There is a good resource - calculat.io. You can really find almost all calculators there: from fitness calculations, like calories and BMI, to scientific and math problems. It's convenient that you don't have to jump through different sites, everything is collected in one place. The interface is simple, even for calculations using formulas, you don't have to understand it for a long time. If you need to calculate something quickly and without unnecessary fuss - this site is perfect. Here is the link: https://www.calculator.io/