How to use it
- Type two of the ratio values: Primary voltage V1 and Secondary voltage V2, or the two turns counts.
- Type one more value: a Primary current I1 or Secondary current I2, or the Rated power S with one voltage.
- Type the Efficiency from the nameplate or datasheet, or leave the default of 95 %.
- Read the Turns ratio and the currents, the Input power and the Losses, then open Working, step by step.
The ideal transformer
The two windings share a magnetic core. The voltage across each winding is proportional to its number of turns, so the voltage ratio equals the turns ratio. Power is the same on both sides in the ideal case, so the current goes the other way round.
V1 ÷ V2 = N1 ÷ N2 = I2 ÷ I1 = a
S = V1 × I1 = V2 × I2
Step-down: a above 1 (the secondary voltage is lower)
Step-up: a below 1- The VA rating is the same on both sides in the ideal case. The low-voltage side always carries the larger current.
Efficiency and losses
No transformer is lossless. The resistance of the windings and the losses in the core turn part of the input into heat. Efficiency is the useful output divided by the input, so the input is larger than the output by the losses.
η = P_out ÷ P_in
P_in = P_out ÷ η
Losses = P_in − P_out- The calculator applies the efficiency to the volt-amperes. That is exact when the power factor is the same on both sides, as for a resistive load.
What the calculator does not cover
- The magnetising current and the leakage reactance, which make the currents of a real transformer differ a little from the ideal values.
- Three-phase transformers and their winding connections (delta and star), which change the line voltages and currents.
- Taps, the voltage regulation under load and the temperature rise. Take these from the nameplate and the manufacturer's data.
Worked example: 230 V to 12 V, rated 600 VA, 95 % efficiency
The turns ratio is a = V1 ÷ V2 = 230 ÷ 12 = 19.17, a step-down transformer.
The rated output current is I2 = S ÷ V2 = 600 ÷ 12 = 50 A. The primary current is I1 = I2 ÷ a = 50 ÷ 19.17 = 2.609 A, which is the same as S ÷ V1 = 600 ÷ 230.
At 95 % efficiency the input is 600 ÷ 0.95 = 631.6 VA, so the losses are 631.6 − 600 = 31.6 VA.
Frequently asked questions
What is the turns ratio?
The turns ratio a is N1 ÷ N2. In an ideal transformer it equals V1 ÷ V2 and I2 ÷ I1 as well. A ratio above 1 steps the voltage down, and a ratio below 1 steps it up.
Why does the low-voltage side carry the larger current?
The power is nearly the same on both sides, so when the voltage falls the current has to rise by the same factor. For 600 VA, the 12 V side carries 50 A while the 230 V side carries 2.6 A.
What efficiency should I enter?
Use the figure from the nameplate or the datasheet. The Wikipedia article on transformers gives 98 to 99 % as typical for distribution transformers, and small transformers are usually less efficient than large ones.
Why is the rating in VA and not in watts?
A transformer heats according to the voltage and current in its windings, not according to the power factor of the load. The rating is therefore given in volt-amperes; multiply by the load's power factor to get watts.
Can I enter only the turns?
No. Turns give the ratio, but you also need one absolute value: a voltage, a current, or the VA with one of those. Otherwise the size of the voltages is not known.
Sources
- Wikipedia: Transformer (ideal transformer equations, efficiency of distribution transformers)
- Wikipedia: Energy conversion efficiency (useful output divided by input)
Figures and text last checked on Tuesday, November 10, 2026.