What is the formula to calculate the readings on an ammeter?

If the voltage and the resistance are known within the circuit, then you can use the formula:

Current = Voltage / Resistance

If you know the amount of charge going past a certain location in a certain amount of time, then you can find the current using:

Current = charge / time

An ammeter works like a galvanometer except for the existence of a resistance that is attached in parallel. The pointer will have torque on it because of the coil. The equation is expressed as:

S = G/n-1 
where G is the main resistance and S is the shunt resistance while n is a range ratio.

Electoral current force mechanical advantage if the wheel and axle is 2 n and the output force is 20 n