I read on social media that a certificate pays a certain rate. Can I rely on that figure?

Do not rely on it alone. Posts and news can be outdated, can quote only a stepped certificate's year-one rate, can mix a total for the term with an annual rate, can refer to a term or payout schedule different from the one you want, or can even be about a Central Bank decision rather than a product rate. Rates change, so the rate to rely on is the one the bank publishes for the product itself, with its date. Before calculating anything: find out the product's exact name, check the published rate by term and payout schedule, the rate type (fixed, variable or stepped), and the minimum and its multiples. Then ask for the return computed on your amount, term and payout schedule. Note that banks must disclose their actual rates, so ask the bank directly or check its official website. And check the product's key-facts statement, which states the rate, its type, the payout schedule and a worked example.