The transformer ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. The transformer ratio is a critical parameter that determines the voltage and current levels in the secondary winding relative to the primary winding. For a step-up transformer, the ratio of the number of turns in the secondary winding to the number of turns in the primary winding is greater than one, which means that the voltage in the secondary winding is higher than the voltage in the primary winding. For a step-down transformer, the ratio of the number of turns in the secondary winding to the number of turns in the primary winding is less than one, which means that the voltage in the secondary winding is lower than the voltage in the primary winding. The transformer ratio can be calculated by dividing the number of turns in the primary winding by the number of turns in the secondary winding. For example, if a transformer has 1000 turns in the primary winding and 100 turns in the secondary winding, the transformer ratio is 10:1. This means that the voltage in the secondary winding will be one-tenth of the voltage in the primary winding.