, roughly 38 sales, before the campaign breaks even. Write that target down first.

Then fix a target return before the first click. If your business wants a return of 3.0, a £2,000 monthly outlay must produce £6,000 of incremental gross profit. That figure keeps month-end conversations factual rather than anecdotal.

Work in figures without VAT. A VAT-registered business normally recovers VAT on the agency fee and on media, so including it would overstate cost. A business that is not VAT-registered cannot recover it, and should add the 20% standard rate to its outlay.

Step 2: separate incremental profit from existing sales

Only sales you would not have made anyway count. A brand already ranking first for its own name still collects paid clicks on that term, and those are not incremental. Ask the agency to report branded and non-branded separately, and to strip out sales that organic or direct traffic would have delivered regardless.

A geo test can settle the question. Run paid search in one region and hold it back in another with a similar sales pattern, then compare the two. Where a test is impractical, ask what share of conversions came from new customers rather than returning ones. The attribution checks explain why platform-reported conversions overlap.

Step 3: build the tracking before the spend

Conversion measurement rests on tags and cookies. In the UK, consent is needed before non-essential cookies are set. Audit the set-up in month one: a broken tag hides a losing campaign or flatters a mediocre one. Check the Google Ads conversion actions and the Google Analytics 4 (GA4) events, and confirm your consent management platform passes the visitor's choice on. Google's documentation on conversion measurement explains how reported conversions are defined, and your profit calculation has to use the same definition.

Missing consent shows up as missing conversions. Where a large share of visitors reject cookies, the reported figure understates real sales and your return calculation reads low. Ask what share of sessions consented, and note it beside the result.

Step 4: calculate return on investment

Use the formula from the takeaways: incremental gross profit divided by agency fee plus media spend. A team paying £400 a month in fees and £1,600 in media has an outlay of £2,000. If the campaign generates £3,000 of incremental gross profit, the return is 3,000 divided by 2,000, which is 1.5. After costs, the team keeps £1,000. Below 1.0, the arrangement loses money.

Do not confuse this with return on ad spend (ROAS), which divides revenue by media spend alone. ROAS ignores the fee and your margin, so a healthy ROAS can still sit alongside a return below 1.0.

Payback timing matters as much as the ratio. If incremental profit arrives over nine months, a return of 1.5 can still leave a cash gap in the first quarter. Ask when cumulative profit first covers the cumulative outlay.

Keep the media and fee lines separate in the report. Blending them hides whether the agency or the auction is driving a change in cost per acquisition.

What belongs in the denominator?

Fee and media, plus the recurring extras: landing page builds, feed management, analytics setup. Charges outside the fee erode return quietly, and the PPC agencies hidden costs page lists the ones buyers most often miss.

How do you handle a wobbly month?

Judge on a rolling quarter. Seasonality and auction pressure move costs around, so compare the quarter's cost per acquisition with the target you set in Step 1 rather than reacting to one weak month.

Which route fits your situation

Match the buying model to your budget and your margin.

Common questions

How often should I recalculate PPC agencies return on investment?

Recalculate monthly with the same formula, but change your judgement only after a full quarter. Keep a rolling twelve-month view for planning.

What counts as a good return on investment?

There is no universal figure: a business on 10% margins needs far more revenue than one on 60%. Set the target from your own gross profit per sale. A return above 1.0 means the arrangement covered its costs, and the target you set in Step 1 says whether that is enough.

Can I compare agencies on return alone?

No, because each agency inherits a different account, budget and market. Compare like for like: same period, same margin assumptions, same tracking.

What is the biggest mistake buyers make?

Counting revenue instead of gross profit. A £100,000 revenue month at 5% margin contributes £5,000, which may not cover a £3,000 fee plus media.