1. Defining performance indicators (KPIs)
Before reaching for the calculator, you need to know what you're measuring. A website doesn't always generate money "directly" (like a shop), but it always has economic value.
- Direct sales vs Lead generation: For an e-commerce site, it's simple (turnover). For a showcase site, the KPI is "conversion" (a form completed, a call clicked).
- Customer Acquisition Cost (CAC): This is the total amount spent to acquire a new customer. Example: If you invest €5,000 in your website and it brings you 50 customers in the first year, your CAC is €100. Is it profitable? That depends on the next section.
- Customer Lifetime Value (LTV): This is the total profit a customer generates over the entire duration of their relationship with the company. If a customer costs €100 to acquire but generates €2,000 over 3 years, your investment is excellent.
2. The mathematical calculation method
There is a universal formula to reassure a decision-maker:
ROI = (Gains generated - Cost of investment) / Cost of investment x 100
- Calculating the gain: For a showcase site, the gain is estimated by multiplying the number of leads by the commercial conversion rate and by the average basket.
- Analysis period: A website is generally amortised over 24 to 36 months. An ROI calculated over just 1 month will often be negative, whereas over 3 years it is massively positive.
3. The "invisible" gains (The ROI of automation)
A well-designed website saves time, and time is money.
- Human time saving: If your website automates appointment booking or answers the 10 most frequent questions via a dynamic FAQ, how many secretarial hours are saved per month?
Calculation: 5 hours saved/week x 52 weeks x €30 (hourly cost) = €7,800 savings per year. The website pays for itself on this point alone.
- Error reduction: A well-structured form prevents manual data entry errors that are costly in after-sales service.
4. Behaviour analysis (Data-Driven ROI)
We don't guess, we measure.
- Conversion Rate: This is the most powerful lever. Going from 1% to 2% conversion (thanks to better UX) doubles turnover without spending a single extra euro on advertising.
- Measurement tools: A clean installation of Google Analytics 4 or Matomo allows you to prove where revenue comes from.
5. Short-term vision (SEA) vs Long-term (SEO)
This is the analogy between "renting" and "owning".
- SEO (Organic search): This is a heavy initial investment that becomes a "passive income stream". Once positioned, traffic arrives "for free". The longer time passes, the more the cost per click (CPC) drops towards zero.
- Independence: A technically well-optimised site (performance, markup) reduces dependence on advertising budgets that increase every year.
Conclusion
The ROI of a website is not judged on the day it goes live, but on its capacity to become an autonomous collaborator working 24/7, without ever taking holidays.