Why working out an ad budget is harder than it looks
Most business owners approach an advertising budget backwards. They decide they can afford £300 a month, hand it to a platform like Google Ads or Meta, and hope something useful comes back. The trouble is that £300 buys a wildly different result depending on your industry, your average order value, and how much competition there is for the same customers. A firm selling £40 handbags and a solicitor bidding on personal injury claims are playing entirely different games with the same pound.
The Ad Budget Calculator exists to work the problem the other way round: starting from what you want the campaign to achieve, and showing what that is likely to cost. This guide explains the reasoning behind the numbers it produces, so you can adjust the inputs with some understanding of what each one is doing.
What the calculator actually does
You give it a goal, typically a number of leads, sales, or website visits you would like in a given period, along with a few figures about your business: your average conversion rate (the percentage of visitors who go on to buy or enquire), your average order value if relevant, and an estimate of what a click currently costs in your market. From those, it works backwards to a recommended monthly spend.
It is a calculator, not a forecast. It applies a formula to the numbers you supply, and if those numbers are rough guesses, the output will be a rough guess too. Real campaigns are affected by seasonality, competitor activity, and how good your landing page is at converting visitors once they arrive, none of which the tool can see. Treat the result as a sensible starting point for a conversation with yourself about spend.
The maths behind the numbers
Three terms do most of the work here, and it is worth being clear on each before you type anything into the calculator.
Cost per click (CPC)
Cost per click, usually written as CPC, is what you pay each time someone clicks your ad. On Google Ads this is set by an auction: you and your competitors bid on the same search terms, and the platform decides who pays what based on bid amount and ad quality. CPCs for a plumber in Manchester bidding on "emergency plumber" will look nothing like the CPCs for a software company bidding on a niche technical term, so a generic average is only ever a rough guide.
Click-through rate (CTR)
Click-through rate, or CTR, is the percentage of people who see your ad and click it. It affects how far your budget stretches indirectly, because a higher CTR usually earns a better position at a lower cost on platforms that reward relevant ads, but the calculator's core arithmetic runs on clicks and conversions.
Conversion rate
Conversion rate is the percentage of visitors who complete the action you actually want, whether that is a purchase, a form submission, or a phone call. This is the figure that turns a click into a customer, and it is also the one business owners most often guess wrong, because it depends heavily on the website the ad sends people to.
Put together, the logic runs: divide your target number of conversions by your conversion rate to get the number of clicks you need, then multiply that by your CPC to get your budget. A target of 50 leads at a 5% conversion rate needs 1,000 clicks; at £2 a click, that is a £2,000 budget for the period.
Using the calculator step by step
- Set a realistic goal. Decide whether you are aiming for leads, sales, or traffic, and pick a number for the period you are budgeting for, usually a month.
- Check your conversion rate. If you have Google Search Console or an analytics tool connected to your site, look at your actual historical conversion rate. If you have no data yet, a cautious estimate is safer than an optimistic one.
- Estimate your CPC. Use figures from your own past campaigns if you have run any, or compare typical costs for your sector with the CPM / CPC Benchmark Comparator, which shows benchmark ranges by industry.
- Enter your average order value, if relevant. This lets the calculator express the result as a return on spend as well as a raw budget figure, which is useful when you are deciding whether the campaign is worth running at all.
- Read the output as a range. Try the calculator with slightly more optimistic and slightly more cautious conversion rates, and see how much the recommended budget moves. If it swings wildly, your conversion rate estimate is the thing to firm up before you commit spend.
A worked example
Say a small business wants 30 new enquiries a month from Google Ads, its website converts visitors to enquiries at around 4%, and CPCs in its market average £3. Working through the formula: 30 enquiries at a 4% conversion rate needs 750 clicks, and 750 clicks at £3 each comes to £2,250 for the month. If the business tightens its landing page and lifts conversion to 6%, the same 30 enquiries need only 500 clicks, bringing the budget down to £1,500 for the same result. This is why the calculator is as useful for testing what happens if you improve your website as it is for pricing up a campaign from scratch.
Common mistakes when setting an ad budget
- Using an industry-wide average CPC. Averages published in general guides often blend very different sectors together. A benchmark for your specific industry, such as the ranges in the CPM / CPC Benchmark Comparator, gets you closer to the real figure.
- Guessing conversion rate optimistically. Founders tend to expect more from their website than the data supports. If you have no analytics history, start with a cautious estimate and revise it once you have real figures.
- Ignoring the landing page. Two campaigns with identical budgets and identical CPCs can produce very different numbers of customers, because the page the ad sends traffic to does most of the converting. Spend on ads without a page built to convert is spend that leaks away.
- Setting the budget once and never revisiting it. CPCs move with competition and season, so a figure that worked in January can be wrong by June. Re-run the calculator periodically rather than treating the first output as permanent.
- Treating the output as a promise. The calculator applies a formula to the numbers you give it. It cannot see your competitors, your ad quality, or how the platform's auction will actually behave, so results in practice will vary from the estimate.
Where paid spend fits alongside everything else
An ad budget is one channel among several, and it is rarely the right starting point for a business with little existing traffic. If you have not yet worked out which promotion channels deserve your time and money before you consider ads, the Promotion Channel Prioritiser is a reasonable place to start, and the accompanying guide to using it explains the reasoning behind its recommendations in the same way this guide has for the budget calculator. Unfamiliar terms used across either tool are explained in full in the promotion glossary, if you would rather look something up than take it on trust.