Enter your primary voltage, secondary voltage, primary turns, and primary current to calculate the turns ratio, secondary current, and impedance ratio of an ideal transformer. Leave any one field blank to solve for it using the transformer equations. Also try the Parallel Resistor Calculator.
Turns Ratio (Np : Ns)
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Secondary Current (Is)
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Impedance Ratio (Np/Ns)²
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Input Impedance (Zin)
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Apparent Power (S)
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Transformer Type
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An ideal transformer is a theoretical model that assumes 100% efficiency — meaning no energy is lost to resistance, core losses, or leakage flux. It perfectly transfers power from the primary winding to the secondary winding, making it a useful baseline for understanding real transformer behavior. See also our find Fresnel Zone Radius with Fresnel Zone Calculator.
A transformer uses two coils (primary and secondary) wound around a shared magnetic core. An alternating current in the primary coil creates a changing magnetic field, which induces a voltage in the secondary coil. The ratio of turns between the two coils determines how much the voltage is stepped up or stepped down.
The turns ratio is Np/Ns, where Np is the number of primary turns and Ns is the number of secondary turns. This ratio also equals Vp/Vs (primary voltage over secondary voltage) and Is/Ip (secondary current over primary current) for an ideal transformer.
A step-up transformer has more secondary turns than primary turns (Ns > Np), increasing the output voltage. A step-down transformer has fewer secondary turns (Ns < Np), reducing the output voltage. Power remains constant in both cases (for an ideal transformer). You might also find our calculate Shaft Torque, Torque (lb·ft) & Torque (lb·in) — Electric Motor Torque useful.
The impedance ratio equals the square of the turns ratio: (Np/Ns)². It describes how a load impedance on the secondary side is 'reflected' to the primary side. This is important in impedance matching applications, such as audio amplifiers.
In an ideal transformer, power in equals power out (Vp × Ip = Vs × Is). Therefore, secondary current Is = Ip × (Np/Ns). A step-down transformer that reduces voltage will proportionally increase the current on the secondary side.
No. Transformers require alternating current (AC) to operate. A changing current is needed to produce a varying magnetic field, which is what induces voltage in the secondary coil. Applying DC would not produce a changing field and could damage the transformer due to excessive current.
A current transformer (CT) is designed to measure or transform high currents to a lower, measurable value, and is connected in series with the circuit. A voltage transformer (VT or PT) steps voltage up or down and is connected in parallel. Both operate on the same ideal transformer principles.