Transformer Calculator

How the Transformer Calculator Works

This transformer calculator uses the fundamental transformer equation V_p/V_s = N_p/N_s = I_s/I_p to derive missing values from the ones you provide. Enter any two of the four main parameters — primary voltage, secondary voltage, primary turns, or secondary turns — and the calculator computes the turns ratio and fills in the missing values. When current or VA rating is provided, it calculates power handling capacity and the corresponding current on the other side of the transformer. The efficiency parameter accounts for real-world losses in the transformer core and windings. An ideal transformer has 100% efficiency, but practical transformers range from 90% to 98% depending on size, core material, and operating frequency. All calculations run instantly in your browser with complete privacy.

Step-Up vs Step-Down Transformers

Transformers are classified by their turns ratio. A step-down transformer has more primary turns than secondary turns, producing a lower output voltage — this is the most common type, used in power supplies to convert mains voltage (120V or 240V) to lower voltages for electronics. A step-up transformer has fewer primary turns than secondary turns, increasing the output voltage. Step-up transformers are essential in power transmission, where generating station voltage (typically 11kV to 25kV) is stepped up to 132kV, 275kV, or 400kV for efficient long-distance transmission. The current relationship is inverse to the voltage relationship: when voltage steps up, current steps down proportionally, conserving power. An isolation transformer has a 1:1 ratio — it does not change voltage but provides galvanic isolation between circuits, which is critical for safety in medical equipment and test instruments.

Transformer VA Rating and Sizing

The VA (volt-ampere) or kVA rating indicates the maximum apparent power a transformer can handle continuously without overheating. Unlike watts, VA includes both real and reactive power components. To size a transformer correctly, calculate the total VA of all connected loads. For resistive loads like heaters and incandescent lamps, VA equals watts. For reactive loads like motors and fluorescent lighting, VA is higher than watts due to the power factor. Always select a transformer with a VA rating at least 20% higher than the calculated load to account for inrush currents, future expansion, and derating at high ambient temperatures. Common residential transformer sizes range from 25 VA for doorbells to 75 kVA for whole-house service. Industrial transformers can reach several MVA for factory power distribution.

Transformer Efficiency and Losses

Real transformers have two types of losses: core losses and copper losses. Core losses (also called iron losses) occur due to hysteresis and eddy currents in the magnetic core, and they remain relatively constant regardless of load. Copper losses (also called I-squared-R losses) increase with the square of the current flowing through the windings. Small transformers typically have efficiencies of 90-95%, while large power transformers achieve 98-99% efficiency. This calculator includes an efficiency field so you can account for these losses when determining actual power output. For precision applications, always verify the manufacturer's efficiency curve at your specific operating point, as efficiency varies with load level and reaches maximum at a particular fraction of full load.