"How much should I invest in ads?" That is the number 1 question business owners ask themselves before they start with online advertising. And the classic agency answer, "it depends", does not help much.
This guide gives you concrete figures, calculation methods and sector benchmarks so you can set the advertising budget that matches your objectives. At ConvertiLab, we run campaigns on budgets from 300€ to 10,000€/month for 150+ business owners.
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What does an online advertising budget depend on for a small or trades business?
An advertising budget depends on 4 factors: your commercial objective (the number of leads or sales you want), your sector (the CPC varies enormously, from 0.30€ in e-commerce to 20€ in legal services), your geographic area (local vs national targeting), and the competition in your market. The best approach is the reverse method: start from your sales objectives and work out the budget you need, rather than starting from an arbitrary budget.
Average CPC by sector in France (2026 reference):
| Sector | Average Google Ads CPC | Average Meta Ads CPC |
|---|---|---|
| E-commerce | 0.50 to 2€ | 0.30 to 1€ |
| Local services | 1 to 5€ | 0.50 to 2€ |
| Property | 2 to 8€ | 1 to 3€ |
| Insurance / Finance | 5 to 15€ | 1 to 5€ |
| Legal | 5 to 20€ | 2 to 5€ |
| SaaS / Tech | 3 to 10€ | 1 to 4€ |
How do you calculate your advertising budget with the reverse method?
The reverse calculation starts from your objectives and works back to the budget you need. The formula: monthly budget = number of clients wanted × customer acquisition cost (CAC). A concrete example for a local small business: a target of 20 clients a month, a site conversion rate of 3%, an average CPC of 2€ → 20 / 0.03 = 667 clicks needed × 2€ = 1,334€/month. The profitability check: 20 clients × an average order of 490€ = 10,000€ of revenue / 1,334€ of budget = a ROAS of 7.5× (excellent).
The 3 figures to calculate before you launch any advertising campaign:
1. Your maximum acceptable CAC:
(average order × margin rate) - fixed costs = the net value of a client
If that value is 200€, your CAC must not exceed 200€ for the campaign to stay profitable.
2. Your minimum viable budget:
To leave the Google Ads learning phase, you need 50 conversions a week = 200 conversions a month. If your conversion rate is 3% and your CPC is 3€, you need 6,667 clicks × 3€ = 20,000€. Out of reach for a small business? Lower the target to 10 conversions a week and accept slower optimisation over 3 months.
3. The break-even point:
Budget invested / (conversion rate × average order) = the number of clicks needed to cover the budget. Below that threshold, every campaign loses money.
Recommended budgets by platform and level of ambition:
| Platform | Test budget | Functional | Competitive |
|---|---|---|---|
| Google Ads | 500€/month | 1,000 to 2,000€/month | 3,000€+/month |
| Meta Ads | 300€/month | 500 to 1,500€/month | 2,000€+/month |
The Google Ads split rule: 70% Search (commercial intent) + 20% remarketing + 10% testing.
The Meta Ads split rule: 60% acquisition (cold audiences) + 30% retargeting + 10% creative testing.
Which budget mistakes cost small businesses the most?
The 5 most expensive budget mistakes: investing too little (200€/month on Google Ads in a competitive sector does not generate enough data), skipping the test phase (always keep 20% back for testing), cutting campaigns too early (a campaign needs at least 50 conversions before the algorithm can optimise), ignoring seasonality (match the budget to the peaks in your sector), and forgetting the management fees (management accounts for 15 to 25% of the media budget). A sixth major mistake is often overlooked: not raising the budget gradually. Doubling an advertising budget in one go resets the learning phase, so increase it by 20% a week at most to preserve the algorithmic optimisation you have already earned.
A realistic total budget to plan for (everything included):
| Component | % of total budget | Example (3,000€/month) |
|---|---|---|
| Media budget (Ads) | 65 to 75% | 2,100€ |
| Management / agency | 15 to 25% | 600€ |
| Tools (tracking, A/B testing) | 5 to 10% | 300€ |
Is it better to run your campaigns yourself or hand them to an agency?
Running your advertising campaigns yourself makes sense if the budget is under 1,000€/month, if you have 5 to 10 hours a week to give it, and if your campaigns are simple (one service, one geographic area). Above 1,000€/month, or for campaigns across several platforms, an agency starts to pay for itself: the optimisation gains more than cover the fees. A Google Ads specialist who takes your Quality Score from 5/10 to 8/10 cuts your CPC by 30 to 40%. A rule of thumb: a specialist agency should generate a net gain (CPC savings plus extra conversions) at least 2× its management fees to be worth it. Always ask for read access to your own Ads accounts, a serious agency has no reason to refuse.
3 levers to improve your advertising ROI:
ConvertiLab case study: a physiotherapist in Strasbourg was investing 400€/month in Google Ads for 8 new patients a month (CAC 50€). After the audit: a slow landing page (4.2s to load), no visible testimonials, a 7 field form. Once the landing page was optimised (1.8s, testimonials, 3 fields), on the same budget he now gets 19 patients a month, with the CAC divided by 2.4 down to 21€, without adding a single euro to the advertising budget.
Frequently asked questions about online advertising budgets
What is the minimum budget to start with online advertising?
Technically, you can start with 5€ a day on Meta Ads (150€/month). But to get usable data and meaningful results, plan on at least 500€/month on a single platform. Below that, you do not have the volume to optimise the campaigns. To work out the exact budget that matches your objectives, use our advertising budget estimator.
How long before digital advertising becomes profitable?
With good targeting, you can be profitable from the first month on Google Ads (strong buying intent). On Meta Ads, allow 1 to 3 months to refine the targeting and the creatives. Retargeting campaigns are usually profitable from the first month because they target prospects who already know your brand.
Google Ads or Meta Ads: which should you choose for your business?
Google Ads targets intent (people are actively looking for your solution). Meta Ads targets interest (people discover your offer). For B2B and local services, Google Ads is often more effective. For B2C and visual products, Meta Ads excels. The ideal in the long run is to combine the two: Google Ads to capture existing demand, Meta Ads to create demand and to retarget.
How do you know whether you are spending too little or too much on digital advertising?
You are spending too little if your CPC is high but you get few conversions, because the algorithm does not have enough data to optimise (Google Ads Performance Max needs at least 50 conversions a week). You are spending too much if your ROAS falls below 1.5×, meaning every euro invested brings back less than 1.5€, which leaves the net margin negative. The target profitability zone is a ROAS between 3× and 7× depending on your product margin. !Optimal advertising budget - ROAS profitability threshold 3x-7x conversions algorithm data
