Transformers are electrical devices with the ability to raise or lower the voltage of alternating current (AC) power. Their manufacturers wrap two wires, interwoven, around an iron (or sometimes air) core. The "primary" side has the wire where the unchanged voltage enters. The "secondary" side has the wire where the new voltage leaves. Through electromagnetic principles, when the original voltage enters from the primary side it causes a magnetic field inside the iron core, which in turn causes a new AC voltage in the secondary coil. The rise or drop in voltage across the transformer is directly related to the ratio of the numbers of turns of each coil: the transformer turns ratio.

Divide the primary voltage by the desired voltage. For example, if 10 volts exist on the primary side and 5 volts are desired on the secondary, then 10 divided by 5 equals 2. This result is the required transformer turns ratio. It means there must be twice as many loops on the primary side as the secondary side.

Divide the number of primary turns by the number of secondary turns, alternatively, to also get the transformer turns ratio, if you have been given that information upfront.

Set the ratio of primary to secondary voltages equal to the ratio of primary to secondary turns to solve for any one of those four values, if you have the other three.