What is formula to find mass?

Answer 1

That depends, based on what you want to figure out the mass.

To measure a mass, it can be done either by inertia (see how much force produces how much acceleration, and apply Newton's Second Law), or by its weight (measure the weight in Newtons, and divide by Earth's gravity - measuring force based on pulling a spring, for example). A spring balance is usually already calibrated for units of mass such as kilograms (implicitly assuming Earth gravity, of course). It is also possible to compare an object's weight to another, known, weight, on other types of balances.