What an ideal transformer does
A transformer transfers electrical energy between two circuits through a shared magnetic core, changing voltage and current levels according to the ratio of turns in each winding while (ideally) conserving power.
The formulas used
For an ideal transformer, the voltage ratio equals the turns ratio: Vs / Vp = Ns / Np. Because an ideal transformer has no losses, input power equals output power, which gives the current relationship Ip = Is × (Ns / Np). A step-down transformer (fewer secondary turns) raises current while lowering voltage, and a step-up transformer does the opposite.
Real transformers have some resistance, leakage inductance and core losses, so actual output will be slightly lower than this ideal calculation. This tool is for general electronics planning and education.