How emf is calculated?

Answer 1

EMF is basically a term for voltage. Two rules that are often used in calculations are:

1) Kirchoff's Voltage Law, which states that the total voltage around a closed loop is zero. Alternately, the voltage provided by a battery (for example) is equal to the voltage drop accross one or more connected resistances.

2) Ohm's Law: V=IR (voltage = current x resistance).

For more details, and example, I would suggest consulting a high school (or first-semester college) book about electricity.

Answer 2

The emf of a battery depends mainly on the chemistry of the cells it uses. Lead-acid, carbon-zinc, alkaline, nickel-metal-hydride, lithium, etc. each have a different voltage per cell. The only general statement you can make that is true for all types is that a battery -- a series stack of cells -- has a total open-circuit emf equal to the product of the emf-per-cell multiplied by the number of cells.