Relative strength index (RSI)

Hi Community,
I’m trying to find a formula to calculate the relative strength index (RSI) based on a stock series of at least 14 days.
I’m already using moving average but I feel to be not enough.
Did someone already developed such of formula?

Thank you in advance for your help.

Denis

DuckDuckGo Search for relative strength index (RSI) first hit gives this.