Transformer Calculator
Calculate secondary voltage, turns ratio, and secondary current for transformers.
Enter primary voltage plus either secondary voltage or turns ratio. Primary current is optional.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
Can't find what you need?
Request a ToolHow to Use the Transformer Calculator
A transformer changes AC voltage levels through electromagnetic induction. The voltage ratio equals the turns ratio: Vp/Vs = Np/Ns. This calculator solves for secondary voltage, turns ratio, and secondary current.
- Enter primary voltage. The voltage applied to the primary winding (e.g., 120 V for US mains, 230 V for European mains).
- Enter secondary voltage or turns ratio. Enter either the desired secondary voltage (to calculate the ratio) or the known turns ratio Np/Ns (to calculate secondary voltage). You only need one of these.
- Enter primary current (optional). If you know the primary current, the calculator also shows secondary current and apparent power. Leave blank if not needed.
- Read the results. Secondary voltage, turns ratio, secondary current, and power are shown where applicable.
About Transformers
An ideal transformer obeys: Vp/Vs = Np/Ns = Is/Ip. Power is conserved: Vp × Ip = Vs × Is (assuming 100% efficiency). Real transformers have losses (typically 90% to 98% efficient), primarily from winding resistance (copper loss) and core eddy currents and hysteresis (iron loss). For sizing, always use the VA rating of the transformer, not watts alone.
Step-down transformers (Np > Ns) reduce voltage and increase current. Step-up transformers (Np < Ns) increase voltage and reduce current. Isolation transformers have a 1:1 ratio and provide electrical isolation without changing voltage. All calculations assume an ideal (lossless) transformer.
Frequently Asked Questions
What is the transformer turns ratio formula?
The turns ratio a = Np/Ns = Vp/Vs = Is/Ip. For a step-down transformer with a 10:1 ratio: secondary voltage = primary / 10, secondary current = primary × 10. For example, a 120 V primary at 1 A with a 10:1 ratio gives 12 V secondary at 10 A (assuming ideal, 100% efficiency). Power in equals power out: 120 W = 120 W.
What VA rating do I need for a transformer?
Size the transformer VA rating at least 20% above the maximum connected load. For a 100 W load, choose a 120 VA (or larger) transformer. For loads with high inrush current (motors, switching power supplies), add a larger safety factor. Transformers can overheat if continuously run at or above their VA rating. The VA rating accounts for both real and reactive load components.
Can I use a transformer for DC voltage conversion?
No. Transformers work only with AC because electromagnetic induction requires a changing magnetic flux, which requires AC (or pulsed DC). For DC voltage conversion, use a DC-DC converter (buck, boost, or buck-boost topology). However, the secondary of an AC transformer can be rectified and filtered to produce DC: this is how most linear power supplies work — transformer steps down the AC voltage, then a rectifier and capacitor convert it to DC.
What is the difference between a step-up and step-down transformer?
A step-down transformer has more primary turns than secondary turns (Np > Ns), reducing voltage. It is used to convert high distribution voltages to usable levels (e.g., 11 kV to 240 V at the grid edge). A step-up transformer has fewer primary turns than secondary (Np < Ns), increasing voltage. It is used to raise generator voltage for efficient long-distance transmission (e.g., 25 kV to 400 kV). An autotransformer uses a single winding with multiple taps and is more efficient but provides no electrical isolation.