Transformer Calculator

The formulas behind the calculator

An ideal transformer’s turns ratio equals its voltage ratio, while the current ratio runs in the opposite direction:

For an ideal transformer, apparent power is equal on both sides:

Knowing the primary and secondary voltage gives you the turns ratio immediately. Add one more value — either primary current, secondary current, or apparent power — and the calculator can determine the remaining values.

What does the turns ratio mean?

The turns ratio compares the number of turns on the primary winding with the number of turns on the secondary winding. Because the turns ratio equals the voltage ratio, it determines whether a transformer increases or decreases voltage.

For example, a 20:1 turns ratio means the primary winding has 20 times as many turns as the secondary winding. The calculator determines the ratio, not the actual number of turns. A 20:1 ratio could represent 200 primary turns and 10 secondary turns, for example.

Worked example

A transformer steps 480 V down to 120 V and draws 10 A on the primary side. The turns ratio is 480 V ÷ 120 V = 4:1. Apparent power is 480 V × 10 A = 4,800 VA. The secondary current is 4,800 VA ÷ 120 V = 40 A. Current increases as voltage decreases, keeping apparent power equal on both sides of an ideal transformer.

VA vs. watts

Transformers are commonly rated in volt-amperes (VA) or kilovolt-amperes (kVA), rather than watts. VA measures apparent power, which reflects the voltage and current the transformer must handle. Watts measure real power delivered to a load.

For an AC load, real power depends on its power factor:

For example, a 2,400 VA transformer can handle 2,400 VA of apparent power, but the real power delivered to a load may be lower depending on the load’s power factor.

VA measures apparent power, while watts measure real power. If you want to estimate the electricity consumption and running cost of an appliance, use our Electricity Calculator.

Step-up, step-down, and isolation

A turns ratio greater than 1:1 means the primary voltage is higher than the secondary voltage, making it a step-down transformer. A ratio less than 1:1 means the secondary voltage is higher, making it a step-up transformer.

A 1:1 ratio is commonly used for an isolation transformer. The voltage remains approximately the same, while the primary and secondary windings remain electrically separated. Isolation transformers can be used for electrical isolation, safety, and noise reduction.

Power vs. energy

A transformer’s VA rating describes apparent power, while watt-hours (Wh) and kilowatt-hours (kWh) describe energy used over time. Power tells you how quickly energy is being transferred; energy tells you how much is transferred over a period of time. For calculations involving power and energy over time, see our Watt-hour Calculator.

Frequently asked questions